Themiste (Themiste) alutacea (Grube, 1858)
According Cutler & Cutler (1988) the type material is listed in the catalogue of the Museum für Naturkunde der Humboldt Universität, Berlin, under the following number: MNHU1033.
According Cutler & Cutler (1988) the type material is listed in the catalogue of the Museum für Naturkunde der Humboldt Universität, Berlin, under the following number: MNHU1033.
Themiste alutacea is easily recognized by its scattered dark hooks, pyriform trunk, and stem-like tentacles with yellow and dark red pigments. An unpigmented introvert collar is present behind the tentacular crown. Internally the contractile vessel has few and long villi.This species lives in warm and temperate waters, inside hard substrata, and until now has been recorded only in the Western Atlantic Ocean.
Themiste blanda. It differs from T. alutacea in having four tentacular stems which divide symmetrically into eight, lacking pigments on the tentacules, and by living in cold water in the northern Pacific (Cutler, 1994).
(Cutler & Cutler, 1988; Cutler, 1994 and Stephen & Edmonds, 1972).
External anatomy
Introvert is about one third the trunk length.
Tentacular crown consists of a circle of branched tentacles around the mouth. Juveniles have four tentacle lobes, two long dorsals and two shorter ventrals (Rice, 1975b). In many adult worms, two of the tentacles may divide near the base resulting in six stems, sometimes five. In small worms the subsequent division of these branches is limited and the slender tentacules, with pigment spots, are distributed along each branch. A live specimen from Florida (see pictures) has 9 tentacles with dark red color stems and yellow and red pigment spots in each of the tentacules. Posterior to the tentacular crown is a white introvert collar. Nuchal organ is white and surrounded by dark red pigments forming a circle around the structure.
In early juveniles one ring of hooks is present, but in adult worms hooks are scattered and can be more than 100, but they are smaller (75-150 µm) when compared to the other species of this genus.
Trunk has a stout, pyriform shape.
Internal anatomy
Trunk wall has a smooth muscle layer.
The spindle muscle is fixed only anteriorly.
Nephridia length is half or as long as the trunk length and opens posteriorly to the anus.
Contractile vessel has few (less than 20) long threadlike tubular extensions.
A pair of retractor muscles arises in the middle or the posterior half of the trunk.
In Fort Pierce, Florida, Themiste alutacea is found inside rocks under the Phragmatopoma reef, associated with Phascolosoma perlucens, or inside vermetid reef conglomerate growing on a vertical seawall, associated with the Phascolosoma nigrescens and Antillesoma antillarum.
Themiste alutacea is found in warm and temperate waters, in depth less than 30 m. In the Western Atlantic Ocean T. alutacea is spread from North Carolina (34º N) to Argentina (42º S).
This species inhabits hard substrata, such as crevices in coral, oyster beds or soft rock. In Fort Pierce, Florida, Themiste alutacea was found under Phragmatopoma reef and inside vermetid reef conglomerate growing on a vertical seawall.
(From Rice, 1975b)
This species follows the developmental pattern III determined by Rice (1976), which includes two larval stage: lecithotrophic trochophore and a lecithotrophic pelagosphera, transforming into a vermiform stage and then into a juvenile form. The egg of T. alutacea is opaque and white with a relatively dense concentration of yolk. The shape of the egg is spherical and measures in average 138 µm in diameter. No jelly coat has been detected, but the eggs are adhesive and attach to the substratum contacted at the time of spawning. The cleavage is spiral and unequal. After 32 hours at 25ºC the lecithotrophic trochophore metamorphoses into a lecithotrophic pelagosphera. The larvae can swim but mostly remain adhered to the substratum by the posterior extremity. This pelagosphera does not have a definitive terminal organ and the form of the body is extremely plastic. Between 7 to 8 days the larvae transform into a vermiform stage.
Costa Rica, St. Croix (Grübe and Oersted, 1858)