Actinopyga caerulea Samyn, Vanden Spiegel & Massin 2006

Very large species; living specimens up to 400 mm long and 140 mm wide mid-body; preserved specimens from 225 to 280 mm long and from 85 to 110 mm wide mid-body. Body loaf-shaped with slight ventral flattening (more or less cylindrical with some distal tapering). Colour in life bluish with patches of white devoid of tube feet at anterior and posterior ends and, discontinuously, along sides (Plate 1). Colour in type material in alcohol largely preserved, but faded to dull brown in specimen from Papua New Guinea. White patches remain clearly visible on all specimens. Body wall smooth, up to 14 mm thick. Mouth ventral, surrounded by 15–18 large, peltate, uniformly bluish-grey tentacles, in turn surrounded by a stout collar of bluish papillae, fused at their base. Anus terminal, guarded by five prominent, calcareous, teeth, each bearing numerous tubercles. Ventral tube feet stout, distributed unevenly, albeit somewhat concentrated in ambulacral areas. Dorsal “papillae” large, conical at base, near cylindrical at top; bluish at base, slightly lighter at top; scattered over ambulacral and interambulacral areas, though absent in white zones. Cuvierian organ absent. Single, club-shaped Polian vesicle, about one seventh of length of preserved animals. Stone canal and associated madreporite not observed in all the specimens studied. Gonad observed only in the specimen from Papua New Guinea. Calcareous ring huge, radial pieces about twice as large as interradial pieces (Figure 1A). Details of surface of calcareous ring obscured by thick layer of tissue.

This species is characteristic of somewhat deeper tropical waters; it has been observed from 12 to 45 m. The species is predominantly a detritus/deposit feeder on coral patches on the outer slope of coral reefs; it forages actively during the day.

Tentacles with rods only; base of tentacles with few, straight to slightly curved, smooth rods, 50–90 mm long (Figure 1B); tip of tentacles with similar but larger rods, up to 500 mm long (Figure 1C, D), occasionally distally branching (Figures 1C, 3A). Ventral body wall with rosettes of various forms, some elongated with endings swollen, others wider and more spiny, 15–65 mm long (Figures 1E, 3B). Dorsal body wall with small rosettes that have their endings swollen, 20–60 mm long (Figures 1F, 3C) and elongated rod-like spiny rosettes, 255–100 mm long (Figures 1G, 3D). The proportion of rosettes with swollen endings versus spiny rod-like rosettes as well as the size of the rosettes are highly variable within a single specimen, depending on site of bivium sampled. The same phenomenon occurs in specimens coming from different geographic localities: holotype from Comoros Islands with more spiny ossicles in dorsal body wall than the specimen from Papua New Guinea. Base of dorsal papillae with rosettes and rodlike rosettes, 25–65 mm long, as well as dichotomously branched spiny rods, 100–160 mm long (Figures 1J, 3E). Tip of dorsal papillae with spiny rods of various form; from simple to complex branching, 50–200 mm long (Figures 1H, 4A). Ventral tube feet with smooth rods, 25–40 mm long, spiny rods, 40–150 mm long, and stout spiny rods, 100–140 mm long, with perforated extremities (Figures 2A, 4B); terminal disc, up to 1,000 mm across, composed of several pieces; centrally several perforated plates with large holes (Figure 4C) surrounded by 10–12 perforated plates with smallest holes at periphery (Figure 4D). Cloaca with spiky rods, similar in shape as those from dorsal papillae, 50–100 mm long (Figures 2D, 4E). Longitudinal and cloacal retractor muscles with simple, smooth, occasionally branched rods, 35–55 mm long (Figures 2B, C, 4F, G). Gonad with spiny, branched rods, 160–250 mm long (Figure 2E).