Amphiura borealis, (G. O. SARS, 1871)

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 : 561-566

AMPHIURA BOREALIS (G.O. SARS, 1871)

( FIG. 11A–K )

This is a brooding species in which the embryos develop inside the bursae. The smallest free-living postlarvae found measure 0.7 mm dd, with five arm segments. The often high, circular dorsal disc is formed by transparent, irregularly rounded, overlapping scales of different sizes, with small round perforations ( Fig. 11A ). No primary rosette can be distinguished. The edges of the RSs are just visible above the arm. The DAPs are pentagonal, wider than long with a wide proximal angle and slightly convex distal edge. There are three arm spines of equal length, shorter than an arm segment. A BS is present at the oral plate edge, and infradental papillae (= MP2) on the DP, widely separated by the large pointed apical tooth; all papillae are rugose ( Fig. 11B ). The ASs are narrow, with concave inner edge, bearing a short conical spine next to the first TPo. The OS is triangular with convex distal edge. The VAPs are elongated pentagonal with obtuse proximal angle, straight distal edge and concave lateral edges, except for the first VAP which is trapezoidal with straight lateral edges. The ventral disc is formed by a large round scale distal to the smaller OS and smaller plates to its sides.

At 0.8 mm dd and 6–9 arm segments, the number of disc scales has increased ( Fig. 11C ). The infradental papillae and BS have grown in size ( Fig. 11D, E ).

At 1.1 mm dd and up to 12 arm segments, the number of dorsal and ventral disc scales has increased further, the RSs are indistinguishable among them, and the infradental papillae are closer together than before ( Fig. 11F, G ).

At 2.0 mm dd, the RSs have formed, pairs separated by an elongated scale except at their distal ends ( Fig. 11H ). The infradentals have moved onto the proximal ends of the oral plates and the tooth is block-like ( Fig. 11I ). The OS has a rounded shape, wider than long, with wide obtuse proximal angle, convex on all edges. The ASS has disappeared. The ventral disc is covered with naked skin between the round scales.

From about 2.2 mm dd, the middle arm spine is flattened and slightly wider than the other two, with serrated edges ( Fig. 11J ). This is the first indication of the often axe-shaped, flattened, second ventralmost arm spine that is characteristic of this species. The ventral inter-radial disc is now covered with skin and scattered small round scales with large round perforations not extending to the scale edge ( Fig. 11K ). Lateral MP have just begun to form at the oral plate. The infradentals are close together, but on several jaws a third tooth is placed between them, which can also be seen in larger animals ( Fig. 11K ). These specimens can be identified using keys to adults.

Adult characters (not figured) include four arm spines, the second ventralmost axe-shaped, and two lateral MP above the BS. The dorsal disc is covered by small, round, imbricating scales, the RS is almost semicircular, 2.5¥ as long as wide, length about onefifth of the dd, pairs contiguous only at their distal end, and separated by two narrow wedge-like plates and a proximal round plate. The DAP is wider than long, oval, with proximal edge more convex than distal edge. The occurrence of the axe-shaped spine varies between individuals, and arms and arm segments in the same individual. Often, the second ventralmost spine on the most proximal segments is only slightly flattened with serrated edges, while the fully axe-shaped spines are found further out on the arm.

Remarks: Postlarvae of A. borealis can be distinguished from other species by the lack of a primary rosette, which gives the dorsal disc a conspicuously different appearance with quite uniform scalation.

AMPHIURA CHIAJEI FORBES, 1843

( FIG. 4A–O )

The smallest individuals available have a dd of 0.4 mm and one arm segment. The dorsal disc is formed by the CPP and five RPPs, all of which are perforated by large round fenestrations which continue onto the plate margins ( Fig. 12A ). The arm segment bears a simple spine; the TP terminates in several short thorns and is perforated by longitudinal rows of holes. The large DP bears a small sharply pointed tooth, the BS is present, and the ASS sits close to the disc edge, pointing outwards ( Fig. 12B ). The paired ASs are contiguous along their proximal halves, running parallel with the arms, slightly curving around the first TPo. The OS is just beginning to form at the disc edge, and the M just visible from above with a pointed projection. The first VAP is longer than wide, with proximal angle, convex distal edge and slightly concave lateral edges.

At 0.6 mm dd and 3–4 arm segments, the dorsal disc is still formed by the primary rosette. The plate fenestrations are smaller than in the youngest animals, smallest in the centre of the CPP ( Fig. 12C ). The plate margins are almost imperforate. DAPs are present on the first three segments, rounded triangular, with convex distal edge, and plates on following segments are widely separated by the LAPs. There are two conical arm spines half as long as a segment. The OS has moved to the ventral side ( Fig. 12D ).

At 0.9 mm dd and about ten arm segments, the IR1 and IR2 are present, but it is unclear which is which ( Fig. 12E ). The RSs are wider than long and contiguous, their proximal edges under the RPP. There are now three erect arm spines, about as long as an arm segment. A pair of small, round infradental papillae has formed on the DP, separated by the triangular pointed tooth ( Fig. 12F ). The narrow ASs are strongly curving inwards with a concave proximal edge around the first TPo, their distal edge bordering the lateral proximal edge of the angle of the OS. The OS is teardrop-shaped, longer than wide. The short ASS sits at the proximal end of the concave part of the AS, pointing towards the first VAP. The VAPs of the second and following segments are of similar shape and size as the first VAP. Two short, wide, ventral disc scales have formed near the disc edge, distal of the OS. The bursal slits have begun to form at the disc edge.

At 1.4 mm dd, the k-plate is present, separating paired RSs at their proximal end, and several small additional plates have formed inter-radially ( Fig. 12G ). The infradental papillae have moved closer together, and the ASS is close to the mouth slit and has flattened with a blunt tip. The OS is now wider than long, with straight or slightly convex distal edge ( Fig. 12H ). The ventral disc is covered with small, overlapping plates. The bursal slits are clearly visible.

At 2.0 mm dd, numerous additional disc scales have formed, but the primary rosette is still clearly distinguishable ( Fig. 12I ). The semicircular RSs are separated proximally by the wedge-shaped k-plate. The DAPs are triangular with convex distal edge, slightly wider than long, with plates on following segments nearly touching. The infradentals have moved onto the proximal ends of the oral plates, the ASS is an oval papilla wider than long and the BS lies deeper in the mouth slit ( Fig. 12J ). The second TPo has moved closer to the mouth slit.

At 2.4 mm dd, additional small disc scales and wedge-shaped SIRs have formed ( Fig. 12M ). Two tentacle scales have formed along the arm, except on the most distal segment ( Fig. 12K ). The second TPo is now in the mouth slit and the ASS has grown to a wide scale ( Fig. 12L ). The BS has moved deeper on the oral plate and the infradentals are close together. Characters are now close to the adult state.

At 3 mm dd, the number of dorsal and ventral disc scales has increased further and the RSs are elongated ( Fig. 12N ). ASS and infradentals have grown ( Fig. 12O ).

Remarks: Postlarvae of A. chiajei can be distinguished from A. filiformis by the uniform, non-bordered structure of the dorsal disc scales. The narrow, imperforate plate margins are only noticeable after removal of the integument. The smallest postlarvae have larger plate fenestrations, which distinguish them from the almost solid disc plates of the smallest postlarvae of A. filiformis (see below), and a more pointed tooth. Additionally, the disc scales of A. chiajei are larger and fewer in number than in individuals of A. filiformis of similar size and the primary rosette remains distinguishable in all stages.

AMPHIURA FILIFORMIS (O.F. MÜLLER, 1776)

( FIG. 4A–L )

The smallest animals found have a dd of 0.35 mm and the arms consist of only the TP. The dorsal disc is formed by the CPP and five RPPs, all of which are almost imperforate with a pattern of small round granules on the surface, except for the plate margins, which are perforated with round fenestrations ( Fig. 13A ). The TP is tapering, perforated by irregularly distributed small holes, and bears two short and a longer terminal thorn. The ventral disc conforms with Hendler’s (1988) stage 2. There is a single rather large, flat, triangular, pointed tooth, a BS on the oral plate and the long ASS next to the large first TPo points outwards and is visible from above at the disc edge ( Fig. 13B ).

From about 0.5 mm dd, the number of arm segments varies. With two arm segments, the RSs are visible above the arm. With three arm segments, a rounded triangular DAP with convex distal edge is present on the second arm segment ( Fig. 13C ). The margins of the CPP and the RPPs are wider than before, multilayered, and higher than the centre of the plate. The second arm segment bears two spines on each side, shorter than a segment. A second VAP has formed, twice as long as wide, lateral edges strongly concave, proximal angle and concave distal edge ( Fig. 13D ). Inter-radially at the disc edge, the OSs are just visible, but the M is still not distinguishable. With seven arm segments, five segments have a DAP, the first one being as wide as the arm and twice as wide as long, the following decreasing in size.

At 0.6 mm dd, the animals correspond to Hendler’s (1988) stage 3, the IR1 is clearly visible below the RPPs, and the infradental papillae have appeared on the DP ( Fig. 13E ). The OS is arrowhead-shaped, with the angle between the narrow ASs ( Fig. 13F ). The bursal slits are beginning to form at the disc edge. The first VAP is long triangular with straight edges. The second and following VAPs are twice as wide as long with proximal angle, slightly convex distal edge and deeply concave lateral edges.

At about 0.8 mm dd, according to Hendler (1988), the ASS has migrated to the distal end of the jaw (distal oral plate in Hendler, 1988). In the present material, no animal of this size was found.

At 1 mm dd, there are three arm spines, the middle one of which is serrated along its edges and shows the first sign of the axe-shape that is characteristic of this species ( Fig. 13G ).

At 1.3 mm dd, the CPP and RPPs with their thicker borders are still clearly visible. Both IR1 and IR2 are present, the distal plate smaller than the proximal plate, suggesting that the IR2 formed below the IR1 ( Fig. 13H ). A small triangular k-plate separates the RSs at their proximal end. These additional disc plates are perforated all over with many round fenestrations in contrast to the primary plates, which are perforated in the centre and on the periphery, with an imperforate area in between. The infradentals have moved onto the proximal oral plates and they have grown in length ( Fig. 13I ). The second TPo has moved closer to the mouth slit. Several ventral disc scales have formed.

At 1.6 mm dd, the CPP and RPPs with their bordered structure are just barely distinguishable among the many additional overlapping disc scales ( Fig. 13J ). The ventral disc is partly covered with naked skin, embedded with scattered scales ( Fig. 13K ). The ASS has grown in length. The most widely used character to separate A. filiformis from A. chiajei is the naked ventral disc and its presence makes the animals identifiable using keys to adults.

In adults, the primary plates are no longer distinguishable ( Fig. 13L ).

Remarks: The postlarvae of A. filiformis are characterized by the distinct border on the primary plates, distinguishing them clearly from other species of Amphiura . Small specimens are often more transparent than the sympatric A. chiajei and the plates are slightly reflective.

FAMILY OPHIACTIDAE MATSUMOTO, 1915

Figures

Figure 4. Ophiacantha anomala postlarval development, stages taken from brooding animal are termed embryo, free-living stages postlarvae. A, B, 0.6 mm dd embryo dorsal; C, 0.9 mm dd embryo dorsal; D, 1.3 mm dd embryo dorsal; E, 1.3 mm embryo ventrolateral; F, 1.6 mm dd; G, 2 mm dd postlarva ventral; H, 3 mm dd juvenile jaw. Scale bars in millimetres.

Figure 11. Amphiura borealis postlarval development. A, B, 0.7 mm dd postlarvae; A, dorsal; B, ventral, note the infradental papillae (IP = MP2); C, D, E, 0.8 mm dd postlarvae; C, dorsal; D, ventral; E, jaw, note that the IP form on the dental plate (DP); F, G, 1.1 mm dd postlarvae; F, dorsal; G, ventral; H, I, 2 mm dd postlarva; H, dorsal, note the radial shields (RS); I, ventral, note the IP on the oral plates; J, K, 2.5 mm dd juvenile; J, arm ventral, note the hatchet-shaped spine (arrow); K, ventral. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres.

Figure 12. Amphiura chiajei postlarval development. A, B, 0.4 mm dd postlarvae; A, dorsal, note the large fenestrations of the primary plates; B, ventral; C, D, 0.6 mm dd postlarvae; C, dorsal; D, ventral; E, F, 0.9 mm dd postlarva, E, dorsal, note the inter-radial plates (IR); F, ventral, note the infradental papillae (IP = MP2) on the dental plate (DP); G, H, 1.4 mm dd postlarvae; G, dorsal, note the k-plate (k); H, ventral, the adoral shield spine (ASS) has transformed into a flat scale; I, J, 2.0 mm postlarvae; I, dorsal; J, ventral, IP have moved onto oral plates; K–M, 2.4 mm dd juvenile; K, arm ventral, note the tentacle scales (TS); L, ventral; M, dorsal; N, O, 3 mm dd juvenile; N, dorsal; O, ventral. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres.

Figure 13. Amphiura filiformis postlarval development. A, B, 0.35 mm dd postlarvae; A, dorsal, note the solid plate stereom; B, ventral; C, D, 0.5 mm dd postlarva; C, dorsal, note the multilayered plate margins; D, ventral; E, F, 0.6 mm dd postlarva; E, dorsal; F, ventral, note the infradental papillae (IP = MP1); G, 1 mm dd postlarva arm ventral, note the serrated middle arm spine; H, I, 1.3 mm dd postlarva; H, dorsal; I, ventral; J, K, 1.6 mm dd postlarva; J, dorsal; K, ventral; L, 5 mm dd adult. Abbreviations: as in Figs 1–3, 5. Scale bars in millimetres.