Aspects of the Biology of Mussels in the Cook Strait Area

Abstract

PART 1. The physiography and hydrology of the Pelorus and Keneperu Sounds, and the distribution of Mytilus edulis aoteanus and Perna canaliculus are described and discussed. Temperatures and salinities in the inner parts of the area are higher in summer and lower in winter than those in the outer parts of the area, nearer the open sea. After heavy rain, salinities in the inner parts of the area may fall below 10 ppt., and this flooding occassionally kills the larger intertidal mussels in some places. Mytilus is limited to the intertidal zone in most places by the activities of mobile predators, but Perna is found both intertidally and subtidally. PART 2. Growth of Mytilus and Perna is considered in relation to depth, light, salinity, temperature, and mussel size; from serial measurement of marked animals, from samples from populations of known age, and from new inferential methods of determining and comparing growth rates from shell shapes on markings, or from contemporaneous sampling of separate populations. Growth is markedly reduced by high temperatures and low salinities, and the largest mussels are most severely affected by environmental stresses. Perna grow to 115mm., and Mytilus to 100mm. in a year. These growth rates are compared with those obtained by other workers, and the rapid growth of mussels in the Marlborough Sounds is discussed. PART 3. The annual cycle of condition, in Mytilus and Perna, defined as Meat Volume/Shell Cavity Volume, is described and discussed in relation to length, sex, reproductive cycle, tidal exposure to air, temperature and salinity. Condition of intertidal Mytilus is higher than subtidal ones, but for Perna, intertidal height does not affect condition. In both species, high temperatures and low salinities adversely affected condition , but morphology and, sex of mussels were not related to the condition cycle. The condition index of Mytilus is correlated with reproduction, but in Perna, the correlation is obscured by the proliferation of interfollicular connective tissue as spawning finishes. PART 4. The reproductive cycles of Mytilus and Perna are described from histological preparations, and discussed in relation to temperature, salinity, sex, and external morphology. There is no morphological difference between the sexes, and external morphology and sex bear no relationship to the development of the gonads. The sexes are separate in both species, but one Mytilus contained both male and female gametocytes. The anatomy and development of the gonad and the morphology of the gametes are very similar in both species. Individual mussels of both species are partial spawners, shedding their gametes over a period of several months. Spawning is infrequent below 13°C. In Mytilus, little gametogenesis occurs above 19°C., and in Perna, little gametogenesis occurs in summer. Interfollicular connective tissue proliferates after spawning, particularly in Perna. PART 5. Anomalies in the vertical distribution of Mytilus and Perna are discussed. Mytilus is limited to the intertidal zone by mobile predators. Field, observations and aquarium experiments in which Coscinasterias calamaria eliminated Mytilus from a mixed Population are described. Selection of Mytilus by the starfish may be based upon Wood's (1968) "ingestive conditioning. Laboratory experiments on the strength of the adductor muscles of Mytilus and Perna are described. Mytilus are easier to open than Perna because their adductors are smaller and weaker.