Themiste (Lagenopsis) lageniformis (Baird, 1868)
Unique among the Sipuncula, this species is known to reproduce by parthenogenesis. The main external characteristics, which differentiate T. lageniformis from it congeners are the flask-shaped body, the purple introvert collar and the absence of introvert hooks. It is considered the second most widely distributed sipunculan species in the Indian River Lagoon where it is found associated with oyster beds, among the empty shells.
Themiste minor huttoni is similar, except that it bears introvert hooks (Cutler, 1994).
External anatomy
(From Baird, 1868; Rice & Stephen, 1970; Cutler & Cutler, 1988, and Cutler, 1994)
Introvert is about one-third to one quarter the length of the remainder of the body. It has a purple (live specimens) introvert collar. In fixed specimens the introvert collar is occasionally pale.
Four stems subdivided into smaller branches, one to three times, compose the tentacular crown. This species has the largest tentacule length, 2-4 mm, within the genus.
No hooks are present in adult worms. Shape of the extended trunk resembles a flask.
Internal anatomy
(From Rice & Stephen, 1970)
Trunk wall has a smooth muscle layer.
The spindle muscle is fixed only anteriorly.
A pair of nephridia hangs freely in the trunk cavity and opens at about the same level as the anus.
Contractile vessel has numerous tufts of short filiform villi, which frequently bifurcate near the basal attachments.
A pair of thick retractor muscles arises in the posterior fifth of the trunk.
A caecum is present at the beginning of the rectum.
Rarely over 35 mm long (Cutler, 1994).
There are six other species found in oyster beds near Fort Pierce with less abundance. They are Antillesoma antillarum, Aspidosiphon sp. Nephasoma pellucidum, Phascolosoma perlucens P. nigrescens, and T. alutacea.
(From Cutler, 1994)
This species is well established in the western Pacific from southern Japan to Australia and Hawaii. It is found throughout the Indian Ocean and has been recorded from South and Southwestern Africa, Cuba, and Florida. Themiste lageniformis is considered the second most widely distributed sipunculan in the Indian River Lagoon (Rice et al., 1995), and can occur in densities as great as 2,100 m2 associated with oyster beds near Fort Pierce (Rice et al. 1995).
This species is associated with oyster beds, where it lives among the shells, or associated with the encrusting fauna on sea walls, among vermetid snails, sponges and tunicates (Rice et al. 1995), or in soft rocks, coral rubble and among mussels (Cutler, 1994).
Themiste lageniformis is the only sipunculan known to have a facultative parthenogenic reproduction, in which females are able to produce eggs that spontaneously activate and develop into normal larvae (Pilger, 1987). The period of spawning extends from July through January in Florida in the Indian River Lagoon, but reaches its greatest frequency during August and September. This species follows the developmental pattern III determined by Rice (1976), which includes two larval stages: lecithotrophic trochophore and a lecithotrophic pelagosphera, transforming into a vermiform stage and then into a juvenile form. The eggs are extremely yolky and have a thick jelly coat, which causes them to adhere to any surface upon contact (Williams, 1977). The metamorphosis occurs approximately one week after spawning (Williams, 1977).