Metamorphic remodeling of morphology and the body cavity in Phoronopsis harmeri (Lophotrochozoa, Phoronida): the evolution of the phoronid body plan and life cycle
© Temereva and Malakhov. 2015
Received: 21 April 2015
Accepted: 1 October 2015
Published: 21 October 2015
Phoronids undergo a remarkable metamorphosis, in which some parts of the larval body are consumed by the juvenile and the body plan completely changes. According to the only previous hypothesis concerning the evolution of the phoronid body plan, a hypothetical ancestor of phoronids inhabited a U-shaped burrow in soft sediment, where it drew the anterior and posterior parts of the body together and eventually fused them. In the current study, we investigated the metamorphosis of Phoronopsis harmeri with light, electron, and laser confocal microscopy.
During metamorphosis, the larval hood is engulfed by the juvenile; the epidermis of the postroral ciliated band is squeezed from the tentacular epidermis and then engulfed; the larval telotroch undergoes cell death and disappears; and the juvenile body forms from the metasomal sack of the larva. The dorsal side of the larva becomes very short, whereas the ventral side becomes very long. The terminal portion of the juvenile body is the ampulla, which can repeatedly increase and decrease in diameter. This flexibility of the ampulla enables the juvenile to dig into the sediment. The large blastocoel of the larval collar gives rise to the lophophoral blood vessels of the juvenile. The dorsal blood vessel of the larva becomes the definitive median blood vessel. The juvenile inherits the larval protocoel, mesocoel, and metacoel. Late in metamorphosis, however, the protocoel loses its epithelial structure: the desmosomes between cells and the basal lamina under the cells disappear. This loss may reflect a reduction of the protocoel, which is a characteristic of some recent phoronids.
Based on our investigation of P. harmeri metamorphosis, we hypothesize that the phoronid ancestor was worm-like animal that possessed preoral, tentacular, and trunk coeloms. It lived on the soft sediment and collected food with its tentacles. When threatened, this worm-like ancestor buried itself in the soft sediment by means of the ventral protrusion into which the loop of the intestine and the blood vessels were drawn. We propose that this behavior gave rise to the body plan of all recent phoronids. The evolution of phoronid life cycle seems having more in common with“intercalation” than “terminal addition” theories.
Metamorphosis is a remarkable event in the life cycle of some metazoans. It occurs in animals with indirect development and characterized by converting a larva, with a particular morphology, into a juvenile, with a different and equally distinctive morphology. Investigation of metamorphosis has great significance for understanding of the bilaterian life cycle and usually is discussed in the light of two main theories about origin of the bilaterian larvae [1–6]. According to “intercalation” theories the larval stages (planktotrophic or lecithotrophic) have evolved as specializations from the ancestral, direct life cycle [2–4]. The opposing “terminal addition” theories propose that the ancestor was holopelagic and that the adult stage was added to the life cycle with the pelagic stage retained as a planktotrophic larva [7–11]. Although investigation of metamorphosis has such a great significance for evolutionary analysis, metamorphosis of some enigmatic animals is still poor studied. One of these enigmatic groups is phylum Phoronida, whose metamorphosis is very unusual and the quickest among all animals with biphasic life cycle [1, 12].
The Phoronida is a small group of marine invertebrates with a biphasic life cycle. Adult phoronids live in their own tube in hard or soft substrata as benthic animals . Their body completely embedded into substrata and only the anterior part of the body is exposed into the water. This portion of the body bears the lophophore – a special part of the mesosome, which performs several main functions including the collecting of food particles, the brooding of embryos, and respiration [13, 14]. Most phoronid species have planktotrophic larvae, actinotrochs, which live in plankton for one or several months [15–17]. By the end of pelagic life, larvae acquire some specific characteristics and then undergo catastrophic metamorphosis, which leads to formation of the definitive body plan .
Adult phoronids have an unusual body plan: their mouth and anus are located very near each other, and the digestive tract is U-shaped. Thus, in adult phoronids, the dorsal side is very short, whereas the ventral side is very long. On the other hand, phoronid larvae have a more standard organization with dorsal and ventral sides of similar length. Currently, there is only one explanation for how the unusual body plan of adults appeared during phoronid evolution . This explanation presumes that a hypothetical ancestor of phoronids inhabited a U-shaped burrow in soft sediment, where it drew the anterior and posterior parts of the body together and eventually fused them . As a consequence of folding, the paired coelomic sacks situated along the ascending and descending portions of the gut contacted each other and fused, forming the lateral mesenteries along the line of contact. In adult phoronids, the trunk coelom is divided into four chambers by five mesenteries . Three of the mesenteries are parts of the anal-oral mesentery, which is common for most Bilateria, whereas the two lateral mesenteries are peculiar to phoronids and are not known in other bilaterians except the closest phoronid relatives, the brachiopods, some of which have one or even two pairs of lateral mesenteries [20, 21].
Although phoronid metamorphosis has been investigated many times by different methods [8, 22–27], a detailed description of the remodeling of the external morphology and the coelomic system during metamorphosis is lacking. Such a description might help to answer the question “What did the phoronid ancestor look like?”
The objectives of this work are to describe the remodeling of the external morphology and to trace the fate of body cavities during metamorphosis in Phoronopsis harmeri. Based on these findings, we will propose a new hypothesis about the evolution of the phoronid body plan. We will also consider some phylogenetic implications of our results and speculate about evolution of the phoronid life cycle.
Competent larvae of P. harmeri Pixell, 1912 were collected with a planktonic net during November of 2011 in Vostok Bay, Sea of Japan. In the laboratory, larvae were kept at 0–1 °C until metamorphosis. Many larvae underwent metamorphosis shortly after capture. For this reason, it was easy to collect metamorphic animals and fix them at 1–3 min intervals up to the newly formed juvenile – about 40 min after the onset of metamorphosis. Then 1-, 2- 3- 4-h-old, as well as 1-, 2-, 3-, 4-, and 9-day-old animals were collected and fixed for future investigations (see below). More than 30 individuals of each stage are investigated by different methods.
Competent larvae, metamorphic stages, newly formed juveniles, and 4-day-old juveniles were photographed using a Panasonic DMC-TZ10 digital camera (Panasonic, Kadoma, Japan) mounted on a binocular light microscope. All of these stages were prepared for histology, scanning electron microscopy (SEM), transmission electron microscopy (TEM), cytochemistry, and confocal laser scanning microscopy (CLSM).
For histology, a 4 % paraformaldehyde (PFA) solution in filtered sea water was used as a fixative. Competent larvae, metamorphic stages of P. harmeri, newly formed juveniles, and 3- and 9-day-old juveniles were incubated in PFA for 8–10 h, rinsed in distilled water, dehydrated in ethanol, and embedded in Paraplast Regular (Sigma). Cross sections (5 μm thick) made with a Leica rotary microtome (Leica RM 2125; Leica Microsystems GmbH, Wetzlar, Germany) were stained with Caracci hematoxylin. Sections were examined with a Zeiss Axioplan2 microscope and photographed with an AxioCam HRm camera.
For SEM, fixed metamorphic stages of P. harmeri that had been dehydrated in ethanol followed by an acetone series were critical point dried and then sputter coated with platinum-palladium alloy. Specimens were examined with a Jeol JSM scanning electron microscope (JEOL Ltd., Tokyo, Japan).
For TEM, metamorphic stages, newly formed juveniles, and 4-day-old juveniles of P. harmeri were fixed at 4 °C in 2.5 % glutaraldehyde in 0.05 M cacodylate buffer containing 21 mg/ml NaCl and then postfixed in 2 % osmium tetroxide in the same buffer containing 23 mg/ml NaCl. Postfixation was followed by en bloc staining for 2 h in a 1 % solution of uranyl acetate in distilled water. Specimens were then dehydrated in ethanol followed by an acetone series and embedded in Spurr resin (Sigma Aldrich). Semithin and thin sections were cut with a Leica UC5 ultratome (Leica Microsystems GmbH, Wetzlar, Germany). Semithin sections were stained with methylene blue, observed with a Zeiss Axioplan2 microscope, and photographed with an AxioCam HRm camera (Carl Zeiss, Oberkoche, Germany). Thin sections were stained with lead citrate and then examined with a JEOL JEM 100B electron microscope (JEOL Ltd., Tokyo, Japan).
For cytochemistry, metamorphic stages of P. harmeri, newly formed juveniles juveniles were narcotised in MgCl2, fixed overnight in a 4 % paraformaldehyde solution in filtered sea water, and washed two times in phosphate buffer (pH 7.4) (Fisher Scientific) with Triton X-100 (0.3 %) (Fisher Scientific, Pittsburgh, PA, USA) for a total of 2 h. Then, the specimens were washed in PBT (phosphate buffer + Triton) and incubated in a mixture of rhodamine-conjugated phalloidin (1:50) (Fisher Scientific, Pittsburgh, PA, USA) for 1 h at room temperature in the dark. They were subsequently washed in phosphate buffer (three times × 40 min), mounted on a cover glass covered with poly-L-lysine (Sigma-Aldrich, St. Louis, MO, USA), and embedded in Murray Clear. Specimens were viewed with a Leica TCS SP5 confocal microscope (IDB, Moscow, Russia). Z-projections were generated using the program Image J version 1.43.
The use of phoronids in the laboratory does not raise any ethical issues and therefore approval from regional or local research ethics committees is not required. The field studies did not involve endangered or protected species.
Morphology of competent larvae
Remodeling of external morphology
Remodeling of the body cavities
P. harmeri larvae have a spacious blastocoel in the trunk between the body wall and the lining of the metacoel (Fig. 4a). During metamorphosis, this portion of the blastocoel undergoes reduction because of degeneration of the larval trunk, which becomes very short and forms a sack around the larval telotroch (Fig. 9b).
During the later stages of metamorphosis (stages about 20–25 min after the onset of metamorphosis), the protocoel appears as a small sack with a small lumen (Fig. 12b, c). The lumen of the protocoel is filled with thick, apical protrusions of cells of the protocoel lining. All cells of the protocoel have a similar ultrastructure. These cubical cells have a large nucleus with a nucleolus, numerous canals of rough reticulum, mitochondria, Golgi apparatus, and vesicles (Fig. 12c). Desmosomes occur between some cells but not others. The protocoel is surrounded by an extracellular matrix, but the basal lamina that underlies the cells of the coelomic lining is absent (Fig. 12c).
The posterior portion of the larval metacoel undergoes reduction: its lumen disappears (Fig. 4a, d). The lining of the posterior portion of the larval metacoel is formed by pseudostratified epithelium. It consists of myoepithelial cells that form numerous basal projections, which are covered by a thick layer of the basal lamina (Fig. 14f). In 4-h-old juvenile, these cells separate from the lamina and float in the metacoel (Fig. 9b). Some of these cells are captured and apparently undergo phagocytosis by other cells of the metacoel lining (Fig. 14e). The new lining of the metacoel consists of myoepithelial cells, which have basal pouches containing the myofilaments (Fig. 14g). These pouches form a thin muscular net of the body wall (Fig. 8a).
The phoronid larva, i.e., the actinotroch, is a remarkable one. It is the only type of primary invertebrate larva with a blood system. In phoronids, the larval blood system consists of a spacious blastocoel, which is located in the collar region and contains several masses of erythrocytes, and several blood vessels extending along the digestive tract . The larval blood system does not function but is completely formed. Such an unusual feature, i.e., the presence of a blood system in the larval stage, can be regarded as a developmental acceleration, which correlates with the presence of a very complex blood system in adults [29–31]. Another example of such an acceleration is the organization of the larval protonephridia, which have a U-shaped duct in Phoronopsis harmeri larvae . In other phoronids, there are the nephridia that become U-shaped during metamorphosis .
In phoronid larvae, the coelom is organized in two different ways . Larvae of Phoronopsis spp. have a closed coelomic cylinder in the preoral lobe under the apical organ, whereas larvae of Phoronis spp. lack this coelomic cylinder . In the latter case, larvae have only a dorsal septum that extends between the dorsal border of the apical organ and the esophagus. This septum is formed by epithelial cells, which are connected via desmosomes (unpublished data).
Remodeling of external morphology
Phoronid metamorphosis is an unusual and rapid process: most of the external remodeling occurs within several minutes [17, 35, 36]. Phoronida is regarded as a group, whose metamorphosis is one of the fastest among all animals with pelagic larvae [1, 12]. Fast metamorphosis allows animals start to feed very soon after larval settlement . Phoronids start to eat immediately after metamorphosis beginning, because they first consume huge part of larval body. The same process of eating of whole larval body was recently described in nemerteans .
Fate of some larval structures during metamorphosis in studied phoronid species
Name of studied species and author of study
Fate of larval organs
“Actinotrocha A” (probably larva of Phoronopsis harmeri) 
transforms into small unpaired remnant that gives rise to the adult epistome
completely turns into definitive tentacle
retained for at least 4 days and then degenerates
? “…the existence of a coelomic epithelium in the juvenile epistome is impossible confirm or deny…”
is partly retained as two lateral remnants, which contribute to the future formation of the epistome
undergo partial reduction: the postroral ciliated band degenerates. Larval tentacles change neither in length nor number
retained for 9 days and then disappears
retained, but greatly reduces in size
retained as fold of the larval episphaere near the mouth and then turns into epistome
completely destroyed; adult tentacles arise from anlagen of definitive tentacles, which are located under the larval
is drawn into the digestive tract and becomes hindgut
cells, which form the lining of the preoral lobe are completely retained and give rise to the definitive protocoel
Phoronis muelleri 
completely destroyed; adult tentacles arise from anlagen of definitive tentacles, which are located under the larval
is drawn into the digestive tract and becomes hindgut
protocoel is present neither in larva nor adult
Phoronis psammophilla 
retained as fold of the larval episphaere near the mouth and then turns into epistome
completely destroyed; adult tentacles arise from anlagen of definitive tentacles, which are located under the larval
retained for several days and then disappears
Phoronis pallida 
undergo great cellular death. Accordingly to figures, the length and number of larval tentacle greatly reduce. Definitive tentacles apparently arise from proximal portions of larval tentacles.
? can not be observed by SEM in juvenile just after the completion of metamorphosis. Apparently, the telotroch is drawn into the digestive tract
“Actinotrocha C” 
thickened proximal portions are retained through metamorphosis and give rise to definitive tentacles
is drawn into the digestive tract
The fate of larval tentacles also differs among phoronids. Larvae of some phoronids have definitive tentacles under the larval tentacles [13, 16, 17, 40]. In these larvae, the definitive tentacles give rise to the adult tentacles, and the larval tentacles are completely engulfed early in metamorphosis. Another mode of tentacle remodeling occurs in P. harmeri in that the larval tentacles become the adult tentacles. The exception in P. harmeri is the larval postoral ciliate band, which undergoes cell death. Thus, the definitive postoral ciliated band arises de novo in P. harmeri (Table 1). The postoral ciliated band is the most important structure for food capture, which occurs in different ways in the larva and the adult . Remodeling of the larval postoral ciliated band in P. harmeri correlates with the change in how food is captured by the adult. At the same time, P. harmeri demonstrates the minimal remodeling of larval tentacles: they change neither in length nor in number. Moreover, muscles of larval tentacles are partly retained and integrated into definitive muscles . These minimal changes of larval tentacles allow the juvenile to use tentacles for food capture in little time after catastrophic remodeling. Phoronids, whose larval tentacles are completely consumed by the juvenile, have to raise definitive tentacles in some time. During this period, the juvenile digests larval preoral lobe, tentacles, and telotroch .
The fate of the larval preoral lobe (= the hood) and the formation of the adult epistome are unclear during phoronid metamorphosis. There are two main views about the fate of the larval preoral lobe. The first view, which has been discussed in many early papers [42–48], supposes that the larval preoral lobe undergoes total reduction, whereas the adult epistome arises de novo. The second view was first suggested by Siewing , who studied the metamorphosis of Phoronis muelleri and discovered that a small part of the larval preoral lobe is retained and gives rise to the adult epistome. This second view has been supported by many studies [27, 49–51]. According to our results, P. harmeri retains two dorso-lateral portions of the larval hood, which together with the larval esophagus form the adult epistome. Thus, in P. harmeri, the central portion of the epistome arises from the dorsal wall of the larval esophagus, whereas the lateral parts of the epistome originate from the larval hood.
Table 1 demonstrates that there are two different descriptions, based on different studies for two phoronid species: P. harmeri [15, 22, 26, 27] and Phoronis muelleri [23, 35, 39]. This difference of descriptions might be explained by the use of different methods: only light microscopy for early studies [22, 23, 35] and electron and laser confocal microscopy for recent researches [15, 26, 27, 39]. The use of modern methods allows to recover new details, which were unknown before.
Remodeling of the body cavities
The formation of the definitive blood system was described in detail in some early papers [44, 52]. These authors mentioned that the cavity of the larval collar gives rise to the adult lophophoral vessel. Both Ikeda and Cowles regarded the collar cavity of phoronid larvae as a coelom. For this reason, they could not explain how the connection was established between the lophophoral and longitudinal vessels. Our previous results  and current results clearly show that the larval collar cavity is the blastocoel, which connects the larval vessels that are slits between the wall of the larval digestive tract and the lining of the metacoel. The formation of the ventral blood vessel, which arises between the larval esophagus and the upper border of the metacoel, was described by Cowles , whose findings are supported by our results. The dorsal vessel of the larva gives rise to the median vessel of the adult. The future development of the definitive blood system correlates with the formation of the septum in the lophophoral vessel and its division into upper and lower portions (afferent and efferent lophophoral vessels) as well as with the formation of two lateral blood vessels, which are descendants of the ventral blood vessel.
The most discussed problem of phoronid metamorphosis is the fate of the preoral coelom. Most researchers have inferred that the definitive preoral coelom develops from the proximal portion of the larval protocoel, which is located at the posterior border of the apical organ [22–24, 51]. According to these authors, the larval protocoel occupies all of the volume of the larval preoral lobe, from its edge to the dorsal septum. Exactly this septum is regarded as anlage of the definitive preoral coelom . As recently shown , some phoronid larvae lack a protocoel. These phoronid species probably lack a protocoel as adults . Interestingly, P. harmeri has a protocoel in larval stages  and in adults . Nevertheless, the protocoel greatly decreases in size during metamorphosis and partly loses its epithelial integrity: late in metamorphosis, some cells of the protocoel lack desmosomes. This may reflect the reduction of the protocoel, which occurs in many other phoronids [39, 53] and perhaps occurred in phoronid evolution .
The phylum Phoronida has been classified into the protostomian clade through molecular phylogenetic analyses [56, 57]. However, phoronid morphology and embryology have more in common with deuterostomes than protostomes [58, 59]. For this reason, the affiliation of phoronids with the protostomian clade cannot be regarded as strictly established. Phoronids have been traditionally considered as a group within clade Lophophorata [14, 60–62], whose unity was strongly criticized [63, 64], but recently is restored by molecular [65, 66] and morphological data [67, 68]. All adult lophophorates have tentacles, which are used for food capture, brooding, respiratory, and as a sensory organ . Most of phoronids have planktotrophic larvae, which live in plankton for several weeks and use tentacles for capture of food particles [13, 16, 17]. According to our data, P. harmeri, which demonstrates numerous plesiomorphic features in organization of different organ systems [54, 69], have tentacles as larvae, juvenile, and adult. The plesiomorphic nature of Phoronopsis together with its retention of larval tentacular musculature  and its direct transformation of larval tentacles into juvenile tentacles suggests that the presence of tentacles is a primary phoronid characteristic, i.e., the phoronid ancestor had tentacles at larval and adult stages. Among brachiopods, which are the closest relatives of phoronids [70, 71], the most primitive species of genus Lingula have planktotrophic larvae, whose tentacles directly transform into the juvenile tentacles. This fact might be regarded as an additional evidence for the presence of tentacles in the last common ancestor of clade Brachiozoa (Phoronida + Brachiopoda).
On the other hand, among phoronids there is only a species with lecitotrophic larvae – Phoronis ovalis . This tiny phoronid is characterized by the production of a few large eggs, which are bred in the mother’s tube. P. ovalis is regarded as the most primitive species among phoronids [70, 72]. In the light of the basal position of P. ovalis, the primacy of lecitotrophic larvae in phoronids seems to be more plausible than the primacy of planktotrophic larvae. In this case, we have to suggest that the phoronid ancestor did not have tentacles and tentacles appeared first in small phoronids in a small number and then increased in number in large phoronids.
Thus, we can see two possibilities for the evolution scenario of the last common phoronid ancestor. If we accept the first scenario, we have to suppose the reduction of larvae with tentacles in some phoronids, most of brachiopods, and all bryozoans. In contrast, if we agreed with second scenario, we have to imagine independent origin of tentacles in all three groups of the lophophorates. In the light of the available evidence, the first scenario seems more plausible than second scenario.
The organization of the coelomic system is traditionally used for the establishment of relationships between large taxa. The presence of three coelomic compartments in phoronid larvae and adults, has been traditionally regarded as an important evidence of close relations between phoronids (and other lophophorates) and deuterostomians. Among lophophorates, three coelomic compartments are discovered in some phoronid larvae [32, 51], in several adult phoronids [54, 55], and in only an adult brachiopod – Lingula anatina . Because phoronids and brachiopods have unusual lateral mesenteries in the trunk coelom, we have recently suggested an idea about the metameric organization of these animals . According to this idea, the last common ancestor of the lophophorates had protocoel, mesocoel, and metacoel. The protocoel and metacoel are specialized coelomic compartments, which occupy the two first segments of the body. The metacoel occupies the trunk and consists of two segments in phoronids and three segments in brachiopods. These segments of trunk are divided by lateral mesenteries. In the light of this idea, the presence of the coelom and segmentation seem to be plesiomorphic condition for the lophophorates, as it likely is for the last common bilaterian ancestor . This idea is completely corroborated with numerous data that the last common ancestor of Deuterostomia, Ecdysozoa and Spiralia had a segmented and coelomate body organization and that morphologically more simply organized taxa evolved by secondary reductions [74–78]. On the other hand, our previous idea is in stark contrast to the traditional “acoeloid–planuloid” hypothesis favoring evolution of Bilateria from a simple body organization toward more complex forms with a last common ancestor resembling a flatworm without segmentation and coelomic cavities [60, 79–82].
Because all lophophorates have tentacles and coelom, which usually is subdivided into several compartments, we can conclude that the last common ancestor of the lophophorates was a coelomic animal with tentacles. Whether these structures were inherited from the last common bilaterian ancestor or acquired independently – this question demands further researches and analyses.
Evolution of the phoronid body plan
To date, the only idea about the formation of the body plan in recent phoronids was suggested by Mamkaev . He supposed that a worm-like phoronid ancestor lived in a U-shaped burrow in soft sediment. Given the shape of the burrow, the body of the worm was also U-shaped. With time, two parts of this U-shaped body of the phoronid ancestor gradually approached each other and finally merged. The lateral mesenteries, which are unique to adult phoronids , appear as a result of this merging. It is important to consider that Mamakev did not study phoronid metamorphosis; his interpretation was based solely on the observation of adult phoronids .
Evolution of the phoronid life cycle
In this research, we used modern methods to provide a detailed description of phoronid metamorphosis. Our results reveal the metamorphic remodeling of some larval organs and body cavities, whose fates were previously unclear. We report that metamorphosis in P. harmeri differs from that in other phoronids studied to date. In P. harmeri, for example, remodeling of larval tentacles involves degradation of the postoral ciliated band. The preoral lobe of the larva is retained as two lateral remnants, which then contribute to the formation of the definitive epistome. The telotroch is not drawn into the digestive tract, as it is in most of other studied phoronids, and it gradually degenerates over 9 days (Table 1). Our report reveals that the P. harmeri protocoel is maintained during metamorphosis and gives rise to the definitive protocoel, which is well-developed in adults . Our new data confirm previous assumption that the phoronid metamorphosis occur in two different ways: with complete and incomplete reduction of larval structures. P. harmeri demonstrates minimal changes of larval structures: in contrast to other studied phoronid species, it retains larval protocoel, tentacles, and digestive tract. Because the protocoel (as any other coelom) functions as a skeleton of the epistome, it is important to retain it in P. harmeri, which has a large movable epistome. Direct transformation of larval tentacles into the definitive tentacles allows the juvenile to capture food particles immediately after the critical point of the metamorphosis.
On the other hand, metamorphosis of all phronids demonstrates a general pattern common to all species studied to date. Phoronid metamorphosis starts from eversion of the deep invagination of larval ventral side of the body (a metasomal sac). The metasomal sac gives rise to the most part of adult body. The terminal portion of the metasomal sac (ampulla) demonstrates enormous mobility and is used by the juvenile for burying into the substrate. The presence of movable ampulla and large metasomal sac in all phoronids may reflect the significance of the ventral side of the body in phoronid ancestor. Many recent invertebrates use the ventral side of the body to escape predators by digging into substrata. For this reason, we have suggested an idea about evolutionary formation of the phoronid body plan due to inheritance of an acquired modus of escaping development.
This research was supported in part by several grants. The collection of material was done with support from the Russian Foundation for Basic Research (#14-04-00238). The TEM studies were conducted with support from the Russian Foundation for Basic Research (#15-04-20045; # 15-29-02601). Cytochemical, morphological, and histological investigations were done with support from the Russian Scientific Fund (#14-04-00262), detailed investigation of remodeling of the external morphology by SEM was done with support from the Russian Scientific Fund (#14-50-00029), and the processing of the paper was supported by Grants of the President of Russia (# MD-5812.2015.4). The work was performed at the User Facilities Center of M.V. Lomonosov Moscow State University under financial support of the Ministry of Education and Science of Russian Federation. We great thank all members of marine biological station “Vostok” and Institute of Marine Biology (Vladivostok, Russia) for help with organization of the field works.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.
- Hadfield MG, Carpizo-Ituarte EJ, del Carmen K, Nedved BT. Metamorphic competence, a major adaptive convergence in marine invertebrate larvae. Am Zool. 2001;41:1123–31.Google Scholar
- Sly BJ, Snoke MS, Raff RA. Who came first—larvae or adults? Origins of metazoan bilaterian larvae. Int J Dev Biol. 2003;47:623–32.PubMedGoogle Scholar
- Raff RA. Origins of the other metazoan body plans: the evolution of larval forms. Philos Trans R Soc Lond B. 2008;363:1473–9.View ArticleGoogle Scholar
- Page LR. Molluscan larvae: pelagic juveniles or slowly metamorphosing larvae? Biol Bull. 2009;216:216–25.PubMedGoogle Scholar
- Nielsen C. How did Indirect development with planktotrophic larvae evolve? Biol Bull. 2009;216:203–15.PubMedGoogle Scholar
- Nielsen C. Life cycle evolution: was the eumetazoan ancestor a holopelagic, planktotrophic gastraea? BMC Evol Biol. 2013;13:171.PubMed CentralView ArticlePubMedGoogle Scholar
- Davidson EH, Peterson KJ, Cameron RA. Origin of bilaterian body plans: evolution of developmental regulatory mechanisms. Science. 1995;270(5240):1319–25.View ArticlePubMedGoogle Scholar
- Peterson KJ, Cameron RA, Davidson EH. Set-aside cells in maximal indirect development: evolutionary and developmental significance. Bioessays. 1997;19(7):623–31.View ArticlePubMedGoogle Scholar
- Peterson KJ, Davidson EH. Regulatory evolution and the origin of the bilaterians. Proc Natl Acad Sci U S A. 2000;97(9):4430–3.PubMed CentralView ArticlePubMedGoogle Scholar
- Peterson KJ, Cameron RA, Davidson EH. Bilaterian origins; significance of new experimental observations. Dev Biol. 2000;219(1):1–17.View ArticlePubMedGoogle Scholar
- Erwin DH, Davidson EH. The last common bilaterian ancestor. Development. 2002;129(13):3021–32.PubMedGoogle Scholar
- Hadfield MG. Why and how marine-invertebrate larvae metamorphose so fast. Cell Dev Biol. 2000;11:437–43.View ArticleGoogle Scholar
- Emig CC. The biology of Phoronida. Adv Mar Biol. 1982;19:2–90.Google Scholar
- Emig CC. Le lophophore-structure significative des Lophophorates (Brachiopoda, Bryozoa, Phoronida). Zool Scripta. 1976;5:133–7.View ArticleGoogle Scholar
- Temereva EN. The digestive tract of actinotroch larvae (Lophotrochozoa, Phoronida): anatomy, ultrastructure, innervations, and some observations of metamorphosis. Can J Zool. 2010;88(2):1149–68.View ArticleGoogle Scholar
- Temereva EN. New data on distribution, morphology and taxonomy of phoronid larvae (Phoronida, Lophophorata). Invert Zool. 2009;6(1):47–64.Google Scholar
- Temereva EN, Neretina TV. A distinct phoronid larva: morphological and molecular evidence. Invert Syst. 2013;27(6):622–33.Google Scholar
- Kovalevsky АО. Anatomy and Life History of Phoronis. Proc St-Petersburg Acad Sci. 1867;2:1–35 (in Russian).Google Scholar
- Mamkaev YV. About phoronids of Far eastern seas. Researches of Far Eastern Seas USSR. 1962;8:219–37 (in Russian).Google Scholar
- Malakhov V, Kuzmina T. Metameric origin of lateral mesenteries in Brachiopoda. Dokl Biol Sci 2006. 2006;40(5):1–3.Google Scholar
- Temereva EN, Malakhov VV. The evidence of metamery in adult brachiopods and phoronids. Invert Zool. 2011;8:87–101.Google Scholar
- Zimmer RL. Reproductive biology and development of Phoronida, Ph.D. thesis. Ann Arbor, Michigan: University of Washington, Seattle 1964 [Available from Xerox University Microfilms]; 1964.Google Scholar
- Siewing R. Morphologische untersuchungen zum archicoelomatenproblem. The body segmentation in phoronis muelleri de selys-longchamps (phoronidea) ontogenese – larve – metamorphose – adultus. Zool Jahrbücher Anatomie. 1974;92(2):275–318.Google Scholar
- Herrmann K. Larvalentwicklung und metamorphose von Phoronis psammophila (phoronida, tentaculata). Helgoländer Meeresun. 1979;32:550–81.View ArticleGoogle Scholar
- Santagata S. Structure and metamorphic remodeling of the larval nervous system and musculature of Phoronis pallida (Phoronida). Evol Dev. 2002;4:28–42.View ArticlePubMedGoogle Scholar
- Temereva EN, Tsitrin EB. Development, organization, and remodeling of phoronid muscles from embryo to metamorphosis (Lophotrochozoa: Phoronida). BMC Dev Biol. 2013;13:14. doi:10.1186/1471-213×-13-14.PubMed CentralView ArticlePubMedGoogle Scholar
- Temereva EN, Tsitrin EB. Organization and metamorphic remodeling of the nervous system in juveniles of Phoronopsis harmeri (Phoronida): insights into evolution of the bilaterian nervous system. Frontiers Zool. 2014;11:35.View ArticleGoogle Scholar
- Temereva EN, Malakhov VV. The circulatory system of phoronid larvae. Dokl Biol Sci. 2000;375(5):712–4.Google Scholar
- Temereva EN, Malakhov VV. The organization and origin of the phoronid blood system. Dokl Biol Sci. 2003;389(4):1–4.Google Scholar
- Temereva EN, Malakhov VV. Ultrastructure of the blood system in phoronid Phoronopsis harmeri Pixell, 1912: 1. Capillaries. Russ J Mar Biol. 2004;30(1):28–36.View ArticleGoogle Scholar
- Temereva EN, Malakhov VV. Ultrastructure of the blood system in Phoronid Phoronopsis harmeri Pixell, 1912: 2. Main vessels. Russ J Mar Biol. 2004;30(1):101–12.View ArticleGoogle Scholar
- Temereva EN, Malakhov VV. The answer to Thomas Bartolomaeus: “Larva of phoronid Phoronopsis harmeri Pixell, 1912 has trimeric coelom organization”. Invert Zool. 2006;2(2):394–402.Google Scholar
- Temereva EN, Malakhov VV. Development of the excretory organs of the Phoronopsis harmeri (Phoronida): from protonephridium to nephromixium. Zool Zhurn. 2006;84(1):28–35.Google Scholar
- Bartolomaeus T. Ultrastructure and relationship between protonephridia and metanephridia in Phoronis muelleri (Phoronida). Zoomorphology. 1989;109:113–22.View ArticleGoogle Scholar
- Herrmann K. Untersuchungen über morphologie, physiologie, und ökologie der metamorphose von Phoronis muelleri (Phoronida). Zool Jahrb Anat. 1976;95:354–426.Google Scholar
- Herrmann K. Tentaculata (Lophophorata). In: Westheide W, Rieger R, editors. Spezielle Zoologie. Tiel 1: Einzeller und Wirbellose Tiere. 2nd ed. Heidelberg, Berlin: Spectrum Akademischer Verlag; 2004. p. 737–54.Google Scholar
- Maslakova S. Development to metamorphosis of the nemertean pilidium larva. Front Zool. 2010;7:30.PubMed CentralView ArticlePubMedGoogle Scholar
- Johnson KB, Zimmer RL. Phylum Phoronida. In: Young C, Rice M, Sewell M, editors. ‘Atlas of marine invertebrate Larvae’. San Diego, CA: Academic; 2002. p. 429–39.Google Scholar
- Bartolomaeus T. Ultrastructure and formation of the body cavity lining in Phoronis muelleri (Phoronida, Lophophorata). Zoomorphology. 2001;120(3):135–48.View ArticleGoogle Scholar
- Silén L. Developmental biology of Phoronidea of the Gullmar Fiord area (west coast of Sweden). Acta Zool. 1954;35:215–57. doi:10.1111/j.1463-6395.1954.tb00035.x.View ArticleGoogle Scholar
- Gilmour THJ. Ciliation and function of the food-collection and waste-rejection organs of lophophorates. Can J Zool. 1978;56:2142–55.View ArticleGoogle Scholar
- Wilson EB. The origin and significance of the metamorphosis of Actinitrocha. Quart J Mic Sci. 1881;21:202–18.Google Scholar
- Caldwell WH. Preliminary note on the structure, development and affinities of Phoronis. Proc Roy Soc L. 1882;34:371–83.View ArticleGoogle Scholar
- Ikeda I. Observation on the development, structure and metamorphosis of Actinotrocha. J Coll Sci Imperial Univ Tokyo. 1901;13:507–91.Google Scholar
- Schultz E. Aus dem Gebiete der Regeneration. VI. Uber Regenerationserscheinungen bei Actinotrocha branchiata Muller. Z Wiss Zool. 1903;75:473–94.Google Scholar
- Selys-Longchamps M:Phoronis. Fauna und Flora des Golfes von Neapel und der meeres-abschnitte. Herausgegeben von der Zoologischen Station zu Neapel. Berlin: von R. Friedländer & Sohn. Monograph 30 1907, s.280.Google Scholar
- Meek A. On the Phoronidea. Report Dove Marine Lab Cullercoats. 1917;6:33–48.Google Scholar
- Veillet A. Description et mecanismes de la metamorphose de la larve actinotroque de Phoronis sabatieri Roule. Bull l’Institut Oceanographique Monaco. 1941;810:1–11.Google Scholar
- Herrmann K. The Regionation of Phoronis muelleri (Tentaculata). Zool Jahrb Anat. 1980;103(2):234–49.Google Scholar
- Herrmann K. Ontogenesis of Phoronis muelleri (Tentaculata) with a special sight for differentiation of mesoderm and phylogenesis of coelom. Zool Jahrb Anat. 1986;114(4):441–63.Google Scholar
- Zimmer RL. The comparative structure of the preoral hood coelom in Phoronida and the fate of this cavity during and metamorphosis. In: Chia FS, Rice ME, editor. Settlement and metamorphosis of marine invertebrate Larvae’. New York: Elsevier; 1978. p. 23–40.Google Scholar
- Cowles RP. Origin and fate of the blood vessels and blood corpuscles of the Actinotrocha. Zool Anz. 1904;27:598–606.Google Scholar
- Gruhl A, Grobe P, Bartolomaeus T. Fine structure of the epistome in Phoronis ovalis: significance for the coelomic organization in Phoronida. Invert Biol. 2005;124:332–43.View ArticleGoogle Scholar
- Temereva EN, Malakhov VV. Organization of the epistome in Phoronopsis harmeri (Phoronida) and consideration of the coelomic organization in Phoronida. Zoomorphology. 2011;130:121–34.View ArticleGoogle Scholar
- Temereva EN. Organization of the coelomic system in Phoronis australis (Lophotrochozoa: Phoronida) and consideration of the coelom in the lophophorates. J Zool. 2015;296(2):79–94.View ArticleGoogle Scholar
- Halanych KM, Bacheller JD, Aguinaldo AMA, Liva SM, Hillis DM, Lake JA. Evidence from 18S ribosomal DNA that lophophorates are protostome animals. Science. 1995;267:1641–3.View ArticlePubMedGoogle Scholar
- Dunn CW, Hejnol A, Matus DQ, Pang K, Browne WE, Smith SA, et al. Broad phylogenomic sampling improves resolution of the animal tree of life. Nature. 2008;452:745–9. doi:10.1038/nature06614.View ArticlePubMedGoogle Scholar
- Temereva EN, Malakhov VV. Embryogenesis in phoronids. Invert Zool. 2012;8(1):1–39.Google Scholar
- Temereva EN, Tsitrin EB. Development and organization of the larval nervous system in Phoronopsis harmeri: new insights into phoronid phylogeny. Front Zool. 2014;11:3. doi:10.1186/1742-9994-11-3.PubMed CentralView ArticlePubMedGoogle Scholar
- Hyman LH. The lophophorate coelomates—phylum Brachiopoda. In: Hyman LH, editor. The invertebrates: smaller coelomate groups. New York: McGraw-Hill; 1959. p. 516–609.Google Scholar
- Emig CC. Phylogenkse des Phoronida, Les Lophophorates et le concept des Archimerata. Zeitsch zool Syst Evol. 1976;14:1–24.Google Scholar
- Emig CC. Un nouvel embranchement: les Lophophorates. Bull Soc Zool France. 1977;102:341–4.Google Scholar
- Helmkampf M, Bruchhaus I, Hausdorf B. Multigene analysis of lophophorate and chaetognath phylogenetic relationships. Mol Phylogenet Evol. 2008;46:206–14.View ArticlePubMedGoogle Scholar
- Hausdorf B, Helmkampf M, Nesnidal M, Bruchhaus I. Phylogenetic relationships within the lophophorate lineages (Ectoprocta, Brachiopoda and Phoronida). Mol Phylogenet Evol. 2010;55:1121–7.View ArticlePubMedGoogle Scholar
- Jang K, Hwang U. Complete mitochondrial genome of Bugula neritina (Bryozoa, Gymnolaemata, Cheilostomata): phylogenetic position of Bryozoa and phylogeny of lophophorates within the Lophotrochozoa. BMC Genomics. 2009;10:167.PubMed CentralView ArticlePubMedGoogle Scholar
- Nesnidal MP, Helmkampf M, Meyer A, Witek A, Bruchhaus I, Ebersberger I, et al. New phylogenomic data support the monophyly of Lophophorata and an Ectoproct-Phoronid clade and indicate that Polyzoa and Kryptrochozoa are caused by systematic bias. BMC Evol Biol. 2013;13:253.PubMed CentralView ArticlePubMedGoogle Scholar
- Temereva EN, Tsitrin EB. Modern data on the innervation of the lophophore in Lingula anatina (Brachiopoda) support the monophyly of the lophophorates. PLoS One. 2015;10(4):e0123040.PubMed CentralView ArticlePubMedGoogle Scholar
- Temereva EN, Malakhov VV: Innervation of the lophophore of inarticulate brachiopod Lingula anatina (Brachiopoda) supports the monophyly of Lophophorata. Dokl Biol Sci 2015, 464(3): In press.Google Scholar
- Temereva EN, Malakhov VV. The intestine of phoronids has epitheliomusculer cells. Dokl Biol Sci. 2002;386(4):469–71.View ArticlePubMedGoogle Scholar
- Santagata S, Cohen B. Phoronid phylogenetics (Brachiopoda; Phoronata): evidence from morphological cladistics, small and large subunit rDNA sequences, and mitochondrial cox1. Zool J Linn Soc. 2009;157:34–50.View ArticleGoogle Scholar
- Cohen BL. Rerooting the rDNA gene tree reveals phoronidsto be ‘brachiopods without shells’; dangers ofwide taxon samples in metazoan phylogenetics (Phoronida; Brachiopoda). Zool J Linn Soc. 2013;167:82–92.View ArticleGoogle Scholar
- Hirose M, Fukiage R, Katoh T, Kajihara H. Description and molecular phylogeny of a new species of Phoronis (Phoronida) from Japan, with a redescription of topotypes of P. ijimai Oka, 1897. ZooKeys. 2014;398:1–31.View ArticlePubMedGoogle Scholar
- Temereva EN, Gebruk AA, Malakhov VV. Demonstration of the preoral coelom in the brachiopod Lingula anatina with consideration of its phylogenetic significance. Zool Anz. 2015;256:22–7.View ArticleGoogle Scholar
- Brinkman H, Philippe H. Animal phylogeny and large-scale sequencing: progress and pitfalls. J Syst Evol. 2008;46:274–86.Google Scholar
- De Robertis EM. The molecular ancestry of segmentation mechanisms. Proc Natl Acad Sci U S A. 2008;105:16411–2.PubMed CentralView ArticlePubMedGoogle Scholar
- Couso JP. Segmentation, metamerism and the Cambrian explosion. Int J Dev Biol. 2009;53:1305–16.View ArticlePubMedGoogle Scholar
- Tomer R, Denes AS, Tessmar-Raible K, Arendt D. Profiling by image registration reveals common origin of annelid mushroom bodies and vertebrate pallium. Cell. 2010;142:800–9.View ArticlePubMedGoogle Scholar
- Chesebro JE, Pueyo JI, Couso JP. Interplay between a Wntdependent organiser and the Notch segmentation clock regulates posterior development in Periplaneta americana. Biol Open. 2013;2:227–37.PubMed CentralView ArticlePubMedGoogle Scholar
- Ivanov AV, Mamkaev YV. Turbellaria, their origin and evolution. Leningrad: Nauka; 1973. p. 221.Google Scholar
- Halanych KM. The new view of animal phylogeny. Annu Rev Ecol Evol Syst. 2004;35:229–56.View ArticleGoogle Scholar
- Hejnol A, Obst M, Stamatakis A, Ott M, Rouse GW, Edgecombe GD, et al. Assessing the root of bilaterian animals with scalable phylogenomicmethods. Proc R Soc Lond B Biol Sci. 2009;276:4261–70.View ArticleGoogle Scholar
- Struck TH, Wey-Fabrizius AR, Golombek A, Hering L, Weigert A, Bleidorn CH, et al. Platyzoan paraphyly based on phylogenomic data supports a noncoelomate ancestry of Spiralia. Mol Biol Evol 2014, doi:10.1093/molbev/msu143.