The New Zealand Onchidiidae and aspects of the biology and the life cycle of Onchidella nigricans
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
The family Onchidiidae is perhaps one of the most neglected the Gastropoda. This is rather surprising since it affinities to both the Opisthobranchia and Pulmonata, so is of importance with regard to the phylogenetic relationships of these two groups. As would be expected, the Onchidiidae have been classed sometimes as opisthobranchs and at others as pulmonates , but the latest work on the subject, that of Fretter (1943), regards them as an early offshoot from the main stem of the opisthobranchs. In the New Zealand region only one Genus, Onchidella, is represented, Apart from early accounts of the systematics of the group, and a more recent one (McDowall 1943, unpublished thesis) on anatomy, there has been no research into any other aspect. This has left an open field for the present study, Recourse to what literature there was on the New Zealand Onchidiidae, revealed the necessity for a re-examination of the taxonomy of the Genus. Descriptions of the six species given in Suter's manual (1913) were notable for their lack of characterization of the species. To clarify the systematics it was found to be necessary to study the external morphology and anatomy of animals from all over New Zealand. Naturally enough this lead to the accumulation of details regarding their distribution. Once the various species could be identified with certainty, all further observations were concentrated on on one species, Onchidella nigricans. An ideal site for research into the ecology and reproduction of this species was discovered at Gladstone Pier in the Lyttelton Harbour. As far as general ecology was concerned, investigations were restricted to studies on the habitat and homing behaviour, while population dynamics constituted an interesting analysis of the aggregations of animals in the crevices to which the animals home. Reproduction was investigated under the headings of breeding and development; breeding covering copul ation and oviposition and development the anatomical changes and reorganisations necessary during the transformation of the egg to a young adult. To confirm the breeding times gametogenesis was studied, this also showing the cyclical changes undergone by the gonads during the year. In the conclusions the relations of the family are discussed in the light of a recent paper by Fretter (1943), and of the research carried out in this thesis.