In addition to Crassostrea gigas, Mikrocytos mackini Farley 1988, a pathogenic intracellular protistan of unknown taxonomic affiliations, produces disease and mortalities in other species of economically important oysters (Crassostrea virginica, Ostrea edulis and Ostrea conchaphila). Preliminary evidence suggests that these alternate species may be more susceptible to infection and the resulting disease than the usual host C. gigas. M. mackin1 ~solated from C. vlrginica and 0. edulis were infective for oysters. Warm temperatures (above 15°C) prevented the development of M. mackini in C. gigas, C. virginica and 0. edulis.

Stained prawn disease (SPD) with clinical signs of black discolouration of the cuticula, especially around the edges of body segments, and black stippling on the surface of the hepatopancreas was caused by a rickettsia-like microorganism with a n affinity for fixed phagocytes and haemocytes. This disease was found in prawns from various localities throughout Howe Sound and at one location in the S t r a~t of Georgia, British Columb~a, Canada. The distribution of SPD wlthin Howe Sound has not changed since it was first detected In 1989. However, the prevalence has declined to about 4 % from a record high of about 15% In July 1990 and March 1991 In areas with these high prevalences, an above-average level of mortality, equated to a decline in survival rates from 57 to 15'X,, was detected. These mortalities were not attributable to fishing pressure because the affected area has been closed to fish~ng since November 1988 due to unacceptable levels of dioxin and furan compounds In shellfish tissue samples. Laboratory studies indicated that the SPD agent can be transm~tted horizontally by cannibalism and via the water [exposure to screened (1 mm pore size) effluent from lnfected prawns] and remained infectious for 10 d or more of storage at -lO°C. About 50% of the prawns that fed on infected prawns (both fresh and after being frozen) and 25 % of the prawns exposed to contaminated water became infected. Most mortalities attributable to SPD occurred between 2 and 4 mo after exposure to the etiological agent in the laboratory.

The abundances and distributions of the molluscs Austrocochlea constricta, Bembicium nanum, Cellana tramoserica, Melanerita melanotragus, Montfortula conoidea, and Morula marginalba were studied on three marine, tidal, rock platforms, which varied in their degree of exposure to the action of ocean waves. The abundances and distributions of A. constricta, B. nanum, C, tramoserica, Mn. conoidea, and Mr. Marginalba were associated with wave action, but this was not the case with MI. melanotragus. The distribution of Hormosira banksii affected the distribution of A. constricta and the distribution of B. nanum was affected by other undetermined, environmental factors. The results of this investigation are discussed in relation to present and future studies on the abundances and distributions of these six species of molluscs.

The densities and patterns of distribution of the six species of molluscs were determined in the three environments: dry rock surface, "intersection" area, and rock pool, at both midday and midnight low tides during each season. Austrocochlea constricta occurred abundantly within the rock pool area, which was a homogeneous environment at certain times and in certain seasons. The density of Bembicium nanum was inversely related to the presence of water, and this species had a clumped pattern of distribution, except in the rock pool during the winter. Cellana tramoserica occurred in similar densities in all three environments during cool weather, but tended to migrate from the rock surface to the "intersection" environment during hot weather. The densities of Melanerita melanotragus, Montfortula conoidea, and Morula marginalba were greatest in the "intersection" environment, but, because of the generally low densities of these three species on the rock platform, few valid conclusions could be made concerning their abundances and distribution.

The seasonal variations in the abundances of Austrocochlea constricta (Lamarck), Cellana tramoserica (Sowerby), and Montfortula conoidea (Reeve) were associated dominantly with seasonal variations in temperature. Temperature influenced survival of Mn. Conoidea, and probably was associated with the dispersal behaviour of G. tramoserica, but its influence on the abundance of A. constricta was unknown. The seasonal variations in the abundances of Bembicium nanum (Lamarck) was most probably associated only with wave action. No association was found between the seasonal abundances of Melanerita melanotragus (Smith) and any factor of the environment, whilst the density of Morula marginalba (Blainville) was too low for significant associations to be determined.