Fei cui yi bei de she shi sheng li yan jiu | Feeding physiology of the green mussel, perna viridis (L.) | 翡翠貽貝的攝食生理硏究

The green mussel, Pema viridis is a tropical euryhaline species which distributes widely from the oceanic water to estuarine water in Hong Kong. The present study investigated the relationship between food availability and feeding behaviour of this species in both laboratory and field conditions. In the laboratory, individuals of P. viridis were exposed to different food suspensions consisting mixtures of sediment and microalgae (the diatom Skeletonema costatum and the green microalga Dunaliella tertiolecta). Clearance rate (CR), filtration rate (FR), pseudofaeces production, ingestion and absorption, were analyzed at these various food quantity and quality. CR decreased exponentially with the increase of the particulate organic matter (POM). As FX increased, pseudofaeces production also increased, and more pseudofaeces was rejected when filtered matter had a lower organic content (f). The pre-ingestive food selection enhanced the rate of particulate organic matter ingestion (OR) and the organic enrichment of ingested matter became more pronounced for diets of low food value, of which most filtered matter was rejected as pseudofaeces. The absorption rate (AR) was dependent on the content of POM and the absorption efficiency (AE) increased as the organic content of ingested matter increased. Short-term changes in food quantity and quality within coastal seas are typical tidal effects, brought about by tidal resuspension and deposition of sediments. Food availability and feeding responses of P. viridis were investigated for two complete tidal cycles during both spring and neap tides. Temporal changes in total particulate matter (TPM), particulate inorganic matter (PIM) and particulate organic matter (POM), were smaller during neap than spring tides. Seston characteristics at different times of a tidal cycle were compared for both spring and neap tides. Only during spring tides were TPM and PIM significantly higher at high tides while POM remained relatively constant (p>0.05). CR of the mussels underwent temporal variations with tides, and was a negative power function of TPM and a positive linear function off, during both spring and neap tides. FR, OR, AR and AE also increased linearly with increases in f. Pseudofaeces were produced only during spring but not neap tides. Feeding rates and AE were highest at low and lowest at high tides (p<0.01). There was no significant temporal change in the wet weight and protein content of the crystalline style with the tidal regime. For the digestive gland, a-amylase activity was higher at spring than at neap tides, and higher during high tides in a tidal cycle. The digestive gland cellulase activity did not change significantly with the tides. For the crystalline style, both the activity of cellulase and a-amylase were not significantly different (p>0.05) between spring and neap tides. Circatidal rhythms in feeding and digestion in this species were likely controlled by temporal variations in food availability in the seawater. By adjusting feeding rates and enzymatic activities, absorption in Pema viridis remained constant, irrespective of the changes in food availability. Seasonal changes in the food environment and feeding responses of P. viridis at two contrasting sites, Kat 0 and Ma Wan were studied for one year. The former site is oceanic whereas the other one is estuarine. Feeding responses at the two sites were similar with low values of CR, AR and AE obtained from February to April, 1998. CR was a negative power function of TPM and a positive linear function of f. Both AR and AE were significantly correlated with TPM and f. The activity of aamylase and cellulase in the digestive gland and crystalline style were studied in February, May, July and October. Seasonal variations were observed for the aamylase and cellulase activity in the digestive gland with lowest values obtained in February. Reciprocal transplantation of P. viridis between Kat 0 and Ma Wan was made to ascertain the plasticity of the physiological responses recorded in the native populations. Physiological responses including feeding rates and scope for growth were determined 10, 30, 90, 150 and 240 days after transplantation. Physiological responses of transplanted mussels showed complete acclimatization to their new environments after 30 to 60 days whereas complete acclimatization of enzymatic activities was not observed until 90 days after transplantation. Although there were some differences in the gill and palp area between native and transplanted populations, the gill to palp ratio was similar. The observed physiological differences between the two populations, therefore, were largely environmentally induced. To conclude, plasticity of feeding and absorption in P. viridis were demonstrated in both laboratory and field studies. In the face of temporal variations in food availability, from "event-scale" variation in food, over periods of days, to long-term, habitat-specific differences in seston concentration, energy gain can be maximized by the production of pseudofaeces, coupled with selection processes both within the gills and on the labial palps, and by the induction and synthesis of digestive enzymes. Differences in physiological responses between populations, however, were largely environmentally induced.

Thesis (Ph.D.)--City University of Hong Kong, 2000