Serotonin-immunoreactive neurons in the ventral nerve cord of Remipedia (Crustacea): support for a sister group relationship of Remipedia and Hexapoda?
© Stemme et al.; licensee BioMed Central Ltd. 2013
Received: 6 February 2013
Accepted: 4 June 2013
Published: 10 June 2013
Remipedia were initially seen as a primitive taxon within Pancrustacea based on characters considered ancestral, such as the homonomously segmented trunk. Meanwhile, several morphological and molecular studies proposed a more derived position of Remipedia within Pancrustacea, including a sister group relationship to Hexapoda. Because of these conflicting hypotheses, fresh data are crucial to contribute new insights into euarthropod phylogeny. The architecture of individually identifiable serotonin-immunoreactive neurons has successfully been used for phylogenetic considerations in Euarthropoda. Here, we identified neurons in three species of Remipedia with an antiserum against serotonin and compared our findings to reconstructed ground patterns in other euarthropod taxa. Additionally, we traced neurite connectivity and neuropil outlines using antisera against acetylated α-tubulin and synapsin.
The ventral nerve cord of Remipedia displays a typical rope-ladder-like arrangement of separate metameric ganglia linked by paired longitudinally projecting connectives. The peripheral projections comprise an intersegmental nerve, consisting of two branches that fuse shortly after exiting the connectives, and the segmental anterior and posterior nerve. The distribution and morphology of serotonin-immunoreactive interneurons in the trunk segments is highly conserved within the remipede species we analyzed, which allows for the reconstruction of a ground pattern: two posterior and one anterior pair of serotonin-immunoreactive neurons that possess a single contralateral projection. Additionally, three pairs of immunoreactive neurons are found in the medial part of each hemiganglion. In one species (Cryptocorynetes haptodiscus), the anterior pair of immunoreactive neurons is missing.
The anatomy of the remipede ventral nerve cord with its separate metameric ganglia mirrors the external morphology of the animal’s trunk. The rope-ladder-like structure and principal architecture of the segmental ganglia in Remipedia corresponds closely to that of other Euarthropoda. A comparison of the serotonin-immunoreactive cell arrangement of Remipedia to reconstructed ground patterns of major euarthropod taxa supports a homology of the anterior and posterior neurons in Pancrustacea. These neurons in Remipedia possess unbranched projections across the midline, pointing towards similarities to the hexapod pattern. Our findings are in line with a growing number of phylogenetic investigations proposing Remipedia to be a rather derived crustacean lineage that perhaps has close affinities to Hexapoda.
KeywordsEuarthropoda Homology Comparative neuroanatomy Ground pattern Phylogeny Immunocytochemistry Speleonectes Godzilliognomus Cryptocorynetes
A variety of independent molecular and morphological studies strongly supports a close relationship of Hexapoda and Crustacea (e.g., [1–3]), known as Tetraconata or Pancrustacea [4–6]. However, which crustacean taxon represents the possible sister group of Hexapoda is still under debate. Quite a number of crustacean taxa were suggested to be closest relatives to Hexapoda in different kinds of analyses (see review ), for example all Crustacea, or subgroups such as Cephalocarida, Copepoda, Branchiopoda, Malacostraca, Remipedia or Xenocarida (Remipedia and Cephalocarida; ). Most of these predominantly molecular studies did not provide a comprehensive crustacean taxon sampling, which resulted in the lack of important groups, e.g., the Remipedia .
Serotonin-immunoreactive (5HT-ir) neurons are suitable for phylogenetic investigations within Euarthropoda for several reasons . The number of 5HT-ir neurons is small, facilitating individual identification of cells and their characteristic neurite morphology. Additionally, the serotonin-immunoreactivity (5HT-IR) has been investigated in a wide range of euarthropod species allowing comparisons over a wide range of taxa (Chelicerata and Myriapoda: e.g., [34–36]; Crustacea: e.g., [33, 37–43]; Hexapoda: e.g., [44–47]). Furthermore, serotonergic neurons have not only been analyzed in Euarthropoda, but also in other invertebrates and vertebrates, which led to the suggestion that the serotonergic system is highly conserved in evolution .
While the general morphology and the 5HT-IR of the remipede brain have already been analyzed [14, 15, 22], here, we describe the neuroanatomy of the ventral nerve cord focusing on the distribution and morphology of 5HT-ir neurons in three species of Remipedia. Our comparisons to already studied euarthropod taxa add new insights to the phylogenetic position of Remipedia and the crustacean sister group of Hexapoda.
General neuroanatomy of the ventral nerve cord of Remipedia
The innervation of the trunk segments is provided by the anterior and posterior segmental nerves (ASN and PSN) that exit each ganglion laterally. The thicker ASN originates at a level between the AC and PC, projecting posterolaterally straight into the appendages (Figure 2A, B). The PSN leaves the ganglion laterally of the PC parallel to the ASN. Subsequently, it splits into three branches, at least two of which run into the appendages (Figure 2A, B).
Additionally, two neurite bundles leave the CON. Both bundles eventually fuse, forming an intersegmental nerve (ISN) that extends further laterally into the periphery (Figure 2A, C). This nerve then divides several times forming a network of neurite bundles distally to the ventral nerve cord. One anterior and one posterior projecting neurite bundle can be distinguished (arrows in Figure 2A). The posterior projecting branch of the ISN extends ventrally and finally splits in numerous radially distributed fine neurite bundles (Figure 2C).
Serotonin-immunoreactive neurons in the ventral nerve cord
In all remipede species examined here, the 5HT-ir neurons of the ventral ganglia are arranged in a stereotyped pattern. Based on their soma position within each ganglion, three groups can be distinguished. In Speleonectes tulumensis and Godzilliognomus frondosus, we detected six pairs of 5HT-ir neurons (Figure 3A-D). These individually identifiable neurons are arranged in two posterior, three medial, and one anterior pair of immunoreactive neurons (PSC, MSC, and ASC, respectively). All these neurons are found in a ventral position of the nervous system. In general, the distribution and projections of 5HT-ir neurons appear similar in all three investigated species. One conspicuous modification is observed in Cryptocorynetes haptodiscus, namely the absence of the ASC (Figure 3E, F).
Number of medial serotonin-immunoreactive neurons in selected ganglia of Cryptocorynetes haptodiscus
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In the three studied species, the somata of the MSCs send one short, unbranched neurite into the neuropil of the hemiganglion (exemplarily shown for Speleonectes tulumensis in Figure 4C; Additional file 4), indicating that these neurons are of the monopolar type. We could not resolve whether these neurons project contralaterally or remain on the ispsilateral side. In some cases, we found the medial projecting primary neurite of the intense labeled MSCs looping back laterally (Figure 4C, red arrows), hinting towards an ipsilateral projection pattern.
In the anterior part of each ganglion, another 5HT-ir cell pair could be detected in Godzilliognomus frondosus and Speleonectes tulumensis (ASC; Figure 3A-C; Additional files 5 and 6). In both species, a primary neurite projects from this pair via the AC into the contralateral hemiganglion and then turns posteriorly (exemplarily shown for Godzilliognomus frondosus in Figure 4B; Additional files 5 and 6). Due to the intense labeling of the neuropil, the projection could not be traced further.
General morphology of the ventral nerve cord
The ventral nerve cord of Remipedia resembles a rope-ladder-like structure (Figures 2A and 5A), most likely a plesiomorphic feature among euarthropods. The MNB has been described in representatives of Malacostraca [51–54] and Mystacocarida , but is missing in all studied representatives of Branchiopoda: Artemia salina, Leptodora kindtii, Triops cancriformis, Leptestheria dahalacensis and Cyclestheria hislopi. A corresponding neurite bundle has also been described in various representatives of Hexapoda , but not in Myriapoda [57, 58] or Chelicerata . Thus, the MNB might be a plesiomorphy of Pancrustacea. Its absence in Branchiopoda could be due to reduction and hence an apomorphy of this group [39, 41].
Serotonin-immunoreactive neurites in peripheral nerves and connectives
All studies on the serotonergic system in the ventral nerve cord of euarthropods mention 5HT-ir neurites or neurite bundles in the CONs, which travel along the ventral nerve cord interconnecting the segmental ganglia and joining the ventral nerve cord and the brain. There are between 15 to 20 parallel longitudinal 5HT-ir neurites in the CONs of Remipedia (Figures 3A and 4D). It is a common feature in Pancrustacea that 5HT-ir neurons project through several segments in ascending or descending pathways (e.g., [60, 61]). Among the many immunofluorescent cell processes, we resolved that the neurites from the PSCs extend anteriorly into the CONs, indicating an ascending pathway. In Decapoda and Syncarida, these neurites are arranged in three distinct tracts called median, central and lateral fiber bundles [60–64]. Furthermore, one single neurite is described in the trunk segments of Mystacocarida , four axons are known from Isopoda  and approximately 24 axons project in the CONs of the chelicerate Limulus polyphemus. Thus, longitudinal 5HT-ir neurites are a common feature among euarthropods, but there are differences in the number and arrangement of 5HT-ir projections between taxa.
Several studies on Euarthropoda describe 5HT-ir neurites within segmental nerves as observed in our study or an immunoreactive plexus surrounding some of the segmental nerves (e.g., Chelicerata: [36, 65]; Crustacea: [33, 38, 40, 60–63]; Hexapoda: [44, 47]). Because serotonin is released into the haemolymph and acts as a circulating hormone (e.g., ) the immunoreactive neurites in the ASN of Remipedia might be projections to peripherally located endocrine release areas as suggested for a copepod .
Comparative neuroanatomy and remipede phylogeny
Although we could only investigate specimens from three species, some general considerations concerning the phylogenetic relationships within Remipedia might be possible. Speleonectes tulumensis and Cryptocorynetes haptodiscus belong to the Speleonectidae, whereas Godzilliognomus frondosus is a representative of the Godzilliidae. However, the phylogenetic relationships within Remipedia are currently being re-evaluated . First results based on an analysis of cytochrome oxidase c subunit 1 sequences from 23 species of Remipedia revealed paraphyletic Godzilliidae and Speleonectidae (Figure 6; compare ). In this tree, the basal clade to all other Remipedia is composed of the genera Godzilliognomus and Godzillius. The three Cryptocorynetes species together with Speleonectes benjamini are positioned in a more derived clade as a sister group to the remaining species of Speleonectes, including Speleonectes tulumensis. The status of the genus Cryptocorynetes has been already discussed by Koenemann et al., who suggested creating a new taxon comprising the species of this genus . Based on the discussed phylogenetic tree, the loss of the ASC might be an apomorphy of the Cryptocorynetes clade (Figure 6).
Euarthropod phylogeny, comparative neuroanatomy, and neural ontogeny
The distribution and morphology of 5HT-ir neurons have been studied in a variety of euarthropod species, especially crustaceans and hexapods. Similarities in the neuroanatomy of 5HT-ir cells led to the suggestion that some of these neurons may be homologous. In this context, ground patterns for the major taxa have been reconstructed for phylogenetic comparisons (Figure 5C) [33, 34, 43, 68].
According to these studies, the serotonergic system in the ventral nerve cord of Crustacea and Hexapoda can be divided into an anterior and a posterior group, each consisting of at the most two bilateral pairs of neurons. The ground pattern of Branchiopoda and Malacostraca possess the same number of neurons: two anterior and two posterior bilateral pairs. However, there are differences in the morphology of the neurites. Each branchiopod neuron possesses an ipsi- and a contralateral projection (bipolar neurons after the definition of [33, 49]), whereas the malacostracan neurons only show one axon with contralateral projections in the anterior group and ipsilateral projections in the posterior group (Figure 5C; compare ). In Pterygota however, the anterior pairs of neurons are missing. The remaining posterior cells project one axon each via the PC into the contralateral neuropil. According to a preliminary description of Harzsch, two anterior and two posterior 5HT-ir pairs and an additional medial neuron are present in the mesothoracic ganglion of the wingless insect Thermobia domestica (Zygentoma) [34, 69]. This observation that needs elaboration in additional taxa of wingless insects suggests that the hexapod ground pattern comprises two anterior and two posterior 5HT-ir pairs, as Thermobia domestica is the most basal hexapod taxon where 5HT-IR has been studied.
The other major lineages of Euarthropoda, the Myriapoda and Chelicerata, differ markedly in their distribution of 5HT-ir neurons in comparison to Crustacea and Hexapoda . In Chelicerata, 5HT-ir neurons form an anterior and a posterior cluster comprising approximately a dozen neurons with contralateral projections. In Myriapoda, 5HT-ir neurons are arranged in groups of at most four neurons in different positions within the ganglia (Figure 5C). Whether the clusters of 5HT-ir neurons in Chelicerata and Myriapoda correspond to the cell pairs in Crustacea and Hexapoda is unclear (discussed in detail in [34, 70]).
Differences in the number, clustering, and distribution pattern might be explained by distinct strategies of neurogenesis among respective taxa. In Chelicerata and Myriapoda, the nervous system is formed from clusters of postmitotic neuronal precursors which migrate from the neuroectoderm, while in Crustacea and Hexapoda individual neural stem cells termed neuroblasts generate defined cell lineages [71–73]. All serotonergic neurons in the ventral nerve cord of two species of locusts, Schistocerca americana and Melanoplus differentialis, and the fruitfly Drosophila are progeny of the neuroblast 7–3 [74–76]. Unfortunately, the lineages of 5HT-ir cells in Crustacea are still unknown. Comparisons between distantly related insect and crustacean embryos revealed a stereotyped pattern of neural precursors and a common plan of neuronal development . This finding initiated a discussion of possible homology among insect and crustacean neuroblasts, despite distinct differences in the segregation pattern from the neuroectoderm [6, 29, 78–80]. Because of their relative position, proliferation pattern and type of progeny, homology of neuroblasts between insects and malacostracans has been suggested [53, 80, 81]. Along these lines, we also propose the hypothesis that the posterior immunoreactive neurons are homologous in Remipedia, Hexapoda and other Crustacea. Similar evidence indicates homology of the anterior 5HT-ir neurons between Remipedia and other Crustacea. This type of neuron appears to be absent in the ground pattern of Pterygota (Figure 5C).
Harzsch and co-workers suggested that differences in the mode of neural ontogeny account for the differences in number of 5HT-ir neurons . As Chelicerata possess clusters of neuronal precursors, it might be reasonable that clusters of 5HT-ir neurons and not single cells result from this mode of neurogenesis. In contrast, the asymmetrically dividing neuroblasts in Crustacea and Hexapoda generate specific lineages that comprise small numbers of individual 5HT-ir neurons.
Following the generally accepted opinion that Chelicerata represents the plesiomorphic mode of neurogenesis, whereas the Hexapoda and Malacostraca represent a rather derived mode, the serotonergic systems of the ventral nerve cord seem to have undergone a simplification process. A comparative analysis of the serotonergic neurons of crustacean and hexapod taxa indicates that the ground pattern of Branchiopoda represents the complete serotonergic system, which is then reduced in more derived taxa from four pairs with bipolar projections in Branchiopoda to four pairs with only monopolar projections in Malacostraca. This loss of bipolar neurons points towards a close relationship of Malacostraca, Remipedia and Hexapoda, all of which possess only monopolar neurons. Remipedia and Hexapoda show only contralateral projections, whereas the posterior neurons of Malacostraca project ipsilaterally.
Interesting is that in almost all molecular studies including recent phylogenomic data, Branchiopoda are inferred as closer related to Remipedia and Hexapoda than Malacostraca (e.g., [8, 9, 28]). However, strong evidence from morphological studies (e.g., [14, 15, 22, 82]) supports an ambiguous, equally likely evolutionary scenario, in which Malacostraca, Remipedia and Hexapoda are closely related. Our study presents new characters that potentially could resolve these contradicting results if extended to more taxa. In this context, the Cephalocarida play an important role, because they were recovered in two recent molecular studies as closest relatives to Remipedia [8, 19]. Based on long branch effects and inhomogeneous substitution rates, this grouping was critically discussed since its first molecular reconstruction . A sister group relationship is also not supported by recent anatomical studies of the brain of the cephalocarid Hutchinsoniella macracantha.
In addition to the ASCs and PSCs, we detected three MSCs in Remipedia. It will be interesting to find corresponding cells in other euarthropod taxa. One medial neuron has been described in the mesothoracic ganglion of the silverfish Thermobia domestica[34, 69] and medial 5HT-ir neurons have also been assigned to the ground plan of Myriapoda . To facilitate meaningful comparisons, it will be necessary to resolve their detailed projection pattern in different euarthropod taxa.
Previous neuroanatomical studies on the remipede brain [14, 15, 22] discovered several synapomorphies with Malacostraca and Hexapoda, such as the layout of the olfactory system including its projection neuron pathway to the protocerebrum and the architecture of the central complex. Here, we resolved the rather simple organized serotonergic system of the homonomously segmented ventral nerve cord indicating that the remipede nervous system comprises a mixture of ancestral and derived anatomical features. However, it is in line with previous neuroanatomical studies [14, 15, 22] in support of a polytomic clade of Malacostraca, Remipedia and Hexapoda.
Remipedia possess a rope-ladder-like arrangement of segmental ganglia, a feature considered plesiomorph for Euarthropoda. The neuroanatomical description of the 5HT-ir cells in the ventral nerve cord of Remipedia supports the proposed homology of these identifiable neurons in Pancrustacea. Due to position and projection pattern, we assume that the anterior and posterior 5HT-ir neurons are homologous between Remipedia, other Crustacea and Hexapoda. Comparing the ground patterns from pancrustacean taxa reveals similarities between Remipedia and Hexapoda. Our findings on the ventral nerve cord support the opinion that Remipedia are a more derived crustacean lineage and do not contradict the phylogenetic investigations that propose a sister group relationship of Remipedia and Hexapoda [9, 20, 82], although the architecture of the remipede brain shows slightly more resemblance to that of malacostracan crustaceans [14, 15, 22]. The relevance of the medial 5HT-ir neurons in the central nerve cord of Remipedia and Zygentoma for phylogenetic analyses has to be addressed in the future. To answer if these neurons can be homologized and if this feature is another argument for a sister group relationship of Remipedia and Hexapoda needs to be investigated in further studies.
Collection and tissue processing
Four individuals of Cryptocorynetes haptodiscus and one of Godzilliognomus frondosus were obtained from Sawmill Sink on Abaco Island, Bahamas. Two specimens of Speleonectes tulumensis came from the Cenote Crustacea on the Yucatan Peninsula, Mexico. Directly after collection, each specimen of Speleonectes tulumensis was cut into three parts in order to assure the penetration of the tissue by the fixative. All other specimens were fixed without any dissection. Animals were fixed in 4% paraformaldehyde dissolved in phosphate-buffered saline (PBS, 10 mM sodium phosphate, 150 mM sodium chloride, pH 7.4) for up to five days. The tissue was washed four times in PBS for at least 30 min each and stored at 4°C in PBS with 0.5% sodium azide until use.
After short incubation with Poly-L-Lysin, the tissue was embedded in 4% agarose dissolved in aqua dest. at approximately 50°C. After cooling to room temperature, the blocks were trimmed and sectioned horizontally in 50–60 μm using a vibratome (Hyrax V 50, Zeiss).
All steps of immunocytochemistry were performed on a shaker with smooth agitation at room temperature. All washing steps were conducted with 0.2% Triton X-100 (PBS-TX 0.2%) for at least 15 min if not stated differently. Horizontal sections were permeabilized for 45 min with 0.3% Saponin dissolved in PBS-TX 0.2%. After three washing steps, nonspecific binding of antibodies was blocked by incubating in 5% normal goat serum (Vector) in PBS-TX 0.2% for 3 h. Subsequently, sections were incubated for 48 hours at 4°C with blocking solution containing the following antibodies: the polyclonal antibody rabbit anti-serotonin (Sigma; dilution 1:2000), the monoclonal antibodies mouse anti-acetylated α-tubulin (Sigma; 1:500) and mouse anti-synapsin “SYNORF1” (DSHB University of Iowa; 1:30) . Three washing steps were followed by incubation with a mixture of the secondary antibodies goat anti-rabbit Cy3-conjugated (Jackson Immuno Research Laboratories; 1:250) and goat anti-mouse Alexa Fluor 488-conjugated (Molecular Probes; 1:250) plus 4′6-diamidine-2-phenylidole-dihydrochloride (DAPI) for counterstaining the nuclei (1 μg/ml) in blocking solution overnight at 4°C. Sections were washed again three times and an additional time in PBS and mounted on glass slides in Mowiol.
In this study, we used a polyclonal rabbit antiserum raised against a serotonin creatinine sulfate complex conjugated to bovine serum albumin as the immunogen (Sigma, cat. no. S5545, lot no. 108K4868), a monoclonal mouse anti-Drosophila synapsin antibody (“SYNORF1”, Developmental Studies Hybridoma Bank, University of Iowa, 1:30) raised against a Drosophila GST-synapsin fusion protein and a monoclonal mouse anti-acetylated α-tubulin (Sigma, cat. no. T6793, lot no. 059K4823, clone 6-11B-1) raised against acetylated tubulin of Strongylocentrotus purpuratus (sea urchin). Because of the difficulty of collecting specimens of Remipedia, only a low number of specimens were available. Thus, no experiments concerning the specificity of antisera could be conducted. However, 5HT-IR was investigated in a variety of invertebrate phyla , including studies on all kinds of Euarthropoda (Chelicerata and Myriapoda: e.g., [34, 36]; Hexapoda: e.g., [85–88]; Crustacea: e.g., [22, 33, 39, 41, 83]). Several of these studies used the same antiserum as in this account [22, 41, 88]. This let suggest a highly conserved antigen and a specific labeling of serotonin.
The antisera used for acetylated α-tubulin labeling is a monoclonal antibody raised against acetylated α-tubulin from the sea urchin Strongylocentrotus purpuratus (Sigma, cat. no. T6793, lot no. 059K4823, clone 6-11B-1). This antibody reacts with acetylated α-tubulin over a wide range of species such as plant, human, pig, monkey, invertebrates, hamster, bovine, chicken, rat, frog, protista and mouse (see datasheet manufacturer) and was utilized in numerous studies on the nervous system of the major crustacean taxa (e.g., Branchiopoda: ; Cephalocarida: ; Malacostraca: ; Maxillopoda: ; Remipedia: ).
The used synapsin antiserum is a monoclonal mouse anti-Drosophila synapsin antibody (“SYNORF1”, Developmental Studies Hybridoma Bank, University of Iowa) raised against a Drosophila GST-synapsin fusion protein. This antibody stained neuropil structures over a wide range of euarthropod taxa, for example Crustacea (Branchiopoda: [55, 89]; Malacostraca: [52, 90, 91]; Remipedia: ), Hexapoda (e.g., [92, 93]), Chilopoda  and the spider Cupiennius. Additionally, in western blots of brain tissues of Drosophila and the crustacean Coenobita (Anomura) identical bands were stained by the synapsin antibody, which suggests that the epitope for SYNORF 1 is strongly conserved between Drosophila and Coenobita. These results and the similar staining pattern of synaptic neuropils in different euarthropod taxa lead to the suggestion that the synapsin antibody reacts with a highly conserved epitope.
The three antibodies stain similar structures in a variety of euarthropod species and were used in a previous study of the nervous system of Remipedia . Thus, we suggest that these antisera in fact label specific structures in the remipede ventral nerve cord.
Microscopy and image acquisition
Confocal images and z-stacks were taken with a Leica TCS SP5 confocal laser-scanning microscope using Leica LAS AF software. Z-series processing including maximum projections of confocal stacks, contrast and brightness enhancement, and movie preparation was conducted with NIH ImageJ, v. 1.46r (Rasband, W.S., ImageJ, U.S. National Institutes of Health, Bethesda, MD, http://rsb.info.nih.gov/ij/). Photographs were arranged using Adobe Photoshop 6.0 (San Jose, CA).
Texas A&M University is an AAALAC, international accredited facility which conducts all vertebrate animal activities involving research, teaching, and testing according to the regulations set forth by the Animal Welfare Act, the Guide for the Care and Use of Laboratory Animals, and the Public Health Service Policy. The work conducted by Dr. Iliffe and colleagues involved the use of invertebrates which are not specifically covered by the regulations previously listed, but are also afforded the highest standard of care when used in research, teaching, or testing. No work on living specimens was conducted in Europe. All procedures in this investigation complied with international and institutional guidelines, including the guidelines for animal welfare as laid down by the German Research Council (DFG).
Anterior serotonin-immunoreactive cell
Anterior segmental nerve
Unpaired longitudinal median neurite bundle
Medial serotonin-immunoreactive cell
- Numbers 1–3:
Neurite bundles originating from the posterior segmental nerve
0.2% Phosphate-buffered saline containing 0.2% Triton X-100
Posterior serotonin-immunoreactive cell
Posterior segmental nerve
TS was supported by a scholarship of the German National Academic Foundation (Studienstiftung des deutschen Volkes). Collection of remipedes from anchialine caves in the Bahamas was funded by grants from the US National Science Foundation (DEB-0315903) and the NOAA National Undersea Research Program to TI. BMvR was funded by DFG research grants Re 3454/1-1 at the NHM in London, UK and Wa 530/34 at the ZFMK in Bonn, Germany. We thank Brian Kakuk of the Bahamas Caves Research Foundation for his assistance with collecting remipedes from Bahamian caves.
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