A Baby Cadborosaur No More. Part 9: ... and the rest!

Woodley et al. (2011) didn't just concern itself with poachers, pipefish, and 'Cadborosaurs'; everything vaguely similar to the Hagelund specimen in the region was considered. Just in case.

Aulorhynchus flavidus from Flickr user jmandecki.

Tube-snouts (Aulorhynchus flavidus) are pipefish-like relatives of sticklebacks (Gasterosteiformes) which fit the Hagelund specimen's proportions, head shape, and coloration. The dorsal, anal, and pelvic fins are small and transparent and thus possible to overlook. The forked caudal fin could be confused for overlapping fins if folded. Lateral scutes are present, albeit not extensive (illustrated here); it could be possible for the scutes and spines before the dorsal fin to suggest more extensive armor to an eyewitness.

Tube-snouts appear to swim primarily with their pectoral fins while keeping their bodies stiff (similar to poachers, sans ground effect), which makes sustained undulatory locomotion seem improbable. The largest known specimen was 18.8 cm in total length (Bayer 1980), which is less than half of the Hagelund specimen's reported length and hugely problematic for Tube-snouts as candidates.

From Wikipedia Commons.

The Green Sturgeon (Acipenser medirostris) reaches sizes far beyond 40 cm. The extensive bony scutes, barbels (= "whiskers"), and elongated body are interesting similarities with the Hagelund specimen. The major problem is that the dorsal, anal, and pelvic fins are prominent and don't seem capable of folding, unlike the other, more derived candidates. Plus, you'd think a sturgeon would be recognizable... but you never know.

From Wikipedia Commons.

Cutlassfishes (Trichiuridae) are interesting candidates as they are unambiguously eel-like, capable of anguilliform locomotion, have vestigial or outright absent pelvic fins, and (unlikely quite a few of the candidates) have teeth.

The Pacific Black Scabbardfish (Aphanopus arigato) and Pacific Scabbardfish (Lepidopus fitchi) both exceed 40 cm and have strongly forked caudal fins; neither fits the coloration, however. No cutlassfishes have scales, let alone plate-like ones, which can be viewed as a critical problem.

bc-spot-prawns-alive from Flickr user Island Vittles.

Staude and Lambert suggested that the Hagelund specimen may be a decapod in an editorial responding to LeBlond and Bousfield's description of 'Cadborosaurus' in Amphipacifica... an amphipod publication. In order for this identification to work, the "whiskers" would be head appendages (antennae, mandibles, maxillae), the "head" would be the carapace, the "fuzz" would be thoracic and abdominal appendages (maxillipeds, pereiopods, pleopods), the "plate-like scales" would be segments, and the tail appendages would be uropods. This is certainly thought-provoking, but it would require Hagelund to somehow fail to distinguish a vertebrate from an arthropod. There also aren't any obvious candidates, with the largest (Pandalus platyceros - pictured above) being around half the size of the Hagelund specimen with a radically different coloration and proportions.

From Wikipedia Commons.

The Hagelund specimen is surprisingly similar to pinnipeds, as it is the only group to possess a similar appendage arrangement (in phocids, at least), have true whiskers and fur, and be unambiguously coded as having a "seal-like face". Various pinnipeds also demonstrate long heads, slender bodies, and sorta similar coloration. Describing a pinniped as "eel-like" and "undulatory" is problematic, and the lack of plate-like scales and much larger size (even when born) are critical flaws. If the Hagelund specimen were to be taken literally and assumed to be a cryptid, a pinniped would be the most likely identification (far more so than 'Cadborosaurus'); of course, a misinterpreted known fish would be far more likely.


References:

Bayer, R. D. (1980). Size and Age of the Tube-snout (Aulorhynchus flavidus) in the Yaquina Estuary, Oregon. Northwest Science 54(4), 306-310. Available.

Hagelund, W. A. (1987). Whalers No More. Vancouver: Harbour Publishing.

LeBlond, P. H. & Bousfield, E. L. (1995). Cadborosaurus, Survivor from the Deep. Victoria, British Columbia: Horsdal & Schubart.

Woodley, M. A., Naish, D. & McCormick, C. A. (2011). A Baby Sea-Serpent No More: Reinterpreting Hagelund's Juvenile "Cadborosaur" Report. Journal of Scientific Exploration 25(3), 495-512.


Previous entries:

A Baby Cadborosaur No More. Part 1: Introduction

A Baby Cadborosaur No More. Part 2a: Hagelund's Account

A Baby Cadborosaur No More. Part 2b: Hagelund's Account - annotated

A Baby Cadborosaur No More. Part 3: Dealing With Traits

A Baby Cadborosaur No More. Part 4: What is 'Cadborosaurus'?
A Baby Cadborosaur No More. Part 5: Hagelund's Specimen And The Cadborosaurus
A Baby Cadborosaur No More. Part 6a: Cold Water on the 'Reptilian Hypothesis'
A Baby Cadborosaur No More. Part 6b: Reptilian Reproduction
A Baby Cadborosaur No More. Part 7: Poachers
A Baby Cadborosaur No More. Part 8a: Pipefish in a Bucket
A Baby Cadborosaur No More. Part 8b: The Bay Pipefish

Tet Zoo Coverage: 
A baby sea-serpent no more: reinterpreting Hagelund’s juvenile Cadborosaurus

How Do Remipedes Disperse?

For those not in the know, remipedes are small, blind, saltwater cave-dwelling (pan)crustaceans remarkable for their 'primitive' polychaete-like body plan, possible close relation to insects, and discovery in 1979. Here's a video with lots of hot remipede action and commentary from the clade's initial descriptor, Jill Yager:

I'd recommend just going to YouTube.

Remipede biogeography has become quite the complex topic, and in retrospect my last post failed to give it any justice. 22 out of the 25 known remipede species* are present in the Caribbean, with 15 being found in the Bahamas alone; Caicos Bank has 4 species while San Salvador Island, Cuba, Hispaniola, and Yucatán all have a single indigenous species (Koenemann et al. 2009). The Bahamas remipedes exhibit remarkable sympatry, with up to 6 species from 3 genera inhabiting a single cave system (Koenemann et al. 2004). It has been speculated that the diversity of remipedes in the Caribbean, and particularly the Bahamas, is due to the complex geomorphology of the caves and changing sea elevation (and thus coastlines) throughout the Pleistocene (Koenemann et al. 2009). It should be cautioned that since remipedes were initially discovered from the Bahamas, sample bias may have exaggerated the disparity of diversity between the Bahamas and other Caribbean locales.

* Species count from Koenemann et al. (2009), which includes species with formal descriptions in preparation. Not included are unidentified and undescribed species, the number of which is ambiguous.


In the Bahamas and Caicos, individual remipede species are occasionally present on multiple islands, including G. robustus from Exuma Cays, Great Bahama Bank and North Caicos (Koenemann et al. 2009). Also notable is Speleonectes lucayensis, the first described remipede, which is present on Andros and Cat Island from the Great Bahama Bank as well as Grand Bahamas and Abaco from the Little Bahama Bank (Koenemann et al. 2009). Iliffe et al. (2010) took note of a pair of Godzilliognomus on either side of the Bahamas Banks, and speculated that dispersal may have occurred through open water, the deep sea, or cave colonization when the banks were forming during the Cretaceous. Very recently it was discovered that remipedes are not restricted to subterranean caves, but are present in sub-marine caves with similar properties located on shallow 'platforms', which apparently increases the potential range considerably and may explain occurrence on multiple islands (Daenekas et al. 2009). It is certainly an intriguing thought that islands which share remipede species have a contiguous, or nearly so, distribution of individuals in between. Since islands outside of the Bahamas and Caicos all have unique remipedes, it can be assumed this phenomenon has limited applicability for planetary-scale dispersal.

So what are those other Caribbean remipedes? All are members of the genus Speleonectes (Koenemann et al. (2009), and morphology-based phylogeny places S. epilimnius (San Salvador), S. gironensis (Cuba), and S. tulumensis (Yucatán) well within the genus (Koenemann et al. 2007). The Bahamas and Caicos are also home to several Speleonectes and other speleonectids (Koenemann et al. 2009), and I think it would be fascinated if all the divergent speleonectids were analyzed through molecular phylogeny to determine relations more certainly and calculate divergence times. If the Speleonectes from outside the Bahamas and Caicos are indeed closely related to those within, it would seem to suggest that colonization occurred very recently, presumably during or after the sea level changes of the Pleistocene.

Speleonectes are also present far outside of the Caribbean, as now two species are known from the Canary Islands. Both S. ondinae and S. atlantida were found in the Túnel de la Atlántida (apparently the world's largest lava tube), despite it being only 1,700 m (~ 1 mile) long, and much simpler in structure and more recently formed (~20,000) than the Bahamas locales which support remipede sympatry (Koenemann et al. 2009). Remipedes are also rarely seen in the tube despite ideal viewing conditions, and Koenemann et al. (2009) suggest they may be found outside the system, although this has not been confirmed. Morphological phylogeny tended to place S. ondinae around Caribbean Speleonectes (Koenemann et al. 2007) and limited examination of DNA demonstrated S. atlantida formed a clade outside S. ondinae, with a difference between the two being larger than a mean intraspecific distance for a few select species (Koenemann et al. 2009). It would be interesting to determine if the species are sister taxa, or if they managed to colonize the tube independently.

Incredibly, there is a species of remipede from Western Australia - and it isn't Speleonectes! Lasionectes exleyi is still a speleonectid and is currently classified as congeneric with L. entrichoma (Koenemann et al. 2009) however morphological phylogeny generally shows Lasionectes to be paraphyletic, and places both species basally in Speleonectidae (Koenemann et al. 2007). Unlike the other cases, this evidence could suggest a relatively ancient dispersal.


Remipede biogeography is still an emerging topic, and for all I know, there could be remipede colonies in between the Caribbean and disjunct locales, or even worldwide. As the Canary Islands species occur in a very recently-formed lave tube, it would suggest that remipedes can occasionally disperse through open waters and colonize ideal locales. It could be possible they haven't been detected yet due to being rare, although confusion with polychaetes is possible. Hermaphrodites would be ideal colonizers, and since it is now known remipedes can live in caves off the mainland, it would greatly increase the chances of dispersal. The exact extent of remipedes and sub-marine caves is intriguing, but it seems likely that contiguous populations between islands are only likely to occur in the Bahamas and Caicos. Why only Speleonectes are dispersed throughout the Caribbean and Canary Islands is curious, so presumably there's some aspect of their life history which aids dispersal.


Instead of having to write on this topic again every couple years, I'm considering setting up one of those fancy blog 'pages' to keep everything strait on remipedes. Then again, I have no idea what that may be getting me into.

References:

Daenekas, J., Iliffe, T., Yager, J., and Koenemann, S. (2009). Speleonectes kakuki, a new species of Remipedia (Crustacea) from anchialine and sub-seafloor caves on Andros and Cat Island, Bahamas. Zootaxa 2016, 51-66. Available.


Iliffe, T., Otten, T., and Koenemann, S. (2010). Godzilliognomus schrami, a new species of Remipedia (Crustacea) from Eleuthera, Bahamas. Zootaxa 2491, 61-68. Available.

Koenemann, S., Bloechl, A., Martinez, A., Iliffe, T., Hoenemann, M., and Oromi, P. (2009). A new, disjunct species of Speleonectes (Remipedia, Crustacea) from the Canary Islands. Marine Biodiversity 39, 215-225. Available.


Koenemann, S., Schram, F., Honemann, M., and Iliffe, T. (2007). Phylogenetic analysis of Remipedia (Crustacea). Organisms, Diversity & Evolution 7, 33–51. Available.

Koenemann S., Iliffe T., and Yager, J. (2004) Kaloketos pilosus, a new genus and species of Remipedia (Crustacea) from the Turks and Caicos Islands. Zootaxa 618, 1–12. Available.

The Cephalocarids

Cephalocarids, literally 'head shrimp', are superficially reminiscent of those other oddball arthropods I covered, the remipedes. Both are 'class'-level crustacean clades discovered in the latter half of the 20th Century and assumed to be 'extremely primitive' on the basis of traits such as elongation, undifferentiated trunk appendages, and weak tagmosis (formation of head/thorax/abdomen by fusing 'segments') (Ruppert et al. 2004). Some have pointed out that arthropods should probably be described as 'jointed' rather than 'segmented' (Valentine 2004), suggesting the similarity of these 'primitive' clades to annelids is coincidental. A (Cephalocarida + Remipedia) clade has been recovered on occasion, and recently a molecular phylogenetic analysis proposed the taxa to be called 'Xenocarida', or 'strange shrimp' (Reiger et al. 2010). Before I delve into the hyper-controversial world of (pan)crustacean systematics, more background is needed on the cephalocarids.

Dorsal view of Hutchinsoniella macracantha taken and modified from Sanders (1955). The proportionally large head (17-18% length - excluding caudal spines) provides the 'class' name; it should be noted that copepods have similar, if not more extreme, proportions. The purported common name 'horseshoe shrimp' derives from the head shape, but I'm not enthusiastic about the descriptive value of that name either.

Cephalocarids appear not to exceed 4 mm in length (Ruppert et al. 2004), so it is not particularly surprising that they were overlooked for so long. The first recorded specimen was found in September 1953 in San Francisco Bay, although that species (Lightiella serendipita) was not properly described until Jones (1961). In August 1953 another specimen was found in Long Island Sound, but the actual holotype for Hutchinsoniella macracantha was recovered in July of the next year from the roughly the same locale (Sanders 1955). Confusing! Anyways, Hutchinsoniella specimens from Long Island Sound and Buzzard's Bay were found on muddy bottoms 9-29 m below the surface, although shortly afterwards they were found down to 100 m on a continental slope (Hessler and Sanders 1964). The Lightiella specimens from San Francisco were only found 3 to 5 feet (~0.9 to 1.5 m) below the lowest water level (Hessler and Sanders 1964). The depth record for cephalocarids as a whole is 1550 m, and they have been recorded from sand in addition to mud (Ruppert et al. 2004).

Major cephalocarid discoveries are still being made, and I would certainly look closely at any sand or mud I happen to dredge up. A new species was recently discovered from Japan (Shimomura and Akiyama 2008) and the clade (in the form of a new species) was just described from Europe (Carcupino et al. 2006). Martin et al. (2002) state the presence of cephalocarids anywhere is notable due to their infrequence and considerable interest in their morphology and phylogenetics - so why is that?

Lightiella incisa taken and modified from Martin et al. (2002). The roundish structures are oocytes - all species are hermaphrodites. 

Sanders (1955) noted that cephalocarids share traits with a number of clades such as branchiopods (number of thoracic and abdominal segments), malacostracans (postcephalic appendage morphology), and copepods (large head) - but concluded that they were at least as 'primitive' as branchiopods and probably related to the 'ancestral stock' that gave rise to those clades. Cephalocarids were assumed to be eyeless, but Burnett (1981) recorded the degenerated compound structures in H. macracantha - and used that as further evidence that cephalocarids were almost unchanged from a 'Paleozoic urcrustaceanlike organism'. I'm not particularly impressed with that line of reasoning. Read et al. (1994) found that cephalocarid muscle overall resembles that of other crustaceans, but (along with ostracods) lacks myofibrils in a condition similar to onychophorans, and was assumed to represent a 'primitive' rather than simplified condition. Considering the incredibly small size of cephalocarids (most appear to be between 2-3 mm) the possibility of simplification cannot be casually tossed aside. Elofsson and Hessler (1990) examined the central nervous system of H. macracantha and were surprised to find it highly developed (albeit missing some structures - like eyes, for the most part) and thought that since it didn't conform with the 'overall primitiveness' of the animal, it must have evolved separately from that of other crustaceans! L. incisa appears to prefer oxygen-rich microzones (Martin et al. 2002) - which would seem appropriate for animals with a complex nervous system. It seems that a lot of these systematic inferences were made with the 'primitiveness' of cephalocarids in mind and fitting the evidence into that preconception - so what happens when phylogeny is examined using modern methods?

From Reiger et al. (2010). Note the position of Xenocarida - the sister group of insects!

Reiger et al. (2005) used RNA sequences to hypothesize insects were terrestrial (pan)crustaceans and that branchiopods were their sister group, with a cephalocarid + remipede clade as the sister group to (Branchiopoda + Hexapoda). The authors speculated that members of this clade had a proclivity for near shore or marginal marine habitats, but were excluded by other pancrustaceans and forced into their present unusual habitats (hexapods live on land and freshwater, branchiopods generally in freshwater, and remipedes live in anchialine caves) (Reiger et al. 2005). A prior study using different genes also recovered a cephalocarid + remipede clade and interpreted it to be due to long-branch attraction, but Reiger et al. (2005) argue that it could be interpreted literally. While Reiger et al. (2005) argue that cephalocarids are derived, they could not answer whether or not their morphology is actually plesiomorphic - but noted that prior placements based on the 'ur-crustacean' hypothesis were somewhat anachronistic. It should be noted that a phylogeny determined from mitochondrial protein coding genes failed to recover 'Xenocarida', but did create a clade of branchiopods, malacostracans, cephalocarids, and insects (Carapelli et al. 2007).

I think what we should take away from the phylogenetic work is that the cephalocarid-as-ur-crustacean hypothesis is looking rather shaky and an extensive examination of morphology and molecular data is needed to resolve the phylogeny with anything resembling rigorous support. Cephalocarids are not known from the fossil record, so when or if those discoveries are made, hopefully it will clarify if the morphology is plesiomorphic, simplified, or some combination of the two. The potentially phylogenetically significant placement of cephalocarids probably means our knowledge of them will expanding in the near future and they won't always be so enigmatic. I certainly intend to squint at marine mud the next time I happen across it...

References:

Burnett, B. (1981). Compound eyes in the Cephalocarid Crustacean Hutchinsoniella macracantha. Journal of Crustacean Biology 1(1), 11-15.

Carapelli, A., et al. (2007). Phylogenetic analysis of mitochondrial protein coding genes confirms the reciprocal paraphyly of Hexapoda and Crustacea. BMC Evolutionary Biology 7(2). doi:10.1186/1471-2148-7-S2-S8

Carcupino, M., et al. (2006). A new species of the genus Lightiella: the first record of Cephalocarida (Crustacea) in Europe. Zoological Journal of the Linnean Society 148(2), 209-220. doi:10.1111/j.1096-3642.2006.00237.x

Hessler, R. and Sanders, H. (1964). The Discovery of Cephalocarida at a depth of 300 meters. Crustaceana 7(1), 77-78.

Jones, M. (1961). Lightiella serendipita gen. nov., sp. nov., a Cephalocarid from San Francisco Bay, California. Crustaceana 3(1), 31-46

Martin, J. et al. (2002). First record and habitat notes for the genus Lightiella (Crustacea, Cephalocarida, Hutchinsoniellidae) from the British Virgin Islands. Gulf and Caribbean Research 14, 75-79. Available.

Reiger, J. et al. (2010). Arthropod relationships revealed by phylogenomic analysis of nuclear protein-coding sequences. Nature doi:10.1038/nature08742

Reiger, J. et al. (2005). Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proceedings of the Royal Society B 272(1561), 395-401. doi: 10.1098/rspb.2004.2917

Ruppert, E., et al. (2004). Invertebrate Zoology: A Functional Evolutionary Approach. Belmont, California: Brooks/Cole - Thomson Learning.

Sanders, H. (1955). The Cephalocarida, a new Subclass of Crustacea from Long Island Sound. Proceedings of the National Academy of Sciences 41(1), 61-66. Available.

Shimomura, M. and Akiyama, T. (2008). Description of a New Species of Cephalocarida, Sandersiella kikuchii, and Redescription of S. acuminata Shiino Based Upon the Type Material. Journal of Crustacean Biology 28(3), 572-579. doi: 10.1651/07-2864R.1

Valentine, J. (2004). On the Origins of Phyla. Chicago, USA: University of Chicago Press.

Remipedia

One of my classes assigns write-ups on invertebrate peer-reviewed literature, so I figured that I might as well cannibalize and extend my original report to post it here. I also gave a short talk on Remipedia that a "lucky" few were subjected to - let's hope this is a more coherent format.


I have an inexplicable intellectual attraction to relictual organisms, making remipedes fascinating to me despite my relative unfamiliarity with Crustacea. I should point out that the popular conception of a "crustacean" is essentially synonymous with Decapoda (shrimp, lobsters, crabs, etc.) and excludes all of the other various clades of mandible-bearing arthropods with two pairs of antennae, two maxillae pairs and division into tagma (e.g. cephalothorax, pereon, pleon). Remipedes are the exception to the latter trait and simply possess a head with a long, homonomously segmented trunk somewhat reminiscent of myriapods (especially chilopods = centipedes), onychophorans and some polychaete annelids. It was long assumed that arthropods evolved from a long-bodied annelid-like ancestor so early remipede morphological workers (i.e. those in the 80's and 90's!) assumed that remipedes lay at the base of the crustacean phylogenetic tree; one worker even used remipedes to root the crustacean phylogenetic tree and placed them in a more basal position than Burgess Shale arthropods (Odaraia, Canadaspis) (Schram and Koenemann 2004). As we'll see later, the only modern consensus about remipede relations is that they aren't at the base of the "crustacean" family tree...

The unpigmented, eyeless, marine cave-dwelling members of class Remipedia were first discovered off the Bahamas in 1979* and were described by Yeager (1981). Remipedes were later found in other Caribbean locales such as the Yucatan peninsula, the Turks and Caicos, Cuba, and the Canary islands - incredibly they've also been found in roughly antipodal Western Australia as well (Yager and Humphreys 1996). Yager (1981) suggested that a remipede had in fact been described before: the enigmatic Carboniferous Tesnusocaris (described in 1955) which also possessed homonomous segments with paddle-like appendages. Emerson and Schram (1990) re-described Tesnusocaris with both a pair of uniramous ventrolateral appendages used for swimming and a midventral pair used for sculling; the authors hypothesized that this two appendage pair state is a "missing link" between uniramous and biramous appendages. Koenemann et al. (2007a) found some aspects of the authors' reconstruction questionable** (the two limb pairs per segment?) but still used their description of Tesnusocaris as an outgroup for their phylogeny of modern remipedes (Nectiopoda) - despite potential weirdness Tesnusocaris still did have distinctively remipedian head appendages. The Mazon Creek assemblage of Illinois (home of Pohlsepia and Tullimonstrum) yielded the remipede Cryptocaris which also has the three pairs of prehensile cephalic appendages (maxillule, maxilla and maxilliped), but was not complete enough for analysis by Koenemann et al. (2007a). So, it looks like Remipedia has an even worse ghost lineage syndrome than octopuses.

* Another class of possibly primitive crustaceans, Cephalocarida, was discovered off Long Island Sound in 1953. Granted, remipedes are ~1.5-4.5 cm in length and cephalocarids are 4 mm, but discovering new classes still sounds like a major surprise.
** They cite a later paper from the same authors in 1991 from the Proceedings of the San Diego Society of Natural History. I'm assuming that it's a more thorough version of their Science article.


Not much is known about the reproduction, life history and behavior of remipedes; what is known is fascinating, if somewhat contradictory. Being blind, remipedes have a gigantic olfactory apparatus and use their second pair of antennae to drive currents past the "fields of aesthetascs" on their first antennae pair; their ability to detect low odor concentrations has been confirmed by observations of their quick attraction to dead fish (Fanenbruck et al. 2004). Remipedes have been observed to be slow swimmers but they are not strict scavengers, they have raptorial mouthparts including a fang-like first maxillae and have been observed engaging in predatory behavior (Kohlhage and Yager 1994, Fanenbruck et al. 2004). The maxillule fangs connect to glands which are presumably involved in injecting prey; while empirical evidence of injection doesn't exist, the probable mechanics of injection have been worked out (van der Ham and Felgenhauer 2007). The potentially injected substance is apparently an oxygen-carrying respiratory pigment (!) - another substance capable of turning the hemocyanin-like compound into a harmful phenoloxidase has yet to be discovered (van der Ham and Felgenhauer 2007). In case you're diving in obscure marine caves, don't worry about remipede bites as they apparently have no adverse affect on people (Koenemann et al. 2007b). While the aforementioned evidence seemingly suggests that remipedes are sluggish marine centipede analogues - lab observations of Speleonectes indicate that they spend almost all of their time (>99%) filter feeding (Koenemann et al. 2007b). Koenemann's website has a video summary (warning, 50 megabytes) of 2-3 months of observed behavior, including some of the rare instances of predation (3!). From the video, it can be noted that the thoracic appendages are always moving (even at rest) and remipedes are capable of a fast "snake-like" strikes and coiling (Koenemann et al. 2007b). Even though these remipedes aren't very big animals (~4 cm), I would hesitate in calling them sluggish (note that some parts of the video are at 5x). It seems likely that remipedes spend most of their time filter feeding in the wild as well and engage in facultative predation/scavenging whenever something comes their way - the lack of these behaviors in the lab could be artifacts due to the environment and/or the (relatively) high abundance of potential prey.


So, what are remipedes?

Despite looking like hypothetical ancestral arthropods, remipedes are obviously quite specialized. If we ignore their confusing mosaic of morphological traits for the time being, molecular evidence gives us a wide range of opinions on their placement. Regier et al. (2005) suggested close kinship with cephalocarids and a somewhat more distant relation with branchiopods (both viewed as morphologically "primitive"), oh and all of those groups were in a clade containing hexapods, i.e. insects and kin! The authors note that all of the members probably had ancestors either near-shore or in marginal (read: really weird) marine habitats, possibly the result of competition from other crustaceans, myriapods and chelicerates (Regier 2005). Cook et al. (2005) used mtDNA to place remipedes in a derived clade with Collembola (hexapods!) - mind you in thus study both hexapods and crustaceans were paraphyletic! Since none of the other studies reach any consistent placement, let's look at morphology.

Unexpectedly, remipedes have an order of magnitude more neurons than other taxa like branchiopods and maxillopods and their complex brains resemble those of malacostracans and hexapods (Fanenbruck et al. 2004). Also unexpected are the recently discovered larvae of remipedes, which happen to be non-feeding (lecithotrophic) in a manner similar to malacostracans such as euphausiaceans and dendrobranchiates (Koenemann et al. 2007c). Remipedes larvae share many traits with the lecithotrophic malacostracans but differ in having three pairs of uniramous cephalic limbs, biramous trunk limbs and caudal rami developing on an anal somite rather than a teslon (Koenemann et al. 2007c). The first trait is especially odd since you would expect a maxilliped to resemble a trunk appendage during development, but this somehow is not the case. Convergence can't be ruled out of course, but the coincidence of similar brain morphology and occasionally similar development is interesting (unless the two are somehow connected). Koenemann et al. (2007c) echo a previous study which tenuously concluded that remipedes, cephalocarids and "most of the maxillopodans and malacostracans" form a clade to the exclusion of other crustaceans.

The previous study Koenemann et al. (2007c) are referencing (Schram and Koenemann 2004) used extinct and extant arthropods (including Tesnusocaris) in their analysis. One of the characters united remipedes with Eucrustacea was gonopore placement on the 6th through 8th thoracic segments - I'm not sure how this was coded in for remipedes. Interestingly, even this analysis found insects to lay within the group traditionally known as "crustaceans" - could it be that molecular and morphological camps are finally starting to agree?



This of course isn't everything on remipedes, but it should at least give an idea of the pioneering work being done on this fascinating group. Well, this took far too long to write, I've got obligations to fulfill like crazy...




References:

Cook, Charles E. et al. 2005. Mitochondrial genomes suggest that hexapods and crustaceans are mutually paraphyletic. Proc Biol Sci. 272, 1295–1304

Emerson, Michael J. and Schram, Frederick R. 1990. The Origin of Crustacean Biramous Appendages and the Evolution of Arthropoda. Science 250, 667-669

Fanenbruck, Martin et al. 2004. The brain of the Remipedia (Crustacea) and an alternative hypothesis on their phylogenetic relationships. PNAS 101, 3868-3873.

van der Ham, Joris L. and Felgenhauer, Bruce E. 2007. The functional morphology of the putative injecting apparatus of Speleonectes tanumekes (Remipedia). Journal of Crustacean Biology 27, 1-9

Koenemann, Stefan et al. 2007a. Phylogenetic analysis of Remipedia (Crustacea). Diversity & Evolution 7, 33–51

Koenemann, Stefan et al. 2007b. Behavior of Remipedia in the Laboratory, with supporting Field Observations. 2007. Journal of Crustacean Biology 27, 534-542

Koenemann, Stefan et al. 2007c. Post-embryonic development of remipede crustaceans. Evolution & Development 9, 117-121

Kohlhage, Klaus and Yager, Jill. 1994. An Analysis of Swimming in Remipede Crustaceans. Philosophical Transactions: Biological Sciences 346, 213-221

Regier, Jerome C. et al. 2005. Pancrustacean phylogeny: hexapods are terrestrial crustaceans and maxillopods are not monophyletic. Proc Biol Sci. 272, 395-401.

Ruppert, Edward E. et al. 2004. Invertebrate Zoology: A Functional Evolutionary Approach. Seventh Edition. Thomson, Brooks/Cole, United States.

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