Amphiura princeps Koehler 1907

Amphiura princeps Koehler, 1907

(Fig. 9)

Type locality. Straits of Magellan, Chile.

Maximum size. dd up to 12 mm (Borges & Amaral 2005).

Material examined. 3 specimens (dd: 5.7– 6 mm) in subtidal. ZUEC OPH 2211, St. 71, 1 spm; ZUEC OPH 2277, St. XXVI, 1 spm; ZUEC OPH 2353, St. XXXIV, 1 spm.

Description. Disc: (dd: 6 mm) pentagonal, covered by numerous small and imbricated scales, approximately 22 between the central primary plate and the edge of the disc. Primary radial plates evident. Radial shields acute proximally, divergent, internally straight and externally slightly curved, and separated by irregular scales (Fig. 9A). Ventral interradius covered with scales smaller than the dorsal ones and strongly imbricated, mainly those near the bursal slits. Bursal slits narrow and long (Fig. 9B). Oral shields spearhead-shaped, distal edge lobed. Madreporite larger than other oral shields and with pores. Adoral shields triangular, broadened distally and united proximally. Two lateral oral papillae, buccal scale higher up on the jaw. A pair of broadened infradental papillae (Fig. 9C).

Arms: dorsal arm plates fan-shaped, twice as long as wide, convex distally and obtuse angle proximally, contiguous (Fig. 9D,F). Ventral arm plates pentagonal, as long as wide, proximal acute angle pointed and distal edge straight, contiguous (Fig. 9E,G). Two tentacle scales, the larger inserted on the ventral arm plate and the smaller on the lateral arm plate (Fig. 9E). Variable number of arm spines, at proximal segments two or three, up to six at 15th segment. At distal segments (20–24), the number of spines decreases. Ventral spines of the first arm segment may be curved hook-like with the pointed end towards the disc (Fig. 9C).

Lateral arm plates (Fig. 9H, I): general outline: ventral portion projecting ventro-proximalwards; ventro-distal tip not projecting ventralwards. Outer surface ornamentation: trabecular intersections protruding to form knobs approximately the same size as stereom pores. Outer proximal edge: surface lined by discernible band of different stereom structure, restricted to central part; without spurs; central part not protruding; surface without horizontal striation. Spine articulations: on same level as remaining outer surface, middle spine articulation larger; distance between spine articulations increasing dorsalwards. Lobes simply separated, equal-sized; lobes parallel, bent, and oriented nearly horizontal; stereom massive; sigmoidal fold absent. Inner side, ridges and knobs: two separate (rarely merged) central knobs; without additional dorsal structure on inner side; single large perforation on inner side.

Vertebrae: zygospondylous of universal type and keeled. Large groove on proximal side of vertebrae dorsally corresponding to distalwards projecting dorso-distal muscular fossae of distal side (Fig. 9J). Zygocondyles dorsalwards converging and zygosphene fused with pair of zygocondyles (Fig. 9K). Dorso-distal muscular fossae transformed distalwards almost projecting beyond zygocondyles (true keel) (Fig. 9L). Zygosphene not projecting beyond ventral edge of zygocondyles (Fig. 9M).

Taxonomic comments. The vertebrae possess a dorsal distal keel, but the dorsal projections and depressions, connected by dorsal accessory muscles, are not as evident as in Hemipholis cordifera and Ophiothrix angulata. The number of arm spines is variable ranging five to six (Bernasconi & D'Agostino 1977). Two or three arm spines were observed in proximal segments, but up to six at 15th segment. Therefore, this feature is regarded as variable and should not be used for species identification (Borges 2006). The six primary plates are often evident, but in some specimens only the centrodorsal is visible. A. princeps is similar to A. joubini, but A. joubini is an Antarctic species characterized by a pointed to spiniform distal oral papilla, oral shields that are broadly triangular to spearhead shaped, long narrow radial shields (four times as long as wide), an absence of disc plates on the ventral surface near the oral shield, and arm spines that can be elongated into a sharp hyaline (glassy) and/or bent to bifurcated tip near the disc (Brogger & O'Hara 2015). It is important to highlight that all records of A. joubini in Brazil should be reviewed (Tommasi 1970; Manso & Absalão 1988; Manso 1989; Absalão 1990; Manso 1993; Pires-Vanin et al. 1997; Capítoli & Monteiro 2000; Capítoli & Bemvenuti 2004; Netto et al. 2005; Oliveira et al. 2010; Pires-Vanin et al. 2014), due to the similarity between A. princeps and A. joubini as suggested by Brogger & O'Hara (2015).

Remarks. Its occurrence on southeastern and southern Brazilian coast is possibly due to colder water masses, e.g. South Atlantic Central Water (Tommasi 1970). It is a eurythermal species with a wide geographic and bathymetric distribution (Borges & Amaral 2005); it is dioecious with the male gonads easily distinguished in spawning season (Mortensen 1936). It lives on sand and muddy bottom (Alvarado & Solís-Marín 2013). A. princeps was collected from sand (medium sand) and rubble bottom with a dredge (2 spms) and van Veen grab (1 spm).

Distribution. Temperate South America (realm), Warm Temperate Southwestern Atlantic (province): Southeastern Brazil (Borges et al. 2002; Borges & Amaral 2005) and Uruguay – Buenos Aires Shelf (Lucchi 1985); Magellanic (province): North Patagonian Gulfs to Channels and Fjords of Southern Chile (Koehler 1907; Clark 1915; Mortensen 1936; Bernasconi 1965; Bernasconi & D'Agostino 1977).

From intertidal to 107 m depth (Alvarado & Solís-Marín 2013; Brogger & O'Hara 2015). The present study samples occurred at depths ranging from 20 to 21.5 m.

Selected references. Koehler (1907): p. 303–305, pl. 12(28–29); Clark (1915): p. 235; Mortensen (1936): p. 285–286, fig. 22, pl.7(10); Bernasconi (1965): p. 150, pl. 1(1), 2(1); Bernasconi & D'Agostino (1977): p. 85–87, pl. 6(1,2); Lucchi (1985): p. 122, fig. 2, 25–26; Brogger & O'Hara (2015): p. 435–436 [as Amphiura princeps]; Bernasconi & D'Agostino (1977): p. 80, pr.7, fig. 3,4; Borges et al. (2002): p. 56, fig. 33a,b; Borges & Amaral (2005): p. 264, fig. a–c; Alitto et al. (2016): p. 4, 5, fig. 3G,H [as Amphiura joubini].