Studies on Animals Closely Associated with Some New Zealand Marine Shellfish

Abstract

Between March 1973 and September 1974, 858 Perna canaliculus (Gmelin), 150 Mytilus edulis aoteanus Powell, 237 Crassostrea qlomerata (Gould) and 153 Ostrea lutaria Hutton, were surveyed for parasites. From these four commercially important shellfish species, a total of two sporozoans, three species of trematode sporocyst, and a copepod were found. A second copepod and pea-crabs were found associated with certain of the shellfish, but the nature of this association is uncertain. During the examination of each shellfish the ratio of the meat volume to internal shell volume was measured. This provided a condition factor for the shellfish, and gave an indication of the effect of the parasite on the meat weight of the bivalve. Perna canaliculus was collected from Ahipara, Wellington Harbour, and the Marlborough Sounds. Spores of a gregarine, Nematopsis sp., were abundant in the Ahipara mussels, common in Wellington and rare in the Sounds. The fellodistomid trematode sporocyst known as Cercaria haswelli Dollfus was found in mussels from all three locations. Laboratory infection experiments established that the cercaria from this sporocyst develops into the trematode Terqestia aqnostomi (manter). Gravid specimens of this trematode were obtained for the first time, from the mullet Aldrichetta forsteri Cuvier & Valanciennes. Two specimens of the bucephalid sporocyst described by Haswell (1903) were recovered and re-described. The copepods pseudomyicola spinosus Raffaele & Monticelli and Lichomolgus.n sp. were associated with the mussels, but their status is uncertain. The post-planktonic stages of the pea-crab Pinnotheres novaezelandiae Filhol are described for the first time, and the seasonal abundance, effect of depth on abundance, and the effect of the crab on the host's condition are described. Differences between the zoea of apparently identical female crabs from different host species are noted and the significance of these is discussed. Because of the difference between the zoea of crabs from P. canaliculus and Atrina zelandica Gray, only the crabs from the former host are refered to as P. novaezelandiae. The pea-crabs found in A. zelandica, C. glomerata, and M. edulis aoteanus, have not been assigned to a species. Mytilus edulis aoteanus is host to Tergestia aqnostomi sporocysts, Pseudomyicola spinosus, and Pinnotheres sp. Crassostrea glomerata was collected from the Bay of Islands. Only one parasite, the copepod Pseudomyicola spinosus, was found in this host. A pea-crab Pinnotheres sp. is occasionally found associated with the oyster. A disease of this oyster, a symptom of which is the formation of necrotic pustules in the adductor mussel, could not be traced to any parasite. This disease is discussed in an appendix. Ostra lutaria was obtained from Wellington Harbour, the Marlborough Sounds, and Foveaux Strait. Sporozoan cysts were found to occur in 10% of the oysters from Foveaux Strait, but were not observed to adversly affect the oyster. The sporocysts of the trematode Bucephalus longicornutus (Manter) occur in the areas sampled. Pseudomyicola spinosus lnfests the oyster in Wellington and in the Sounds, but not in Foveaux Strait. It was concluded that there were no serious pathogens likely to infect the shellfish farms growing these species, and that there was little farmers could do at present to reduce the effect on the host of the symbionts already present in the shellfish beds. A checklist and bibliography of all the parasites infecting New Zealand marine molluscs is included is an appendix.