Phormosoma placenta Thomson, 1872

Gondim, Anne Isabelley, Moura, Rafael Bendayan De, Christoffersen, Martin Lindsey & Dias, Thelma Lúcia Pereira, 2018, Taxonomic guide and historical review of echinoids (Echinodermata: Echinoidea) from northeastern Brazil, Zootaxa 4529 (1), pp. 1-72 : 12-13

Phormosoma placenta Thomson, 1872

Figure 4 A–G

Phormosoma placenta Thomson, 1872: 494 ; 1874: 732–737, pl. 62, figs 1–6 and pl. 63, figs 1–8.― Tommasi, 1972: 25.― Ventura et al., 2007a: 250, figs a–b.― Campos & Moura, 2008: 136 (table), 139.― Campos et al., 2010: 237, 288. Phormosoma sigsbei A. Agassiz, 1880: 75 .

Material examined. 2 spms, 15°18,873′S 38°41,685′W, Oceanographic vessel Thalassa, 11.VI.2000 [EqMN2341].

Description. Test thin, flexible, contour circular (TD = 67.4 mm). Preserved test becomes totally flattened ( Fig. 4A, C ). Apical system monocyclic, with plating often reduced and genital pores surrounded by number of small platelets ( Fig. 4B ). Periproct large ( Fig. 4A, B ). Interambulacra about two-thirds wider than ambulacra at ambitus ( Fig. 4A ). Primary spines long, thin, straight, and hollow. Aboral surface with some fleshy spines (live specimens). Secondary spines thin and elongate (like needles). Spines from oral surface similar to those from aboral surface, but shorter and with rounded tips. Ambulacral plate trigeminate, with large central element bearing one or two primary tubercles and pair of ambulacral pores, and two demiplates housing one pore-pair each ( Fig. 4E ). Interambulacral plates wide, with one to three primary tubercles forming subequal rows ( Fig. 4F ). Primary tubercles perforated and non-crenulate ( Fig. 4E, F ). Aboral tubercles irregularly arranged. Oral surface flat, covered by numerous and large tubercles with sunken areoles ( Fig. 4C ). Peristome large, nearly circular and densely covered by overlapped plates ( Fig. 4C ).

Pedicellariae. Only tridentate ( Fig. 4G ) and ophicephalous pedicellariae are found in P. placenta . The first type occurs in smaller numbers than the second type. Both are very numerous and distributed irregularly over entire test, being particularly abundant on the oral surface towards the edge of the peristome.

Colour. In living specimens, the aboral surface is gray with purple spots and oral surface purplish ( Bell 1892; Mortensen 1977). The tube feet are purple, and the spines are nearly colourless ( Thomson 1874). Specimens become brownish-yellowish in alcohol ( Borrero-Pérez et al. 2012).

Distribution. Bahamas, North and South Carolina, Virginia, Florida, Gulf of Mexico, Mexico, Honduras, Dominica, Bermudas, Jamaica, Cuba, Haiti, Nicaragua, Puerto Rico, Panama, Colombia, Venezuela, and Brazil ( Alvarado 2011; Solís-Marín et al. 2013). In Brazil from BA, ES and RJ ( Ventura et al. 2007a; Campos et al. 2010). From depths of 38 to 4100 m ( Mironov 2014; Smithsonian Database), but it is very rare at depths shallower than 500 m ( Serafy & Fell 1985).

Remarks. Phormosoma contains four species, of which only P. placenta occurs in the Atlantic Ocean. Three subspecies are recognized for P. placenta : P. placenta placenta Thomson, 1872 a typical form in the North and East Atlantic, P. placenta sigsbei A. Agassiz, 1880 in the West Atlantic, and P. placenta africana Mortensen, 1934 in South Africa ( Mironov 2014). These subspecies are distinguished mainly on the basis of the number of coronal plates and in the size and disposition of the tubercles on aboral surface. Thomson (1872) published an abstract in which he established the genus Phormosoma and the species P. placenta . However, in this work, the author provided only a short description of the genus and the locality of the material used to designate the species. Thomson (1874) presented a detailed description and provided several illustrations of the taxonomic characters of the species. One of the most conspicuous characters in live specimens of Phomosoma is the presence of aboral fleshy spines, which may partially or totally obscure the dorsal surface of the test or form a “fence” along the periphery of the urchin. According to Emson & Young (1998), virtually all the primary spines of P. placenta are to some extent fleshy, as all the oral and lateral spines have a small fleshy enlargement near the tip and the blunt oral walking spines are enlarged by a thick fleshy covering. The main function of these spines most probably is defence ( Emson & Young 1998).

Ecological notes. Individuals of P. placenta seem to aggregate and this pattern was attributed to their slowmoving deposit-feeding mode of life ( Grassle et al. 1975). Pawson (1982) found only mucus-consolidated mud balls in the guts of this species. Tyler & Gage (1984) believe that the species must have a relatively rapid growth rate (up to a TD of 40 to 45 mm). At this point, gonad development begins and there is a slowing of somatic growth, resulting in an accumulation of adults in the 55 to 75 mm TD range. Above 75 mm TD, the population numbers decline, and generally few individuals larger than 85 mm TD are found. Tyler & Gage (1984) recorded specimens with 45 mm TD possessing developing gonads, while specimens between 47 and 55 mm TD showed no evidence of reproductive development. According to Pearse (1969), the lecithotrophic larvae of P. placenta are not demersal and develop at or close to the surface. Young & Cameron (1987) conducted a study on flotation rates of lecithotrophic eggs from P. placenta and pointed out the enhanced physiological tolerance in embryos of deepwater echinothuriids relative to larvae of shelf and coastal echinoderms. According to Benavides-Serrato & Borrero-Pérez (2010), this species is abundant in the Colombian part of the Caribbean.

Figures

FIGURE 4. Morphological characters of Phormosoma placenta (EqMN2341). (A) Aboral view. (B) Detail of apical system; (C) Oral view; (D) Peristome; (E) Ambulacral plates; (F) Interambulacral plates; (G) Valve of tridentate pedicellariae.