Ecological studies on the bivalve Limaria hians (Gmelin)
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Several ecological studies were carried out on the bivalve Limaria hians and the biogenic beds created by their byssal threads. Specimens used were collected from two populations located on the west coast of Scotland. Age, growth and population dynamics were analysed using several methods. Limaria hians were tagged using plastic tags or the chemical dye calcein, left for 1 year and growth marks in the chondrophore counted. To establish growth parameters marked or caged L. hians were placed in situ and examined after 1 and 2 years. In addition, quantitative collections of L. hians were made from the field every 2 months over an 18 month period, their length measured and numbers recorded. Sub-samples from these collections were used to analyse the sex ratio of the populations. It was found that a single annual growth mark was deposited in the winter and age could be determined by counting chondrophore growth marks; giving a maximum age of 10 years. Measurements of growth using different techniques gave a growth performance index between 2.51 and 2.61. Spawning of L. hians occurred in May and June resulting in peak settlement during July and August. A significant change in sex ratio with respect to shell length was found, indicating protandry. Indirect examination of age was attempted using stable isotopic analysis of δ18O and δ13C contained within the shell. To overcome difficulties found with the stable isotope technique, laser ablation mass spectrometry analysis was used to measure the ratios of trace elements. Two specimens each had a single valve placed in epoxy resin. Sequential lines were drilled along the surface of the valves and the carbonate gathered for analysis by mass spectrometry. From the remaining valve of each specimen a thin section was prepared and laser ablation performed. Limaria hians incorporated δ18O in equilibrium with the surrounding sea water allowing an estimation of ambient temperature at times of shell deposition. A sinusoidal pattern was seen in the isotopic analysis, yet it was not possible to accurately age the specimens by this method, although the approximate time of year when growth mark deposition occurred was obtained. A relationship was seen between the trace element Mg and seawater temperature; however, it is not believed that Mg/Ca ratios in L. hians can be used as a proxy for sea temperatures. Use of these techniques with L. hians is considered limited on account of the animal’s thin and friable shell. An investigation to determine the biodiversity of L. hians beds was carried out for two populations during the winter and summer periods. Cores were taken semi-randomly and the organisms removed from the nest material. Flora and fauna were identified and where possible enumerated. Univariate and multivariate analyses of the data investigated any differences between sites and times. A total of 283 species were found consisting of 16 phyla. Univariate analysis revealed significant differences between the species richness of the two populations, whilst multivariate analysis illustrated differences in the assemblage compositions between sites and times. This study indicated that, in terms of richness and diversity, L. hians beds are at least as rich as horse mussel and serpulid reefs. The rate of regrowth of L. hians nest material, following a simulated dredging impact was examined on an extensive L. hians bed. Within an area of complete coverage of the sea bed by a turf of L. hians nest material, the turf was cleared by diver from 10 0.25 m2 replicate plots and the sediment subsequently raked to simulate the passage of a scallop dredge. The areal extent and pattern of nest regrowth was recorded after 6 and 12 months. Over the initial 6 autumn and winter months treatment plots displayed a mean regrowth of 9.2% of the cleared area, increasing to 15% in the second 6 month spring and summer period. However, no significant difference in growth was found between these periods. After 12 months half the treatment plots exhibited <25% nest cover and none of them contained nest of a thickness comparable to the surrounding bed. Conversion of regrowth rates within the treated plots to the rate of nest advance along a linear front, gave a value of 3.2 cm per year, highlighting the susceptibility of this species-rich biotope to scallop dredging from which activity it may take a century to recover. The pressing need for the conservation status of this biotope to be reassessed is stressed. Its status as a biogenic reef should be accepted internationally and appropriate protective measures put in place.