Concentricycloidea
(Sea daisies)
Phylum: Echinodermata
Class: Concentricycloidea
Number of families: 1
Thumbnail description
Small disc-shaped echinoderms with a circular skeleton and water vascular system
Evolution and systematics
The unusual echinoderm class Concentricycloidea (sea daisies) was erected for the genus and species Xyloplax medusiformis by Baker, et al., (1986) because of its radical departure in morphology from any other known living echinoderm (starfishes, brittle stars, sea urchins, sea cucumbers, and sea lilies). Concentrically arranged skeletal structures and single series of tube feet arranged in a circle were novel in living echinoderms. Given these special features, it was not considered possible, on the basis of morphology, to accommodate the genus in any existing echinoderm class. Rowe, et al., (1988) did consider, however, that Xyloplax was derived from the Asteridea (starfishes), possibly from an ancestral precursor to the order Valvatida. In 1988, a second species, X. turnerae, was discovered which confirmed the unique morphology of the class.
The morphological differences between each of the existing orders of the Asteroidea are quite minor compared with the radical rearrangement of the axial skeleton, water vascular system, and the form of the spermatozoa in the genus Xyloplax. So, where to place this enigmatic taxon within the Echinodermata?
Recently attempts have been made to clarify the morphology and examine the evolutionary history of Xyloplax using cladistic and phylogenetic analyses of DNA sequences.
A case has been made for the Concentricyloidea to be included in the class Asteroidea based on a form of cladistic analysis of rDNA from one species, X. turnerae. Despite regarding Xyloplax as "morphologically enigmatic," the analysis placed Xyloplax as a sister taxon of Rathbunaster, a morphologically completely unrelated genus of the starfish order Forcipulatida. The sequencing has not been subsequently replicated in X. turnerae, and no DNA has yet been successfully sequenced from X. medusiformis. Also, there is a question regarding the effect of initial preservation of some of the samples of X. turnerae in formalin, which usually precludes or confuses accurate DNA analysis. There is an urgent need to apply standard and repeatable phylogenetic analysis of rDNA control region sequences to samples from both X. turnerae and X. medusiformis, and to as wide a range of examples of the Asteroidea as possible, to further elucidate the systematic position of this morphologically highly divergent taxon.
If the Concentricycloidea is to be regarded as a taxon at some level within the Asteroidea, then a re-defining of that class to accommodate the two Xyloplax species is essential. This would also require a revision of the higher systematics of the somasteroids and ophiuroids. Until more fresh material of Xyloplax is collected and examined both morphologically and genetically, and comparisons are made with a much larger number of taxa, the class Concentricycloidea must remain distinct to accommodate the extraordinary unique morphology of the two species of Xyloplax.
The class Concentricycloidea contains one order, Peripodida, and one family, Xyloplacidae.
Physical characteristics
Sea daisies have a small, discoidal, radially symmetrical echinoderm. They are circular or slightly sub-pentagonal in outline, without arms, and weakly inflated. A sea daisy has a delicately plated dorsal surface which delimits the disc-like body with a single peripheral series of flattish, oar blade-like spines. The ventral surface is dominated by a skeletal mouth frame in the form of a ring of ossicles that supports either a thin velum or a peristome opening into a shallow stomach. There are 10 large gonads, and a single peripheral ring of tube feet.
They range in size from 0.03 to 0.54 in (0.9–13.6 mm) in diameter, including the marginal spines. They are translucent, with underlying skeletal structure and gonads visible.
Sea daisies have an outer water vascular ring system with lateral canals, ampullae and podia, not arranged in five radiating arms. An inner ring was originally thought to be also part of the water vascular system as it contained coelomocytes similar to those in the water vascular system but it may be part of the haemal system; however, this is not yet proven. The reproductive system is unique amongst echinoderms. The sexes are separate, and the gonoducts of each pair of 10 gonads are fused to form a single, interradial duct leading to a gonopore that opens in the females at the body margin, and in the males, slightly beyond the margin as the penial projection, which is most likely used for copulation. The highly filiform nature of the spermatozooa also suggests that these extremely modified cells do not contact the external environment, and that copulation takes place.
Distribution
Sea daisies are found only off New Zealand and the Bahamas.
Habitat
Sea daisies are found only on sunken wood below 3,280 ft (1,000 m).
Behavior
Not much is known. But in order to move to new habitats, the sea daisies may move by pulsating medusoid actions of the velum or stomach, or drift "parachute-like" in the demersal plankton.
Feeding ecology and diet
Not much is known about the diet of sea daisies, but it is suspected to be composed of bacteria and dissolved organic material on sunken wood, and detritus, and possibly micro-mollusks.
In the case of X. medusiformis, which lacks a stomach, direct absorption must occur through the velum and general body tissues. Shell fragments found in X. turnerae, which possesses a primitive stomach, may indicate utilization of whole food.
Reproductive biology
The sexes are separate in Concentricycloids, and the disc-like body of the species, and the radial arrangement of gonopores (female) and penial projections (male), makes it mechanically possible, although somewhat unlikely, for one female to be serviced by five males, or alternatively, five females could surround one male, or an alternate male/female chain of reproductively active individuals might occur.
Oocytes are ovoid and become increasingly yolk-filled with growth. The largest develop towards the blind end of the gonad, and measure up to 0.00709 in (180 µm) in length.
Spermatozoa are streamlined, elongate cells over 0.00106 in (27 µm) long and composed of a finely tapered segmented astrodome, tail attachment area, free flagellum, nucleus, and a single, elongate mitochondrion posterior to the nucleus.
The two Xyloplax species are dioecious and sexually dimorphic: males are smaller than females. Different reproductive strategies are adopted by each species: X. medusiformis is an intra-ovarian, non-placental, viviparous species, and X. turnerae is presumed to be capable of depositing fertilized eggs externally. In X. medusiformis, reproduction is asynchronous between the 10 gonads—not all gonads show equal development of oocytes. The developing juveniles are clearly recognizable as cone-shaped or flattened individuals with developing tube feet, a well-developed ectoneural nerve, and a vestigial gut (which is lost in immediate prenatal specimens).
No larval stages have been observed, but it is likely that they exist in X. turnerae, as developmental stages have not been observed in the ovaries of that species.
Conservation status
Xyloplax medusiformis is known only from nine specimens taken off the coast of New Zealand. It is therefore thought to be endemic to that region. Xyloplax turnerae is known from several hundred specimens collected off the Bahamas; it may well be endemic to the tropical west Atlantic Ocean. Neither species is known to be threatened.
Significance to humans
None known.
Species accounts
Medusiform sea daisyTurner's sea daisy
Resources
Books:Janies, D., and R. Mooi. "Xyloplax Is an Asteroid." In Echinoderm Research, edited by M. Candia Carniveli and F. Bonasoro. Rotterdam, The Netherlands: A.A. Balkema, 1998.
Pearse, V. B., and J. S. Pearse. "Echinoderm Phylogeny and the Place of Concentricycloids." In Echinoderms Through Time, edited by A. Guille, J. P. Feral, and M. Roux. Rotterdam, The Netherlands: A. A. Balkema, 1994.
Smith, A. B. "To Group or Not to Group: The Taxonomic Position of Xyloplax. In Echinoderm Biology, edited by R. D. Burke, P. V. Mladenov, P. Lambert, and R. L. Parsely. Rotterdam, The Netherlands: A. A. Balkema, 1998.
Periodicals:Baker, A. N., F. W. E. Rowe, and H. E. S. Clark. "A New Class of Echinodermata from New Zealand." Nature 321, no. 6073 (1986): 862–864.
Healy, J. M., F. W. E. Rowe, and D. T. Anderson. "Spermatozoa and Spermiogenesis in Xyloplax (Class Concentricycloidea): A New Type of Spermatozoon in the Echinodermata." Zoologica Scripta 17, no. 3 (1988): 297–310.
Rowe, F. W. E., A. N. Baker, and H. E. S. Clark. "The Morphology, Development and Taxonomic Status of Xyloplax Baker, Rowe, and Clark (1986) (Echinodermata: Concentricycloidea), with Description of a New Species." Proceedings of the Royal Society of London B 233 (1988): 431–459.
[Article by: A. N. Baker, PhD]
