Ophiura sarsii

Thuy, Ben & Stöh, Sabine, 2011, Lateral arm plate morphology in brittle stars (Echinodermata: Ophiuroidea): new perspectives for ophiuroid micropalaeontology and classification, Zootaxa 3013, pp. 1-47 : 12-15

Ophiura sarsii , based on proximal and median to distal LAPs of ten specimens of different growth stages (disk diameters ranging from 2.46 mm to 12.48 mm) ( Fig. 6 : 3-8):

Overall shape of LAPs: distal LAPs smaller and of more compact aspect in small specimens; height:width ratio of proximal LAPs increases from 1 in smallest examined specimen to 1.6 in largest specimen. Median to distal LAPs in smaller specimens clearly elongate; nearly as high as wide in larger specimens. Proximal edge with conspicuous kink in proximal LAPs of all examined growth stages. Ventral portion of LAP protruding distalwards in proximal LAPs from all specimens; position of protruding part varying from the most ventral in the smallest specimen to nearly central in the largest one.

Ornamentation: tubercles covering almost entire outer surface, largest near spine articulations, in central part of outer surface merged into vertical striae; size difference between tubercles within a row and vertical striation weakly developed in LAPs of smallest examined specimen. Diffuse, poorly prominent and not protruding spur near ventro-proximal edge of proximal and median LAPs in largest specimens and proximal LAPs in medium-sized specimens; lacking in smallest specimen.

Spine articulations: three spine articulations sunken into dorsal portion of distal edge of LAP in all examined growth stages; absolute size of spine articulations increasing with specimen size. Spine articulations spread over three quarters of distal edge in proximal LAPs of smallest specimen, and limited to dorsal half in proximal LAPs of largest specimen.

Inner side of LAPs: bow-shaped, well defined slender ridge in the middle of inner side in proximal LAPs of all growth stages; size of ridge increases with specimen size; horizontally elongate knob in median to distal LAPs of smallest specimen developing into round knob in medium-sized specimens and into comma-shaped, vertically elongate ridge in larger specimens. Tentacle notch entirely on distal edge in LAPs of all examined growth stages; slightly more ventral in smallest specimen. Perforations not discernible.

Remarks: A simplified view of ontogenetic changes in LAP morphology might suggest similar patterns as those related to the position within the adult arm, considering that juvenile LAPs are less differentiated than adult ones. This, however, is only partly the case. In fact, while the spurs on the outer proximal edge in both Ophiacantha fraterna and Ophiura sarsii are lacking in the smaller specimens, the conspicuous vertical striation in O. fraterna is constantly developed in all examined growth stages. Similarly, the ridges and knobs on the inner side and the position, arrangement, relative size and shape of spine articulations show very little ontogenetic variation within the studied size range. Surprisingly, the diffuse oblique area of dense stereom on the ventral portion of the outer surface is a character developed in the juvenile LAPs of Ophiacantha fraterna which gradually fades with specimen size and completely lacks in the largest examined specimen.

Whereas in Ophiura sarsii , juvenile proximal LAPs share a certain resemblance with median to distal adult LAPs, the example of Ophiacantha fraterna clearly shows that this is not necessarily always the case. In contrast, juvenile proximal LAPs rather resemble downscaled and ventro-dorsally compacted versions of their adult proximal equivalents.

Figures

FIGURE 6. Lateral arm plates (LAPs) in external (a) and internal (b) views of one adult specimen of Ophiura sarsii from Toyama Bay, Japan, and three different growth stages of Ophiura sarsii from off Iceland. 1 – 2: proximal (1) and median (2) LAPs of adult specimen from Japan; 3 – 4: proximal (3) and median (4) LAPs of adult specimen from off Iceland, dd = 12,5 mm; 5 – 6: proximal (5) and median (6) LAPs of juvenile specimen from off Iceland, dd = 7,8 mm; 7 – 8: proximal (7) and median (8) LAPs of juvenile specimen from off Iceland, dd = 2,5 mm. dd: disc diameter.