Siphonosoma cf. vastum (Selenka & Bülow in Selenka, 1883)

Fig. 4

Type locality of nominal species

Jaluit, Marshall Islands (Selenka 1883: 103–104, pl. 12, fig. 171, pl. 13, fig. 179).

Material examined

MEXICO • 1; Oaxaca, San Agustín Bay; 15°41′21″ N, 96°14′11″ W; 24 Oct. 2004; SCJ leg.; depth 3 m, under rock in sand; UMAR-SIPU 105 .

Description

Trunk 150 mm in length in preserved specimens (Fig. 4A), 300 mm in living specimens. More than 60 filiform tentacles. Introvert with dispersed orange papillae (Fig. 4C) and reddish region with rectangular papillae (Fig. 4D). Trunk with light brown body wall (Fig. 4E). More than 100 rings of claw-shaped hooks (Fig. 4G). Twenty-six anastomosing longitudinal muscle bands (Fig. 4H). Two long, tiny and dark brown free nephridia occupying 40% of trunk, about 35% attached to body wall. Two pairs of retractor muscles; dorsal pair attached to five bands starting from second band after ventral nerve cord, ventral pair attached to nine bands (Fig. 4B). Multiple rectal caeca covering 90% of rectum (Fig. 4F). Spindle muscle attached posteriorly. Nephridiopore almost at same level of anus.

Remarks

The species is similar to Siphonosoma australe australe (Keferstein, 1865), described from Sydney, Australia, and to S. vastum (Selenka & Bülow in Selenka, 1883), described from Jaluit, Marshall Islands (Table 3). Siphonosoma cf. vastum differs from S. australe australe in that the former species has multiple rectal caeca in 90% of the rectum, which are absent in the latter. According to Cutler et al. (1992), S. vastum is able to disperse across the entire Tropical Pacific Ocean. However, the nominal species is different from S. cf. vastum from Mexico in many diagnostic characters, mainly the coverage of the multiple rectal caeca in the rectum and the attachment of the muscles in the body wall (Table 2). This species is a possibly a new species; however, more specimens are needed to describe it.

Habitat

Subtidal (3 m); in sand.

Distribution

San Agustín Bay, Oaxaca, Mexico.