Seasonal patterns of investment in reproductive and somatic tissues in the squid Loligo forbesi

Tendances saisonnières de l’investissement reproductif et somatique chez le calmar, Loligo forbesi

¹ Department of Zoology, School of Biological Sciences, University of Aberdeen, Tillydrone Avenue, Aberdeen AB24 2TZ, UK
² Highland Statistics Ltd., 6 Laverock Road, Newburgh, Aberdeenshire, AB41 6FN, UK

Corresponding author: jennifer.smith@abdn.ac.uk

Accepted: 29 June 2005

Abstract

Despite many previous studies on the life history of the squid Loligo forbesi, the roles of internal and external factors in growth and maturation have not previously been investigated in detail. The present study takes advantage of the availability of statistical techniques, namely generalised additive models, which permit simultaneous evaluation of the effects of multiple explanatory variables while avoiding the need to assume linear relationships. This has the further advantage that the size of body parts can be entered directly into the models rather than expressed as indices (e.g., gonado-somatic index). The present analysis is based on measurements taken from monthly market samples during 1989-1991 and aims to infer patterns of reproductive and somatic investment though comparison of large numbers of individuals at different (post recruit) life-cycle stages. Results indicate that, once the effect of body size is removed, gonad weight in both sexes is affected by month and digestive gland weight, consistent with seasonal triggering of maturation, and with energy for gonad growth being derived from food. There is also evidence of a negative relationship between somatic and ovary growth in females, possibly indicating remobilisation of somatic tissue to grow the ovary. In males, relationships between variables tended to have more complex forms, probably related to the existence of two or more growth patterns leading to different sizes at maturity. As found in previous work, maturation begins around 1-2 months earlier in males, suggesting that sensitivity to external triggers is controlled by a sex-dependent internal factor.

Résumé

La vie du calmar Loligo forbesi a déjà fait l'objet de nombreuses études, cependant l'action des facteurs internes et externes sur la croissance et la maturation n'avait jusqu'à présent pas été analysée en détail. Cette étude s'appuie sur une méthode statistique, en l'occurrence les modèles additifs généralisés, qui permet d'évaluer les effets de multiples variables explicatives, tout en évitant d'avoir à faire l'hypothèse de relations linéaires. Cette méthode possède l'avantage de pouvoir intégrer directement la taille des organes dans les modèles, plutôt que d'y faire référence à travers des indices (comme l'indice gonado-somatique ou le rapport poids de la glande digestive / poids du corps). L'analyse, présentée ici, repose sur des mensurations faites durant la période 1989-1991 et vise à mettre en évidence divers types d'investissement reproductif, à travers l'analyse d'un grand nombre d'individus recrutés et observés à des stades différents parmi les recrues.