Phascolion (Phascolion) strombus Montagu

Phascolion (Phascolion) strombus (Montagu)

Sipunculus strombus Montagu, 1804: 74 –76.

Type locality: Atlantic coast of northern Europe.

Remarks: The 10 specimens are 9–20 mm long and bear well­developed tentacles and hooks, and in most cases distinct U­ or V­shaped holdfast papillae towards the posterior end of the trunk.

Edmonds (1980) erected a new species from Australia that he called Phascolion cronullae, but Cutler & Cutler (1985) demoted this to the status of a subspecies of P. strombus, and this was maintained in Cutler (1994). The distinction between these two geographically isolated populations was based on:

(a) Relative size of the two retractor muscles and their point of origin. In the nominate form the ventral is much smaller, commonly 10–20% the diameter of the dorsal, but may be up to 35% of the size of the dorsal and have their origins at different anterior/posterior levels. In the Australian form they are more similar in size with the ventral 50–75 % the size of the dorsal, both originating at nearly the same level near the posterior end of the trunk.

(b) Anterior papillae around the base of the introvert. In both populations there is a significant array of large dark mammilate or columnar papillae. The nominate form is alleged to have a single nipple­like tip, while the Australian form has 1–4 tips, around 25% having more than one. We now conclude that these character states are not taxonomically useful or fixed and that there is more variation within this widespread and eurytopic species than previously acknowledged. In this sample, the ventral retractors ranged from 20– 66% of the size of the dorsal muscle and a few multi­tipped papillae could be found on most, but not all, of these. Since many had been removed from gastropod shells, the posterior end of the trunk was not always present, making precise observations about muscle origins impossible.

Distribution: Very common and eurytopic in the North Atlantic and Arctic oceans, most numerous between 200 and 2000 m. It is also found in deep water in the Caribbean, plus scattered records from the Mediterranean, Red Sea, Gulf of Aden, Madagascar, and South Africa; two Antarctic records; and records from Argentina and Chile. Other Pacific Ocean records come from the South Pacific, New Zealand, and Japan. Remarkably, it is known from depths of 1–4030 m. The only previous record from New Zealand was that of Edmonds (1976) when he described P. t o r t u m, later synonomised with P. strombus. A few Antarctic reports have been called into question by Cutler et al., (2001). Thus, these records provide more evidence that this species, while morphologically quite plastic, is present near New Zealand.