ON THE BIOLOGY OF THE MESOGASTROPOD TRICHOTROPIS CANCELLATA HINDS, A BENTHIC INDICATOR SPECIES

¹ Department of Zoology, University of Glasgow, Glasgow, W. 2, Scotland, and the Friday Harbor Laboratories, University of Washington, Seattle 5, Washington

1. Trichotropis cancellata Hinds is a member of the Trichotropidae which, with the Capulidae and Calyptraeidae, constitutes the Superfamily Calyptraeacea. As in other species of the genus, the shell is covered with unusually thick Periostracum prolonged into characteristic spiral rows of spines. Older shells are always deeply eroded apically.

2. The mantle cavity possesses the typical organs of a mesogastropod. Particles collected by the enlarged, but not otherwise specialized, ctenidium are carried by ciliary currents, representing modification of only one out of three groups of cleansing tracts, under the right side of the head to the grooved proboscis. Enlargement of the monopectinate osphradium is to be associated with greater intake of sediment in the augmented inhalant current.

3. The gut is unusual in possessing both a glandular region in the oesophagus and a crystalline style. The former, primitive, structure is usually lost when a style is present. This provides further evidence of the relatively recent adoption of the ciliary feeding habit.

4. Like other Calyptraeacea, T. cancellata is protandric. The penis first appears in animals over 0.5 cm. long. Spawning (at Friday Harbor) is probably from mid-February to mid-May, the animals functioning as males when one year old and between 1.5 and 2.4 cm. long.

5. During the second year, change to the female condition occurs with modification of the pallial reproductive duct to form a capsule gland, the eggs now produced by the gonad being deposited in gelatinous capsules when the animals are two years old and reach a maximum length of 4.2 cm. The great majority, if not all, then die.

6. The process of egg laying, although not of copulation, has been observed.

7. Unlike allied animals, including T. borealis, the penis is retained and enlarges throughout life. This may be due to long retention of sperm in the gonadial duct, more probably to continuation, sometimes until egg-laying, of some production of sperm in the gonad. There is evidence that self-fertilization may occur.

8. Habits involve the simple process of moving as high as possible and then remaining quiescent while feeding on suspended matter drawn in with the increased inhalant current. Only when dislodged is activity resumed. Despite the presence of eyes, habits appear uninfluenced by light.

9. T. cancellata is thus admirably adapted for life on a firm but unstable substrate of dead, largely bivalve, shells. It flounders on soft substrates. Probably no gastropod shell is so richly covered with such a diversity of attaching organisms. Both the spinous covering and the habits encourage settlement of organisms up to the size of tube worms, barnacles and ascidians, the total bulk of which may greatly exceed that of the shell.

10. T. cancellata is an indicator of a restricted type of bottom condition. Much may be learnt from it about the manner in which the limpet form and habit has been acquired in the Mesogastropoda. Assumption of the habit of life, represented here by ciliary feeding and protandry, clearly precedes that of the limpet form.

This article has been cited by other articles:

				E. V. Iyengar Suspension feeding and kleptoparasitism within the genus Trichotropis (Gastropoda: Capulidae) J. Mollus. Stud., February 1, 2008; 74(1): 55 - 62. [Abstract] [Full Text] [PDF]