Aspects of stress with particular reference to mytilid mussels and their parasites

Doctoral Thesis


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University of Cape Town

Eight new species of digenea were found in a survey of Aulacomya ater, Choromytilus meridiana/is, Perna perna and Mytilus galloprovincialis. Metacercaria notobucephala sp. nov., a bucephalid from Choromytilus has locality-specific prevalences ranging from 0.23% to 4.46%. It is a severe reproductive stress and reduces the sperm count by a factor of 1000. Metacercaria notobucephala also causes Choromytilus to lose ninhydrin positive substances. The gymnophallid Metacercaria perchorupis sp. nov. occurs in 90.7% to 100% of Choroinytilus and in 74.83% of Perna. Metacercaria A sp. nov., a renicolid, occurs in 78.86'yo to 90.33% of Choromytilus and 44.74% of Perna. Metacercaria B sp. nov., a lepocreadid, infects from 18.3% to 43.17% of Choromytilus and 3.51% of Perna. The combined effect of these three digenea causes 10% of the variation in emersion survival time of male Choromytilus (P = 0.02). Longer surviving mussels had fewer cysts; this suggests that these parasites constitute a somatic stress. Metacercaria columbinesis sp. nov., a zoogonid, was found in 28.6% of Mytilus. The fellodistomid Metacercaria maculatopsis sp. nov. occurred in 0.4% of Choromytilus. The gymnophallid, Metacercaria ater sp. nov. was found in 0.125% of Aulacomya. A rapid technique of semi-permanent staining and fixing with acetic orcein was developed to aid the description of these worms. Juvenile pycnogonids, Nymphonella sp., were found in 0.15% to 7.5%. ofChoromytilus. Nemerteans, nematodes, copepods, polychaetes and other minor symbionts are quantified in these mussels. Mastigocoleus sp., a shell boring cyanophyte, colonises Mytilus with prevalences from 1% to 94% and can reduce shell strength by 43%. The relationship between stress (amount of alga on the shell) and strain (degree of damage) is demonstrated: 61.8% of variation in damage is attributable to the extent of infection (P = 0.001). Stress is defined as an agent that reduces the fitness of the organism. The degree of reduction is termed strain. Fitness is an ontological statement about the organism. Schemes are proposed for rendering these concepts operational. Individual and ecosystem concepts of stress are not homologous and therefore cannot be substantively integrated. In contrast, psychological and physiological stress are deemed integrable: a scheme for this is proposed. Physical (salinity), chemical (ammonia & phenol) and biological agents (scavenging whelks) are shown to be substantively integrable as stresses since they all inhibit shell gaping and byssus production. Mussels demonstrate clear and appropriate closure responses when in the presence of salinity, ammonia levels and Burnupena: all agents that are likely to be of historical selective significance. In contrast, phenol, an unfamiliar agent, elicits an inconsistent response. It is hypothesised that this difference may help distinguish natural stresses from synthetic pollutants. As predicted, the valve opening dynamics of heat stressed mussels were qualitatively and invariably different from those of non-lethal voluntary movements. Dying dynamics have increasing velocity; voluntary dynamics show maximum velocity initially. Analysis of stimuli effects, often interpreted as eustresses, show that assays covering distal as well as proximal effects and those that tend towards whole body integrations are more likely to detect the agent as deleterious. Thus any notion of positive deflections or eustress must be treated with great caution and subject to longer term tests and with more complete integrations if it is to be accepted.

Bibliography: p. 436-482.