Ophiacantha fraterna

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

Ophiacantha fraterna , based on proximal, median and distal LAPs of seven specimens of different growth stages (disk diameters ranging from 2.46 mm to 7.6 mm) ( Fig. 5 ):

Overall shape of LAPs: LAPs in smaller and thus assumedly younger specimens smaller than in larger ones; height:width ratio in proximal LAPs varies from 1 in smallest specimen to 1.4 in largest one; distal LAPs clearly elongate and proximal LAPs of rounded aspect in all examined growth stages; constriction slightly better developed in smaller specimens, in particular in distal LAPs.

Ornamentation: vertical striation developed in all LAPs of all examined growth stages; remarkably similar in development and areal extent, except in distal LAPs of the smallest specimen. Diffuse oblique area of dense stereom on the ventral portion of the outer surface clearly developed in all LAPs of smallest specimen; progressively fading in larger specimens. Spurs on outer proximal edge and corresponding area of dense stereom on inner distal edge in proximal LAPs developed in all studied growth stages.

Spine articulations: shape, position, arrangement and size pattern of spine articulations constant in all examined growth stages; absolute size of spine articulations increasing according to growth stage; number of spine articulations increasing from five to six in proximal LAPs and from four to five in distal ones from the smallest to the largest examined growth stages.

Inner side of LAPs: development of perforations, ridges and knobs on inner side of proximal LAPs remarkably constant in all examined growth stages; distal LAPs in smaller specimens with undifferentiated horizontal ridge on inner side. Tentacle notch constantly developed in all LAPs independent of growth stage.

Figures

FIGURE 5. Lateral arm plates (LAPs) in external (a) and internal (b) views of three different growth stages of Ophiacantha fraterna. 1 – 3: proximal (1), median (2) and distal (3) LAPs of adult specimen, dd = 7,6 mm; 4 – 6: proximal (4), median (5) and distal (6) LAPs of juvenile specimen, dd = 5,1 mm; 7 – 9: proximal (7), median (8) and distal (9) LAPs of juvenile specimen, dd = 3,8 mm. dd: disc diameter.