Hyocrinidae sp. 1
(Copyright Julian Gutt/AWI)
Bourgueticrinina sp. 1
Pentametrocrinus sp. 1
Featherstar sp. 1
(Copyright Julian Gutt/AWI)
Crinoids are echinoderms sister group to all other extant classes. They have an extensive fossil record and are dated back 500 million years. Crinoids are well distributed at all depths, from shallows to the deep ocean. They are found in a great variety of habitats, from tropical reefs to Antarctica, on unconsolidated or hard substrata. Some of them can crawl on the seabed to avoid predators, others are able to swim, while a number of them are sessile (Messing, 1997 ; Roux et al., 2002).
Crinoids are classically divided in two easily distinguishable groups : the stalkless comatulids and the stalked crinoids.
Figure 1 - Stalked crinoids can be divided in 4 major groups and 13 families (Hess 2011). The Cyrtocrinids include organisms with a stalk reduced to a single ossicle (Neogymnocrinus) and organisms that display no stalk at all and are fixed directly to the calyx (Holopus and Cyathidium). An extensive group, the pentacrinitid, is easily identifiable by its heteromorphic stalk bearing cirri regularly placed all along the stalk (with the exception of Proisocrinus). A third group, the hyocrinids, is composed of organisms displaying an homeomorphic stalk (Fig. 2) and generally five arms. A fourth group, the bourgueticrinid, is composed of organisms displaying a xenomorphic stalk Fig. 2) and 5, 10 or up to 60 arms.
Figure 2 - Comatulids (or feather stars) discard their postlarval stalk but keep the proximalmost columnal that transform into a centrodorsal. The centrodorsal often bears cirri that help comatulids to cling on other organisms or on the substratum. Comatulids are divided into 18 families that are primarily distinguished on the basis of the pinnules (position of pinnules along the arm, shape of the pinnulars, number of pinnulars, length of the pinnules). Comatulids usually display five radials (this number can increase to ten) from which arms arise. Arms can be divided or not, from one to many times.
However, recent molecular work (Rouse et al., 2013 ; Hemery et al., 2013 ; Roux et al., 2013) has demonstrated that comatulids and stalked crinoids are not monophyletic. These results also have demonstrated that the character used to distinguish species are often not homologuous and are convergent or iterative features.
Identification from images
Crinoid identification requires features only accessible when the specimens are available for magnification through SEM or light microscopy. Most of the characters used are not accessible on images. However, a good knowledge of the local fauna in addition to in situ pictures associated to sampled specimens may greatly improve identifications using images. This is because the number of species potentially present is generally reduced, and colour or behavioural features may be added to classical morphological characters.
Obvious features are colour and general posture. Other features may be found on the stalk, the crown of arms and pinnules, and when the image is of better resolution, the attachment disk, pinnulation, proeminence of the calyx or length and composition of brachitaxis may be accessible.
List of potential species
- Ptilocrinus (Ptilocrinus) clarki Roux & Lambert, 2011
- Ptilocrinus (Ptilocrinus) pinnatus Clark, 1907
- Gephyrocrinus messingi Roux & Lambert, 2011
- Hyocrinus bethellianus Thomson, 1876
- Thalassocrinus alvinae Roux, 2002
- Hyocrinus foelli Roux & Pawson, 1999
- Bathycrinus aff australis Clark, 1907
- Bathycrinus mendeleevi Mironov, 2008
- Bathycrinus volubilis Mironov, 2000
- Discolocrinus thieli Mironov, 2008
- Bathycrinus equatorialis Clark, 1908
- Pentametrocrinus sp.
- Bathymetra abyssicola (Carpenter, 1888)
- Thalassometra bispinosa (Carpenter, 1888)
The first character to be looked at is the presence of a stalk. The composition of the stalk may be homeomorphic (all columnals are of the same kind), heteromerphic (some columnals are bearing cirri), or xenomorphic (the stalk has a distal, middle and proximal parts easily distinguished). In the arae of interest, only homeomorphic and xenomorphic stalks can be seen. If the stalk is absent and the animal is attached to the substrate using cirri, then this is a comatulid.
The number of arms may also be of interest. Stalked crinoids from the area may bear 5 or 10 arms, 10 arms being indicative of a Bourgueticrinina. Comatulids as well have a varying number of arms; They usually display 10 or more. Rarely 5 which are indicative of a few genera, and in the case of the deep central Pacific, indicative of a representative of the genus Pentametrocrinus. Other characters are usually impossible to distinguish on in situ pictures.
When the arms are spread to form a fan, pinnules may be seen. They may appear long or short. Long pinnules in association with homeomorphic stalks are indicative of Hyocrinidae. Short pinnules in association with a xenomorphic stalk are indicative of Bourguticrinina.
Clark, A.H. & Clark, A.M. (1967). A monograph of the existing crinoids, Vol. 1: the comatulids, part 5, Suborders Oligophreata (concluded) and Macrophreata. Bulletin of the United States National Museum, 82, i–xv, 1–860.
Eléaume, M., Améziane, N. & Park, Y.-H. (2004). Re-evaluation of the systematics of two deep-sea species of Thalassometra (Echinodermata: Crinoidea) and its biogeographical implications. Journal of Natural History, 38, 1949–1968.
Hemery, L.G., Roux, M., Améziane, N. & Eléaume, M. (2013). High-resolution crinoid phyletic interrelationships: a preliminary study. Proceedings of the 14th International Echinoderm Conference. Cahiers de Biologie Marine. 54(4), 511-523.
Hess H., Messing C.G. & Ausich W.I. (2011). Treatise on Invertebrate Paleontology, Part T, Echinodermata 2, Revised Crinoidea, 3 (P.A. Selden ed). Paleontological Institute: University of Kansas. 261 pp.
Messing, C.G. (1997). Living comatulids. In: Waters, J.A., Maples, C.G. (eds.). Geobiology of Echinoderms. Paleontological Society Papers, 3, 3–30.
Messing, C.G. & Dearborn, J.H. (1990). Marine Flora and Fauna of the Northeastern United States, Echinodermata: Crinoidea. NOAA Technical Report NMFS 91.
Mironov, A.N. (2000) New taxa of the stalked crinoids of the suborder Bourgueticrinina. Zoologicheskii Zhurnal 79(6):712–728.
Mironov, A.N. (2008). Stalked crinoids of the family Bathycrinidae (Echinodermata) from the eastern Pacific. Invertebrate Zoology 5(2):133-153.
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Roux M. (2002). Two New Species of the Genus Thalassocrinus (Echinodermata: Crinoidea: Hyocrinidae) from the Pacific Ocean. Species Diversity 7 (2): 173-186.
Roux, M., Eléaume, M., Hemery, L.G. & Améziane, N. (2013). When morphology meets molecular data in crinoid phylogeny: a challenge. Proceedings of the 14th International Echinoderm Conference. Cahiers de Biologie Marine. 54(4), 541-548.
Roux M. & Lambert P. (2011). Two new species of stalked crinoids from the north-eastern Pacific in the genera Gephyrocrinus and Ptilocrinus (Echinodermata, Crinoidea, Hyocrinidae). Effects of ontogeny and variability on hyocrinid taxonomy. Zootaxa, 2825: 1-54.
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Roux M. & Pawson D.L. (1999). Two new Pacific Ocean species of hyocrinid crinoids (Echinodermata), with comments on presumed giant-dwarf gradients related to seamounts and abyssal plains. Pacific Science, 53(3): 289-298.
Speel J.A. & Dearborn J.H. (1983). Comatulid crinoids from the R/V Eltanin cruises in the Southern Ocean. Antarctic Research Series, 38: 1–60.