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Source: https://theses.gla.ac.uk/85341/ Parent: https://theses.gla.ac.uk/view/subjects/QH.html

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The abundance and distribution of British Tellinidae

Wilson, James G. (1976) The abundance and distribution of British Tellinidae. PhD thesis, University of Glasgow.

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Printed Thesis Information: https://eleanor.lib.gla.ac.uk/record=b1628415

Abstract

Tellina tenuis da Costa and Tellina fabula Gmelin (Bivalvia: Tellinacea) appear to be closely related and display considerable internal and external similarity. They are found together on many sandy shores in Britain and Europe; a typical situation exists in Kames Bay, Millport, where T. tenuis is littoral, T. fabula is sublittoral, and their ranges overlap little. There is extensive published information to suggest that the situation in the Bay has hardly changed over the past 50 years. The factors which govern their distributions were investigated for two reasons. Firstly, to find the environmental influence on the two species' ranges, and secondly, to apply the findings to palaeoecological interpretation of fossil mollusc assemblages in which these Tellinids occur. Distribution, lethal tolerances, rhythms, sediment analysis, burrowing and morphology were the factors investigated. The distribution experiments proved that T. tenuis and T. fabula were largely randomly dispersed with a slight tendency toward aggregation, particularly in the laboratory. Mark and recapture experiments with T. tenuis showed that adult movement was limited to a few cm from their burrowing start position - due, perhaps, to the angle of burrowing. Attempts to monitor the spatfall met with little success; coring experiments suggested that the 1+ year groups were firmly established in their respective habitats. T. fabula was around 5°C less tolerant than T. Tenuis to any increased temperature regime. T. fabula was also more susceptible to changes in salinity or to desiccation, and it appeared that T. tenuis would be more resistant than T. fabula to any environmental stress. Neither species displayed pronounced rhythms, but there were slight traces of a circadian rhythm, the length of which, in T. fabula only, was inversely correlated with day length. The level of activity in both species was correlated with maximum predicted tidal range during the experiment, but not with tidal range on the day of collection. This suggested some endogenous component in largely exogenous rhythms. Sediment analysis showed that there was little difference in grain size composition of beach and sublittoral sand. Sublittoral sand was, however, significantly less penetrable, less permeable, and less porous than littoral sand. No distinction could be made in the burrowing behaviour (speed, digging cycles) provided that the animals were the same size, but T. tenuis burrowed significantly deeper than T. fabula. Burrowing of T. tenuis was affected by a change from littoral to sublittoral sand, but T. fabula was not affected by the reciprocal change. Size and burrowing depth were correlated only in field populations of T. tenuis. T. fabula may be distinguished from T. tenuis by the presence of diagonal striae on the right valve; what purpose they serve is unclear. The shell of T. fabula was significantly thicker than that of similar-sized T. tenuis but it was impossible to identify medium sized animals on length/width, length/height, or length/weight ratios alone. The results suggested that T. tenuis was considerably better suited to living under adverse environmental conditions, since it was bigger, burrowed deeper and was physically more tolerant than T. fabula. T. tenuis may be penalised for its adaptability by reduced efficiency, compared with T. fabula, under optimum conditions (i.e. in the sublittoral zone). However, the unknown effect of larval settlement patterns and larval tolerances must be considered before drawing firm conclusions regarding the control of the species' distributions.

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