Ophiopleura borealis, DANIELSSEN & KOREN, 1877

Stöhr, Sabine, 2005, Who’s who among baby brittle stars (Echinodermata: Ophiuroidea): postmetamorphic development of some North Atlantic forms, Zoological Journal of the Linnean Society 143 (4), pp. 543-576 : 555-561

OPHIOPLEURA BOREALIS DANIELSSEN & KOREN, 1877

( FIG. 7A–N )

The smallest postlarva in the samples has a dd of 0.6 mm and two arm segments. The disc is strongly domed with a large pentamerous CPP, surrounded by five smaller rounded triangular RPP with distal angle. In the inter-radial spaces the bulging ASs are visible. The first free arm segment consists of two inflated LAPs, which meet in midline, bearing a single smooth ventral spine each. The terminal arm segment is flask-shaped with inflated base and narrow distal part, round fenestrations running along its length ( Fig. 7A ). The DP is twice as wide as long, bearing a pointed tooth. A low BS is just visible on the oral plate edge. The first VAP is twice as long as wide, with proximal angle, concave lateral sides and straight distal edge. The swollen first ASs of two adjacent arms meet below the jaw ( Fig. 7B ). The M is visible dorsally in an inter-radius, projecting outwards with the pore opening ventrally at the tip of the plate ( Fig. 7B ).

At 0.8 mm dd, the round CPP is more than twice as large as the RPPs. Inter-radial plates (IR0) have formed as smaller rounded triangular disc plates above the RPPs ( Fig. 7C ). There are now four arm segments, still consisting of only LPs without DAPs. The M is still dorsal in position.

At 1 mm dd, the disc is about as high as wide. Additional smaller almost rectangular plates have formed as secondary radial plates between the IR0, the edges of the IR1 are visible between and below the RPPs, and the second to fourth arm segments bear triangular dorsal plates, longer than wide with convex distal edge, each DAP widely separated from the following plate by the LAPs ( Fig. 9D ). Short triangular VAPs with convex distal edge have appeared beyond the first segment, widely separated from each other by the LAPs. The M has migrated ventrally and forms the largest OS, with the other four just visible at the disc edge ( Fig. 7E ).

At 1.2 mm dd, the RSs have begun to form beneath the RPPs ( Fig. 7F ). In addition to the BS on the oral plate there is a smaller MP at each lateral edge of the DP.

At 1.8 mm dd, IR2 have formed, and the RSs are now large and oval, pairs forming a wide angle, proximally separated by the RPP, distally by the first DAP, contiguous only at a narrow point ( Fig. 7G ). MP and tooth are as before, with ASs comma-shaped and OS teardrop-shaped ( Fig. 7H ). A ventral inter-radial disc plate, almost rectangular, that is wider than long, has formed distal to the OS, framed by narrow genital plates to each side. The second TPo lies outside the mouth slit and is surrounded by 3–4 small block-like scales. The following pores are scale-less.

At 2.6 mm dd, the dorsal disc scaling becomes increasingly complex; the k-plate has formed distal of the RPP. The primary plates are still distinguishable, but now widely separated by additional plates ( Fig. 7I ). The second TPo has moved close to the mouth slit and is now surrounded by 4–5 small scales. The third TPo bears a small papilla-like scale, whereas the following segments are still without TS. Bursal slits are present to either side of the first arm segment, bordered by a genital plate extending from the oral shield to the disc edge. Three additional ventral disc scales have formed at the disc edge. The second and following VAPs are wider than long, hexagonal with proximal and distal angles ( Fig. 7J ).

At 5 mm dd, the animals exhibit adult characters. The CPP is still distinct, but the other primary plates can no longer be distinguished among the many additional scales that have formed. The DAPs are wider than long with convex distal edge and contiguous on proximal segments, becoming more elongated distally. The skin thickens increasingly, obscuring the plates ( Fig. 7K ). There are three short arm spines and proximally two small round TSs, decreasing distally in number ( Fig. 7M ). The DP bears a large pointed tooth and a small round lateral papilla to each side. Four smaller block-like lateral papillae of about equal size and shape have replaced the BS. The first TPo opens into the mouth slit. The large OS is teardrop-shaped and separated from the first LAPs by the narrow ASs. The ventral disc is covered by small, round, overlapping scales. The genital plate bears a row of small, round papillae. The genital slit does not fully extend to the disc edge ( Fig. 7L ). There are 1–2 small, round TSs on the proximal segments and three short adpressed arm spines ( Fig. 7M ). The VAP is about twice as wide as long, with convex distal edge, separated from the following VAP by the large LAPs ( Fig. 7N ).

Remarks: The postlarvae of O. borealis are remarkably different from their adult conspecifics. They are, however, easily recognized among other species. The order of appearance of the dorsal plates differs from the general pattern as presented by Ludwig (1899) and later authors, who described the IR2 as forming above the IR1. In O. borealis the IR2 forms at the disc edge below the IR1 and there is an early set of plates, here termed IR0, which have not been found in other species and are probably not homologous with the IR1. In addition, secondary radial plates form prior to the IR1 and together with the IR0 they form a circle above the RPPs, separating them from the CPP quite early in development.

OPHIURA ALBIDA FORBES, 1839

( FIG. 8A–E )

The smallest postlarva previously described had a dd of 0.6 mm ( Sumida et al., 1998). The BIOIce material includes individuals of 0.4 mm dd and three arm segments ( Fig. 8A ). The dorsal disc is formed by the pentagonal CPP and five RPPs. The CPP has larger round fenestrations in the centre and smaller further out, where they are overlaid by a second layer, while the plate margin is almost imperforate. In some individuals, the CPP is completely multilayered, with smaller perforations all over. There are two short appressed arm spines. The TP is slightly bulbous, hollow and terminating in several short thorns. The oval DP bears a pointed tooth and the oral plate bears a wide, blocklike BS ( Fig. 8B ). The ASS sits on the distal end of the AS near the disc edge, pointing outwards and visible from above. The first VAP is slightly longer than wide, with proximal angle, straight lateral edges and convex distal edge. The second VAP is not even half the size of the first, triangular, wider than long, and with convex distal edge.

At 0.5 mm dd, and four arm segments, the edges of the RS are just visible above the arm and the IR1 has begun to form ( Fig. 8C ). The first DAP is rounded triangular, wider than long. A small OS can be seen at the disc edge between the distal parts of the ASs ( Fig. 8D ).

At 0.7 mm dd, the IR1 has formed ( Fig. 8E ). Larger individuals have been described in previous works. From about 3 mm dd, characters are close to the adult state.

Remarks: Ophiura albida postlarvae are quite similar to O. robusta , from which they can be distinguished by the stereom structure of the primary plates and the shapes of the TP, DAP and first VAP as discussed under that species.

OPHIURA OPHIURA (L., 1758)

( FIG. 9A–H )

A detailed description of the development of this species was given by Webb & Tyler (1985). The smallest individuals in the BIOIce material have a dd of 0.3 mm, with a flat disc, and the arm consists of only the TP ( Fig. 9A ). The jaw bears a sharply pointed tooth and the ASS points outward ( Fig. 9B ).

At 0.5 mm and two arm segments, the dorsal disc is formed by the strongly domed pentagonal CPP and five RPPs, all of which have a complicated structure of fenestrations and an almost imperforate border ( Fig. 9C ). The M is clearly visible in a dorsal interradius. The LAPs are swollen, giving the animal a robust appearance ( Fig. 9D ). A second VAP has formed.

At 0.7 mm dd and four arm segments, the first DAP has formed ( Fig. 9E ). The proximal three arm segments bear small VAPs ( Fig. 9F ).

At 1.8 mm dd, the strongly domed CPP is the most distinguishing character ( Fig. 9G ). The CPP retains its domed shape until the animals are about 4 mm in dd ( Fig. 9H ). From about 3 mm dd, the animals can be identified using keys to adults, bearing in mind that the characteristic pores separating the proximal VAPs are not distinct in small specimens.

Remarks: The Icelandic individuals of O. ophiura are characterized by a high disc in smaller postlarvae (but not the smallest) and a strongly domed CPP in larger animals, which is more prominent than in individuals from other areas. The LAPs are more swollen and bulbous than in previously described individuals ( Webb & Tyler, 1985). These distinct features of O. ophiura postlarvae make them easily distinguishable from other sympatric species of Ophiura .

OPHIURA ROBUSTA (AYRES, 1851)

( FIG. 10A–L )

The smallest individuals have a dd of 0.45 mm and three arm segments, the distalmost just beginning to form. The dorsal disc is formed by the pentagonal CPP and five RPPs ( Fig. 10A ). The CPP is perforated by larger round fenestrations in the centre and smaller ones peripherally. The first DAP is triangular, about as wide as long, with convex distal edge. There are two short adpressed arm spines. The TP is about as long as an arm segment, terminating in several thorns. The DP is twice as wide as long and bears a triangular pointed tooth ( Fig. 10B ). The oral plate bears a low wide BS to each side. The OSs can be seen at the disc edge between the distal ends of the ASs, the M among them distinguishable from below by its large hydropore. The ASS is conical, pointing outwards with its tips just visible inter-radially from above. The first VAP is about twice as long as wide, with a spear-head shape and concave lateral edges.

At 0.55 mm dd and still three arm segments, the RSs are just visible above the arm, below the edge of the RPP ( Fig. 10C ). The first DAP is longer than wide. The DP is slightly smaller, still not bearing a papilla ( Fig. 10D ).

At 0.85 mm dd, the IR1 has formed at the disc edge and the IR2 is a small triangle above the IR1, separating the RPPs at their distalmost end ( Fig. 10E ).

At 1 mm dd, the RSs are beginning to become separated distally by the proximal angle of the first DAP ( Fig. 10F ). The second and following DAPs are longer than wide, teardrop-shaped with convex distal edge and proximal angle. A lateral MP can be seen at the DP ( Fig. 10G ). Three ventral disc scales have formed distal of the OS. The ASS has decreased in size. The hydropore on the M is off-centre.

At 1.3 mm dd, the k-plate is present, separating the RSs proximally ( Fig. 10H ). An additional MP has formed at the proximal end of the oral plate next to the BS ( Fig. 10I ). At the first TPo there are two small oval TSs on the AS and a third on the first VAP; the following TPos bear a single oval TS. The ASs border the proximal angle of the OS, separating it from the first VAP. The OSs are about as wide as long with strongly convex distal edge. The hydropore has moved further to one side.

At 1.8 mm dd, many additional disc scales have formed, among them the SIRs below the CPP. The first DAP has begun to attain a heart-shape ( Fig. 10J ). The DAPs of succeeding segments are now contiguous. The OS is wider than long, there are three smaller round MP, and the BS is twice as wide as long. The second TPo lies outside the mouth slit, bordered by five small rounded scales. Following TPos bear a single oval scale each ( Fig. 10K ).

At 2.9 mm dd, the first DAP has attained its final heart-shape ( Fig. 10L ), separating the RSs distally, while the k-plate separates them proximally. The arm comb papillae can be seen under the distal edge of the RS. There are three arm spines, the dorsalmost almost twice as large as the other two. Characters are now close to their adult state.

At 5 mm dd (not figured), the animals show fully adult characters. The CPP is still distinct, while the RPPs are no longer distinguishable. The DAPs are wider than long, overlapping so that their proximal angle is no longer visible. There are three lateral MP and a single tooth. The first TPo is surrounded by five TSs, the second TPo has two TSs, and all others only one. The OS is wider than long with straight distal edge. The VAPs are wider than long, with straight distal edge; following plates are separate.

Remarks: The smallest postlarvae of O. robusta are difficult to distinguish from O. albida . In mixed samples, the more slender habitus and the different structure of the primary plate stereom distinguish O. robusta . It is also somewhat further developed at smaller sizes with more plates present. The TP of O. robusta is longer and not as bulbous as in O. albida and the first VAP is longer and of a different shape. From just under 2 mm dd, the heart-shape of the first DAP becomes obvious. Although O. albida also may develop a heart-shaped first DAP, it is not as distinct as in O. robusta . Both species can be distinguished from Ophiura sarsii Lütken, 1858 by their distinctly smaller CPP.

FAMILY AMPHIURIDAE LJUNGMAN, 1867

Figures

Figure 7. Ophiopleura borealis postlarval development. A, B, 0.6 mm dd postlarva; A, dorsal, note the high disc, swollen lateral arm plates and dorsal madreporite; B, ventral, note ventral hydropore at madreporite tip (between ASs); C, 0.8 mm dd postlarva dorsal, secondary inter-radial plates have formed above the radial primary plates (IR0); D, E, 1 mm dd postlarva; D, dorsal, secondary radial plates (SRP) have formed above the radial primary plates; E, ventral, the madreporite is now ventral; F, 1.2 mm dd postlarva dorsal; G, H, 1.8 mm dd postlarva; G, dorsal, secondary inter-radial plates (IR2) have formed below the first inter-radial plates (IR1); H, ventral, a second mouth papilla has formed on the dental plate; I, J, 2.6 mm dd juvenile; I, dorsal, the k-plate separates the radial shields proximally; J, ventral; K–L, 5 mm dd juvenile; K, dorsal, note the thick skin on the disc only partly removed; L, ventral; M, arm lateral, note two widely spaced short arm spines; N, arm ventral. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres.

Figure 8. Ophiura albida postlarval development. A, B, 0.4 mm dd postlarva; A, dorsal, note the multilayered structure of the disc plates; B, ventral; C, D, 0.5 mm dd postlarva; C, dorsal; D, ventral, note the shape of the first ventral arm plate (VAP1); E, 0.7 mm dd postlarva. Abbreviations: as in Figs 1–2, 5. Scale bars in millimetres.

Figure 9. Ophiura ophiura postlarval development. A, B, 0.3 mm dd postlarva; A, dorsal, note the flat disc; B, ventral; C, D, 0.5 mm dd postlarva; C, dorsal, note the high disc, madreporite still dorsal; D, ventral; E, F, 0.7 mm dd postlarva; E, dorsal; F, ventral; G, 1.8 mm dd postlarva dorsal, note the domed central plate (CPP); H, 4 mm dd juvenile dorsal, the central plate is only slightly domed. Abbreviations: as in Figs 1, 3. Scale bars in millimetres.

Figure 10. Ophiura robusta postlarval development. A, B, 0.45 mm dd postlarvae; A, dorsal, note the structure of the plates; B, ventral, note the shape of the first ventral arm plate (VAP1); C, D, 0.55 mm dd postlarva; C, dorsal; D, ventral; E, 0.85 mm dd postlarva dorsal; F, G, 1 mm dd postlarvae; F, dorsal; G, ventral; H, I, 1.3 mm dd postlarvae; H, dorsal; I, ventral; J, K, 1.8 mm postlarvae; J, dorsal, note the heart-shaped first dorsal arm plate (DAP1); K, ventral; L, 2.9 mm dd juvenile dorsal, arm combs have formed and the DAP1 has its final heart-shape. Abbreviations: H, hydropore; k, k-plate; SIR, secondary inter-radial plate; others as in Figs 1–3, 5. Scale bars in millimetres.