The enigmatic Remipedia
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Stamm Arthropoda
Unterstamm Crustacea 1
Klasse Remipedia
Ordnung Nectiopoda
Verbreitung siehe hier
Besonderheiten Vergleichsweise neuer Angehöriger des Unterstammes Crustacea, dessen Klassifikation noch nicht abgeschlossen ist.
Schutzstatus Einige Arten sind mancherorts auf der Roten Liste, mehr werden wahrscheinlich folgen.
Fotografie einer neuen unbeschriebenen Art von Speleonectes, dorsale Ansicht (original)

 

Introduction

The crustacean class1 Remipedia is relatively new to science. The first specimens were discovered during a cave diving expedition on Grand Bahamas Island in 1979. The significance of this finding became immediately apparent.


1The literature gives controversial views regarding the taxonomic position of Remipedia. Although it is occasionally even questioned if remipedes are crustaceans at all, the group is generally defined as class, sub-class or order within the Crustacea. These varying taxonomic assignments depend on conflicting views (sometimes derived from different national schools of thought) on higher systematic categories in the phylum Arthropoda.

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Habitus
At first glance, remipedes do not appear to resemble any other of the major groups of Crustacea. In fact, a remipede does not look much like a crustacean at all. Its trunk does not exhibit a structural subdivision into two or more distinct tagmata (pereon + pleon, or thorax + abdomen), a classic diagnostic feature of Crustacea. Superficially, the long, homonomously segmented trunk of Remipedia (see title) is similar to that of some Myriapoda.
However, upon closer examination, remipedes reveal a number of derived characters that they seem to share with (many) crustaceans. These include, among others, the morphology of some of the cephalic appendages, paddle-shaped trunk limbs composed of two branches, and a posterior body terminus with caudal rami.

Although the exact phylogenetic position of Remipedia within the Arthropoda has not been determined yet, we can conclude that Remipedia are unique among the diversity of both fossil and extant ‘crustaceomorph’ body plans, and represent indeed a "class apart".

Remipedes are hermaphrodites, bearing female gonopores on the seventh trunk segment and male pores on segment 14. Their head region is armed with three pairs of prehensile limbs (Fig. 1).
 
Fig. 1 (left): Ventral view of the head region of a new species (of the genus Pleomothra) featuring powerful prehensile cephalic limbs (photograph by Thomas Iliffe).
Click here to enlarge the photograph.
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Behavior - Foraging and Reproduction

Because of these powerful raptorial appendages it has been suggested that remipedes are obligatory predators. This view was supported by occasional reports from cave divers, who observed remipedes hunt and catch comparatively large cave shrimps. However, our recent studies of remipedes maintained in tanks could not confirm this hypothesis. Six specimens - observed over a 2-3 months period - spent more than 99% of observation time filter- or particle-feeding. A variety of offered prey organisms were largely ignored, and predation could be observed in only three cases.

There are still many aspects of the biology of Remipedia we do not know at all. For example, it is not known how ‘old’ this group is from a paleogeographic point of view. We are not certain which crustaceans (or arthropods) are their closest relatives. And there is literally nothing known about the modes of reproduction and development in remipedes. All these unresolved issues have added to the enigmatic aura that surrounds this group of arthropods.

 
Right-click here and "save as..." to download the complete video of various behaviour patterns in good quality (29 MB).
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Distribution

Remipedes are exclusively known from anchialine caves, for example, submerged tunnels or channels that connect inland brackish or freshwater systems with ocean waters. In the larger Caribbean region, many anchialine cave systems formed originally as dry caves that submerged after the last ice ages (Fig. 2a).

All currently known taxa of Remipedia occur in tropical or subtropical belts, mostly in the Caribbean region (Koenemann et al., 2003). A single species is known from Western Australian and the Canary Islands, respectively (Fig. 2b).
 

Fig 2a (left): Remipedia are exclusively known from submerged cave systems
(shown here: Carwash Cenote, Yucatan Peninsula). The impressive stalagmites and stalactites in front of a diver illustrate that this anchialine cave must have been a dry cave for a long period of time. The cave system ultimately submerged as a result of rising sea levels after Pleistocene glaciations. Photo courtesy of Thomas Iliffe.
Fig 2b (right) : Global distribution of Remipedia. Red circles represent individual species.
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Systematics

The class Remipedia contains two orders. The Enantiopoda, with the Carboniferous fossils Tesnusocarisgoldichi Brooks, 1955, and Cryptocaris hootchi Schram, 1974, and the order Nectiopoda, which embraces all extant remipedes. Since their discovery in 1979, 17 species of nectiopod Remipedia have been described. At present, the order Nectiopoda is composed of three families:

  • The Godzilliidae Schram, et al., 1986, with three monotypic genera;
  • The Speleonectidae Yager, 1981, comprising nine species in three genera;
  • A new, monotypic family proposed Koenemann et al. (accepted, 2005).
Fig. 3: Phylogenetic analysis of 17 described taxa Remipedia, including a proposed new family from North Caicos Island and the Carboniferous fossil Tesnusocaris as an out-group; data matrix composed of 33 morphological characters. The two families, Speleonectidae and Godzilliidae, are distinguished by different font colors. Black branches represent topologies that remained stable even after small changes applied to characters and/or coding options; red branches indicate unstable relationships, including the position of the new family.

During recent diving explorations, eight species of remipedes that were as yet unknown to science could be collected. However, the discovery of new taxa revealed several serious inconsistencies of the current taxonomic structure within the order Nectiopoda. For example, a new, apparently higher taxon from the Turks & Caicos Islands could not be assigned to either one of the existing families, Speleonectidae and Godzilliidae (Fig. 5). At present, we are conducting comprehensive phylogenetic investigations of all remipedes, including 16 described and nine as yet undescribed species. To determine the relationships among Remipedia as well as their position within the Crustacea, we will analyze a selection of five mitochondrial and four nuclear (ribosomal and protein-encoding) genes in addition to a matrix of 30-40 morphological characters. Additional investigations carried out in our lab include behavioral studies and paleogeographic analyses.

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Acknowledgements

The research projects presented herein were carried out in collaboration with Frederick R. Schram (Emer. Prof., University of Amsterdam, Netherlands) and Prof. Thomas M. Iliffe ( Texas A&M University at Galveston, USA).
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References
Brooks, H.K. 1955. A crustacean from the Tesnus formation of Texas. J. Paleont. 29: 852-856.
Koenemann, S., Iliffe, T.M. & van der Ham, J. 2003. Three new species of remipede crustaceans (Speleonectidae) from Great Exuma, Bahamas Islands. Contributions to Zoology72 (4): 227-252.
Schram F.R.. 1974. Late Paleozoic Peracarida of North America. Fieldiana, Geology 33: 95-124.
Schram, F.R., Yager, J. & Emerson, M.J. 1986. Remipedia. Part I. Systematics. Mem. San Diego Soc. Nat. Hist. 15: 1-60.
Yager, J. 1981. A new class of Crustacea from a marine cave in the Bahamas. Journal of Crustacean Biology 1: 328-333.

 

Prof. Dr. Stefan Könemann, Dezember 2005


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