Biology and systematics of the Nemertine Prostomatella obscura (Schultze, 1851)

The nemertean Prostomatella obscura (Schultze, 1851), hitherto only reported from Baltic localities, is reported for the first time from North America, occurring in saltmarsh pools along the Minas Basin, Nova Scotia and in the saltmarsh at Rowley, Massachusetts. The anatomy of P. obscura is described in detail for the first time and its significance in relation to the genera Prostoma and Tetrastemma, considered to be closely allied, is discussed, as is its significance in terms of nemertean systematics in general. The ecology and life history, previously only superficially known, are discussed. Distinctive features of P. obscura are: double-cup ocelli, of which the anterior pair lack apparent innervation; bilobed testes; a nephridial system extending for most of the body length and served by only a pair of nephridial pores; viviparity; sexual dimorphism, with males smaller; and its saltmarsh habitat. Features that have been used in some specific and generic diagnoses for other nemerteans were investigated and it was found that for P. obscura the number of proboscis nerves is variable, between 11 and 16; the intestinal caecum major may or may not be bifurcated to form anteriorly directed pockets; the dorsal blood vessel can be derived from the left or right lateral blood vessel; and the puedence of a frontal gland. P. obscura breeds from late September through November and gives birth to fully-formed juvenile worms from late April through May. Present data indicate that at least a portion of the population inhabits the salt-marsh year round and that there are marked behavioral responses to the environment in choice of microhabitat and possibly timing of parturition.