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SEPG 2218 Date Awarded 2003
The estuarine bivalve Tagelus plebeius: Recent ecology and past climate
Dr. Betina J. Lomovasky
Abstract
Analysis of acetate peels of shell sections of the stout razor clam Tagelus plebeius from Mar Chiquita coastal lagoon (37º 32’S, 57°19’W, Argentina), revealed the presence of internal growth bands, a series of repaired shell margin breaks and different types of abnormal calcifications on the inner surface of their valves. T. plebeius showed a maximum age of 14 years and annual periodicity of growth line formation confirmed with an annual mark-recaptured field experiment. Individual growth was described best by the von Bertalanffy growth model with the parameter values H¥ = 23.48 mm, K = 0.173 y –1, t0 = -2.095 (R2 = 0.852). Shell damage and subsequent repair was observed in 73 % of the specimens analysed (70.3 % of them with scars in both valves around the shell margin, 54.5 % with the posterior shell area damaged in one or both valves, 30 % with more than one damage). There was only few sediment grains incorporated to the shell matrix when scars occur around the shell margin. A field experiment suggests that this pattern of shell damage results from natural re-burrowing (vertical movements) activities. However, 10 % of the individuals showed an inner shell alteration forming a blister full of sediment grains on the area of the shell pallial sinus. These blisters were associated with repaired shell breaks that occurred at the posterior part of the valve, which was confirmed with the field experiment. The development of such blisters could be an indirect consequence of sub-lethal predatory attack by the American oystercatcher Haematopus palliatus, given that this species breaks the posterior part of the shells of stout razor clams when extracting them from the sediments. The third type of shell alteration (94.3 % of the shells) was an orange to brown coloration in the inner shell surface concurrently with irregular carbonate deposition that, in some cases, results in the formation of localized calcium carbonate concretions. This pattern is associated to the presence of metacercariae of Gymnophallid parasites. In conclusion, Tagelus plebeius is able to repair its shell after damage produced by different agents. These repairs are very distinctive and, thus, they can be used as evidences of interactions between this clam and its environment (e.g. sediment characteristics) and the associated community (e.g., predators; parasites) in extant and extinct populations, but also to identify biases in the fossil record that could affect palaeoecological analysis.
Full report: SEPG2218
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