Laboratory validation of periodicity of incrementation in statoliths of the South African chokka squid Loligo vulgaris reynaudii (d'Orbigny, 1845): a reevaluation

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The periodicity of increment formation in statoliths of the South African chokka squid Loligo vulgaris reynaudii (d'Orbigny, 1845) is tested using the results of laboratory statolith marking experiments. Statoliths of live squid were marked by oxytetracycline injection, and the squid released into tanks in the laboratory. Two of the squid survived for a sufficient period to permit the application of a second mark. The statoliths were extracted from squid recaptured after a known period and prepared for examination by light and UV fluorescence microscopy. Statoliths from 102 squid were obtained from the experiment, of which 75 (36 males and 39 females ranging from 1 to 29 days postmarking) were successfully prepared for examination. Three replicate counts of the number of increments deposited after marking were recorded from each statolith. The quality of each statolith preparation was quantified during each replicate reading, and these values were used to filter the data to exclude the results from average and below average preparations. The statoliths of 16 squid fulfilled the filtering criteria (11 males and 5 females, 1–29 days and 1–13 days postmarking, respectively). Replicate increment counts were felt to be adequately precise to use the mean increment count as an estimate of the number of increments deposited after marking. The daily increment hypothesis was tested using simple linear regression and statistical power analyses. The data obtained from good statolith preparations strongly support the daily increment hypothesis, although this conclusion should be treated with caution with regard to female squid. Estimates of the statistical power of the conclusions were relatively low, primarily as a result of small sample size. Increment deposition rates in male chokka squid in the laboratory were comparable to those of male squid in a previously reported field study, suggesting that the results of laboratory experiments can be extended to squid in the wild. A possible difference in the readability of statoliths between the sexes is apparent and requires further investigation.