An Interpretation of the Molluscan GenusAmphidesma in New Zealand

Summary

Amphidesma, a genus of heterodont bivalv molluses belonging to the Mactracea, is characteristic of the sandy beaches of New Zealand; it occurs in a similar habitat in Australia, Chile, Argentina, Brazil, and in a number of places in the Northern Hemisphere.

A history of the nomenclature of these molluscs, variously described as Amphidesma Lamarck, 1818, or as Mesodesma Deshayes, 1832, is given; it is voncluded that Donacilla , based on De Blainville’s publication of Lamarck’s informal name of "Donacille," is the correct name to he used for this group of molluscs. The name Amphidesma (although a subjective synonym of Semele Schumacher, 1817) is being retained only for the purposes of the present paper.

A consideration of speciation within Amphidesma in New Zealand, from a morphometrical study of some 35,000 speeimens, shows that a cline exists from northern New Zealand to the south with a gradual lessening in height to length ratios, and increase in length of posterior end to height and to length ratios with higher latitude. The ends of the cline are marked by the "typical" A. subtriangulatum and A. forsterianum in the north and south respectively. The cline is not simple since intermediate forms may be found in many otherwise uniform populations especially in the middle region of the cline embracing the southern part of the North Island and the northern part of the South Island, and also at the Chatham Islands. The development of these forms is correlated with events in the late Tertiary and early Pleistocene, especially·with the influence of the warm East Australian Current, allowing immigration of the Peronian fauna to New Zealand and with the retreat and advance of Subantarctic waters over New Zealand in the late Pliocene.

The development of Amphidesma has been correlated with the marine biotic provinces of Ne’W Zealand. The north end-point of the cline is the "typical" Amphidesma subtriangulatum in the hydrologically stable subtropical Aupourian Province and the southern end-point with "typical .. A. forsl erianum in th t: hydrologically stable subantan.:tie Forsterian Province. The intermediate poptdalions of mid-New Zealand and the Chatham Islands, showing great variability, lie in the hydrologically unstable Cookian and .VInriorian Provinces where factors of larv[’ll dispersal have been so variable that the present pop11lations clearly reflect these environmental conditions.

Three species of Amphidesma are found in New Zealand. A. australa (easily separable subgenerically as Paphies) and A.ventricosum, the Toheroa of commerce are both quite distinct. The third “species" is composed of a complex or group of populations regarded as variants of a polytypic spceies with the clearly marked end-points to which the names A. subtriangulatum and A. forsterianum have been applied . The use of nonLinnacan "neutral terms" for informally describing these units is Proposed.

The present account is offered as a preliminary to a major definiti,· revision of this genus now being undertaken.