Ophiocten gracilis

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 : 7-9

Ophiocten gracilis ( Figure 4 ); proximal and median LAPs of ten individuals of close to adult size (disk diameter between 5.31 mm and 9.7 mm, with an average of 7.81 mm):

Overall shape of LAPs: proximal ones higher than wide and of rectangular aspect; median ones as high as wide, of squarish outline; dorsal and ventral edges of LAPs in most cases parallel; kink between halves of proximal edge forming an obtuse angle; ventral third of LAP widest, with the ventro-distal edge of the LAP projecting distalwards beyond the row of spine articulations; in some median LAPs difference in width between ventral third and remaining plate very small; weak constriction in most proximal LAPs and, to a lesser extent, in median LAPs, reflected in very weakly concave dorsal edge.

Ornamentation: well developed vertical striation covering almost the entire outer surface; stripes mostly parallel and gently bent concentrically around the kink of the proximal edge of the LAP in most plates; in some cases, stripes becoming irregularly oblique in the ventral half of the outer surface; striation fainting dorsally and especially ventrally on most plates, rapidly developing into finely meshed stereom close to the proximal edge of the LAP; ornamentation very similar in both proximal and median LAPs but irregularities in striation not necessarily consistent, regular stripes in proximal LAP sometimes associated with irregular ones in the median LAP and vice versa. No spur discernible on the outer proximal and the inner distal edges of the LAP.

Spine articulations: sunken into dorsal half of distal edge of LAP and slightly covered by striated outer surface; constantly three spine articulations of approximately equal size in both proximal and median plates; ventral gap between spine articulations mostly larger than dorsal one; dorso-distal edge of LAP fragile and projecting beyond two most dorsal spine articulations; most ventral spine articulation nearly at same height as tentacle notch.

Tentacle notches well developed, approximately in the centre of the distal edge of the LAP close to most ventral spine articulation in all plates, and ventrally bordered by strongly protruding part of distal edge of LAP.

Inner side of LAP: two oblique, oval, well defined and prominent knobs of finely meshed stereom in proximal LAPs; little variation in size of knobs; very thin and poorly defined ridge connecting knobs in some cases, documenting the transition from a single united structure to two separate knobs; in median LAPs knobs similarly well defined and prominent but merged into a single sickle-shaped structure in most plates; sometimes knobs in median LAPs incipiently or almost completely separated.

Perforations on inner side of LAP: diffuse area of coarsely meshed stereom between tentacle notch and dorsal knob, with little variation in areal extent and development.

Remarks: The assessment of size-independent within-species variability of LAP morphology reveals that in both examined species, the general outline of the plate is rather variable but seems to follow recognisable patterns, e.g. proximal LAPs of Ophiacantha bidentata are always wider dorsally than ventrally, with a kink in the middle of the proximal edge. The general shape of a LAP, in combination with its robustness, therefore might serve as a basis for a first, rough distinction between isolated LAPs. In contrast, the ornamentation on the outer surface of the LAPs, irrespective of minor variations in its development, is relatively constant, which, in combination with its potentially high diversity, highlights it as a promising feature for species distinction. Similarly, the development of the ridges and knobs on the inner side of the LAP as well as the shape, position, arrangement and size pattern of spine articulations are remarkably constant, thus potentially serving as reliable distinctive characters. Interestingly, the number of spine articulations is variable in Ophiacantha bidentata while being absolutely constant in Ophiocten gracilis , suggesting that its use as a distinctive character is case-sensitive. Perforations on the inner side of the LAP are rather variable and thus probably not suitable for species distinction in most cases. However, as will be discussed in the section on variability between species, the type of perforation (single or several ones in a row) constitutes a potentially powerful character for higher-level classification. Interestingly, LAP size appears to be independent of disk diameter in O. bidentata . As can be seen on Figure 3 , there is positive but very weak correlation between height:width ratio and disc diameter in the studied specimens of O. bidentata . Apparently, individuals with same disk diameter can vary considerably in arm height, possibly as a result of arm regeneration. The overall shape and characteristics of the plates are not affected, but inferences of the size of an individual cannot be made based on the size of the LAPs.

Figures

FIGURE 3. Height: width ratio of proximal lateral arm plates (extracted from 10 th segment counted from first real arm segment incorporated into the disk) of Ophiacantha bidentata plotted against disc diameter.

FIGURE 4. Lateral arm plates (LAPs) in external (a) and internal (b) views of four adult, similarly sized specimens of Ophiocten gracilis. 1: proximal LAPs, dd = 9.7 mm; 2: median LAPs, same specimen as on 1; 3: proximal LAPs, dd = 8.1 mm; 4: median LAPs, same specimen as on 3; 5: proximal LAPs, dd = 9.3 mm; 6: median LAPs, same specimen as on 5; 7: proximal LAPs, dd = 8.6 mm; 8: median LAPs, same specimen as on 7. dd: disc diameter.