Phascolosoma (Phascolosoma) nigrescens (Keferstein, 1865)
This species is considered to be a widespread circumtropical species, inhabiting mainly soft and friable rocks, and crevices of dead coral boulders or buried in coarse sand. The main characters that distinguish this species from its congeners is the dome-shaped preanal papillae, and the exclusive clear streak expanded near the midpoint of the vertical and the middle of the horizontal portions of the hook visible when a hook is examined via transmitted light.
(From Cutler, 1994 and Stephen and Edmonds, 1982)
External anatomy
Introvert is longer than the trunk length, may have dark pigment bands, and has dome- shaped papillae, smaller than those on the trunk, widespread over the surface.
Tentacular crown in live specimens from Florida (see pictures) has 28 dark red, slender, flattened tentacles with white tips, surrounding a yellow red bilobed nuchal organ.
Hooks are arranged in more than 100 mostly incomplete rings, measuring 35-90 µm tall. A secondary tooth, usually small, but may be is absent. A clear streak expanded near the midpoint of the vertical and the middle of the horizontal portions of the hook is exclusive to this species and is visible when a hook is observed via transmitted light.
Trunk has a light brown color and is commonly 20-40 mm long but sometimes larger. The entire trunk is uniformly covered with dome shaped papillae, which are larger on the base of the introvert and more closely set at the posterior end of the trunk.
Internal anatomy
Trunk wall divided in anastomosing longitudinal bands.
The spindle muscle is attached immediately anterior to the anus and at the posterior end of the trunk.
Nephridia are one-half of the length of the trunk, attached to the trunk wall for three quarters of their length, and opening at about the level of the anus.
The contractile vessel may have vesicular swelling.
A pair of retractor muscles arises from the middle third of the trunk.
In Fort Pierce, Florida, Phascolosoma nigrescens is found within vermetid reef conglomerates on vertical seawalls, associated with the Phascolosoma perlucens and Antillesoma antillarum.
This species is very widespread in shallow waters of Indian, Pacific and Atlantic oceans (generally between 30º N and 30º S) (Cutler, 1994).
According to Rice (1975a) P. nigrescens was one of the less abundant species in the Caribbean, with only 0.4% of the total specimens collected, occurring within soft and friable rocks, in crevices of dead coral boulders, buried in coarse sand or wedged between the branches of Porites rubble.
(From Rice, 1975b)
This species follows the developmental pattern IV determined by Rice (1976), which includes a lecithotrophic trochophore and a planktotrophic pelagosphera larva that often remains for several months in the plankton. The shape of a mature egg in P. nigrescens is flattened ellipsoid, wider in the frontal than sagittal planes, and the egg envelope is thick and penetrated by narrow pores. Unlike P. perlucens’ eggs, P. nigrescens has a relatively slight depression at only the animal pole and is somewhat rounder than those of other species from the same genus. The egg size in average is 140 by 91 µm. The cleavage is spiral and unequal. After approximately 3 to 3 ½ days at 25ºC the lecithotrophic trochophore metamorphoses into a planktothrophic pelagosphera larva.