Hermit Crab Worm

Polydora commensalis Andrews

Polydora commensalis Andrews, 1891a, pp. 25–35, 2 pls.—1891b, pp. 291–292, pl. 15: fig. 27.—Cowles, 1930, p. 344.—Berkeley and Berkeley, 1936, pp. 469–471; 1952, pp. 18–19, figs. 29–30.—Annenkova, 1938, p. 178, fig. 14.—Hartman, 1941, p. 308; 1945, p. 32; 1961, p. 29; 1969, pp. 133–134, 4 figs.—Rioja, 1943, p. 229.—Hartman and Reish, 1950, p. 28.—Uschakov, 1955, p. 272, fig. 93; 1965, p. 250, fig. 93—Wells and Gray, 1964, p. 73.—Hatfield, 1965, pp. 356–368, figs. 1–5.—C. Berkeley, 1968, p. 560.—Blake, 1969a, pp. 815–816, fig. 3; 1969b, pp. 21–24, figs. 16–18.

Polydora sp. Berkeley, 1927, p. 420.

Polydora ciliata brevipalpa Zaks, 1933, p. 129.

MATERIAL EXAMINED.—Maine (Damariscotta River), Massachusetts (Cape Cod Bay, coll. SEP; North Falmouth, as P. tubifex coll. J. P. Moore, ANSP 1516), Connecticut (Noank), North Carolina (Beaufort).

DESCRIPTION.—Specimens measure up to 30 mm in length, are 2.5 mm in width, and have over 100 segments. The prostomium is rounded or weakly emarginate on its anterior margin; the entire head is usually contracted into the first setiger (Figure 11a). There is no caruncle. Four small eyes are arranged in a nearly straight line. The palps are unusually short and covered with numerous, fine papillae, each with a sensory hair. The color in life is dark tan with a greenish cast on numerous segments. Red blood vessels are prominent.

Setigers 1–4 have well developed noto- and neuropodial lobes with fascicles of long capillary setae (Figure 11a). Setigers 3 and 4 have some additional short capillary notosetae. Bidentate hooded hooks begin on setigers 12–17 (rarely 10–17). The hooks are accompanied by a few capillary neurosetae. The hooks have a main fang surmounted by a shorter secondary tooth (Figure 11d). The secondary tooth is reduced in hooks from far posterior setigers and, when viewed from certain angles, is difficult to see (Figure 11e). There are 5 or 6 hooks in anterior setigers, increasing to about 13 in middle setigers, and reduced to 4 or 5 posteriorly.

The setae of setiger 5 are modified to include a row of large curved spines and a small tuft of capillary neurosetae. The heavy spines have a characteristic long lateral flange or sheath (Figure 11b, c).

Branchiae are long and straplike, with membranous margins. They begin on setiger 6 and continue to the end of the body.

The posterior end of the animal is dorsoventrally flattened (Figure 11f). The anus is surrounded by a ring of small papillae, the number of which is variable, depending upon the age of the animal. In young juveniles 4 papillae may be present, increasing to 14 in the largest adults. An intermediate stage with 8 papillae is shown in Figure 11f.

DISTRIBUTION.—East and west coasts of North America; North Japan Sea.

Dipolydora commensalis is a species of polychaete worm in the family Spionidae. It has a commensal relationship with a hermit crab and occurs on the lower shore of coasts on the western side of the Atlantic Ocean.