Phascolosoma arcuatum (Gray, 1828)

Adrianov, Andrey V. & Maiorova, Anastassya S., 2012, Peanut worms of the phylum Sipuncula from the Nha Trang Bay (South China Sea) with a key to species, Zootaxa 3166, pp. 41-58 : 46-48

Phascolosoma arcuatum (Gray, 1828)

( Fig. 3 A–B )

Material. South Vietnam: Mekong River mouth, intertidal, mangrove, 30 specimens.

Description. Trunk 60–90 mm long and about 10 mm wide; light yellow-brown or brown-greyish skin with large domelike dark-brown papillae distributed over the entire trunk and more closely set at the anterior and posterior end of the trunk. Introvert 2 X trunk length; hooks up to 70 μm in height, without secondary teeth, warts or toes; with streak (apical canal) widely expanded basally; arranged in numerous (up to 100) complete and incomplete rings. Dorsal retractor muscles originate 10%, ventral retractors 15% of trunk length from the posterior end. Retractor muscles appear to coalesce to a single muscle with four roots in preserved specimens. Gut with 30–50 loops; spindle muscle attached posteriorly. Nephridia about 1/3 of trunk length.

Discussion. Eight other species of the subgenus Phascolosoma are known from the South China Sea. These are well distinguishable from the P. arcuatum by their hook structure. In contrast to P. nigrescens , P. pacificum and P. stephensoni , P. arcuatum lacks basal warts. P. pacificum has, in addition, nephridia equal to the trunk length. P. nigrescens differs from P. arcuatum by presence of hook streak with an enlarged median swelling, and hooks of P. stephensoni are characterized by the presence of smooth narrow streak, clear crescent, and and a distinctive triangle. P. g l a b r u m has hooks with a large rounded bump on the concave side and basal toes. The remaining four species in the subgenus, P. agassizii , P. albolineatum , P. perlucens and P. scolops , vary from P. arcuatum by a lower number (<50) of complete and incomplete rings of hooks. Moreover, in P. albolineatum the tip of each hook is bent at an obtuse angle that is a unique character for this species, while hooks of P. agassizii lack a basal expansion of the streak. Hooks of P. perlucens are characterized by the presence of a large rounded hump on the concave side, and those of P. scolops have a distinct triangle, well separated from the narrow streak.

Phascolosoma arcuatum is a common species in the Indo-West Pacific (from India and northern Australia to south China and Taiwan) found in brackish water environments such as intertidal muddy bottoms and mangrove estuaries.

Figures

FIGURE 3. A – B. Phascolosoma arcuatum. A) lateral view, retracted introvert; B) hook. C – D. P. nigrescens. C) ventral view, introvert everted; D) hooks of P. nigrescens. E – F. P. s c o l o p s. E) lateral view, introvert everted; F) hook (black arrow – triangle of hook; black arrowhead-clear streak). G – I. P. stephensoni. G) lateral view, retracted introvert; H) hook; I) the row of hooks (black asterisk-basal warts). Scale bars: A, 10 mm; B, 50 µm; C, 10 mm; D, 50 µm; E, 5 mm; F, 50 µm; G, 5 mm; H – I, 50 µm.