Who’s who among baby brittle stars (Echinodermata: Ophiuroidea): postmetamorphic development of some North Atlantic forms Author Stöhr, Sabine text Zoological Journal of the Linnean Society 2005 2005-04-30 143 4 543 576 https://academic.oup.com/zoolinnean/article-lookup/doi/10.1111/j.1096-3642.2005.00155.x journal article 10.1111/j.1096-3642.2005.00155.x 0024-4082 5432948 HISTAMPICA DUPLICATA (LYMAN, 1875) ( FIG. 14A–D ) The smallest individuals found measure 1.5 mm dd and the dorsal disc is formed by the primary rosette, wide RSs, secondary radial plates, the k-plate and several IRs ( Fig. 14A ). All disc scales are evenly perforated by small round fenestrations; the primary plates have a wide, thickened, perforated border and a perforated centre, but an imperforate area in between. Triangular DAPs, about as wide as long, separated from the following plate, are present on all arm segments. There are three conical arm spines, the dorsalmost of which is as long as a segment. The oral plate bears a wide BS, and a flat, triangular, pointed tooth arises from the short DP ( Fig. 14B ). The ASs are distally flaring, separating the teardrop-shaped OS from the LAP. A conical ASS is present near the second TPo, some distance from the oral slit. At 2.9 mm dd, the animals possess adult characters with numerous disc scales. The tooth is block-like, wider than long, the DP bears a small round oral papilla followed by two larger round papillae on the oral plate and a third similar one on the AS , probably homologous to the ASS ( Fig. 14C ). Remarks : Although few and only relatively large juveniles were found, the great similarity to A. filiformis is obvious. The primary plates have a similar bordered structure and the tooth of the small postlarvae is also quite similar in both species. In the BIOIce material both species have not yet been found in sympatry, with H. duplicata mainly occurring below 1000 m depth and A. filiformis being restricted to the upper 300 m (my unpubl. data). Off the Swedish west coast, H. duplicata does not occur, which eliminates the possibility of error in the identification of A. filiformis postlarvae from that area. OPHIOPUS ARCTICUS LJUNGMAN, 1867 ( FIG. 15A–H ) The smallest individual found measures 0.55 mm dd, with a single arm segment and the TP (not figured). The dorsal disc is domed, formed by the evenly perforated plates of the primary rosette, M protruding in an inter-radius. The tooth is minute, and a low BS is present on the oral plate. The ASS is flat triangular, pairs pointing towards each other across the first VAP. Figure 14. Histampica duplicata postlarval development. A, B, 1.5 mm dd postlarva; A, dorsal, note the thicker plate margins; B, ventral; C, D, 2.9 mm dd juvenile; C, dorsal; D, ventral. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres. At 0.7 mm dd, the postlarvae have two arm segments, each bearing two spines on either side ( Fig. 15A ). The CPP is distinctly pentagonal, overlapping the rounded pentagonal RPPs. The TP is strong, hollow, tapering and ending in short thorns. At the disc edge, between the ASs, the OSs can be seen, the M with a protruding cone ending in the hydropore. The tooth is wide and flat, triangular. The oral plate bears a wide low BS ( Fig. 15B ). The ASS is wide and flat triangular, pairs pointing towards each other, not visible from above. The TPo on the arm bears a conical pointed scale. The first VAP is elongated pentagonal, with proximal angle, concave lateral edges and straight distal edge. At 0.9 mm dd and three arm segments, the RSs are just visible above the arm ( Fig. 15C ). The DAP on the first two segments is rounded triangular, longer than wide with slightly convex distal edge. The OSs are triangular, wider than long, still close to the disc edge, separated from the first VAP by the ASs ( Fig. 15D ). The base of the flat ASS is almost as wide as the spine is long. The second and following VAPs are of similar size and shape as the first VAP. At 1.3 mm dd, the IR1 have formed below the RPPs and the IR2 at the disc edge, and the k-plate is present ( Fig. 15E ). Additional inter-radial scales are forming next to the IR1. The MP2 has formed on the DP ( Fig. 15F ). At 1.4 mm dd, triangular SIRs have formed at the edge of the now round CPP ( Fig. 15G ). The first DAP is smaller than the following plates, its proximal part covered by the RSs. There are three conical arm spines, about half a segment long. A small round MP3 has formed on the oral plate proximal to the BS ( Fig. 15H ). The MP on the DP have moved closer together. The ASS has transformed into a round scale close to the mouth slit. The ventral disc is formed by small round overlapping plates. The OS is teardropshaped, still separated from the arm by the long narrow ASs. There is no bursal slit. Each TPo bears a single round scale. From about 2 mm dd, keys to adults can be used. Figure 15. Ophiopus arcticus postlarval development. A, B, 0.7 mm dd postlarva; A, dorsal, note the projecting madreporite; B, ventral, note the wide flat tooth (T); C, D, 0.9 mm dd postlarva; C, dorsal; D, ventral (a stray arm spine lies on a jaw); E, F, 1.3 mm dd postlarva; E, dorsal, note k-plate (k) and first inter-radials (IR1); F, ventral, note the second mouth papillae (MP2) on the dental plate; G, H, 1.4 mm dd postlarvae; G, dorsal, note the secondary inter-radial plates (SIR); H, ventral. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres. In adults (not figured), the ASS has grown into a large round scale and becomes part of the row of MP, and the BS forms a leaf-like pointed papilla. Numerous small scales cover the dorsal disc; the CPP is much larger. Remarks : The small postlarvae of O. arcticus have a compact appearance with distinctly straight plate edges. The order of appearance of the IR1 and IR2 is opposite to that in Ophiura , but similar to Ophiopleura . The shape of the tooth is similar to small A. filiformis . FAMILY OPHIOTHRICIDAE LJUNGMAN, 1867