Cephalopod growth: historical context and future directions

Abstract

Research into the biology and management of cephalopods continues to accelerate rapidly. This has been especially so in the past decade. The discovery of new and bizarre species (Vecchione et al. 2001), the advancement of large-scale studies based on geographic information systems (GIS; Waluda et al. 2001), the use of new technology to track individuals and study activity in situ (O’Dor et al. 2002), the challenges of managing growing cephalopod fisheries (Lipinski et al. 1998), the patterns of growth at a cellular level (Semmens and Moltschaniwskyj 2000), the physiological constraints on life spans (Zielinski and Portner 2000) and the potential use of individuals as ecosystem indicators (Jackson and Domeier 2003) are all examples of advances in this field in recent years. Furthermore, understanding and describing cephalopod growth continues to pose challenges and difficulties. While ageing structures in squid, such as, statoliths and gladii to some extent, have proven to be valuable tools for deciphering and describing growth, we are still facing challenges with other groups, such as, cuttlefish and octopus. Even when ageing data are available and we can construct size-at-age plots, this does not necessarily reveal the form of growth. Individual growth rates can be so plastic that attempting to fit curves to such scatter plots is not straightforward or helpful. In the context of these challenges, a workshop was convened on cephalopod growth and held in association with the 2003 Cephalopod InternationalAdvisory Council (CIAC) symposium on the island of Phuket in Thailand. The purpose of this workshop was to bring together experts from around the world, to review and evaluate the current state of knowledge, to work on techniques to advance the field and to provide direction for the future. This special issue is a direct, but independently derived, result of that workshop and attempts to provide a synthesis of our current knowledge and to provide stimulus for future work. Why attempt to understand the growth of cephalopods? Is it really that important? The significance of cephalopods in the world’s oceans can be appreciated by the trend of increasing cephalopod catches worldwide, which is in marked contrast to traditional finfish fisheries (Caddy and Rodhouse 1998). Perhaps a more satisfying rationale for studying the dynamics of cephalopod populations has been given by Clarke (1983) who has estimated that sperm whales alone consume >100 million tonnes of squid annually and that this figure probably exceeds half of the biomass of humanity. If this is the estimate of just squids eaten by all individuals of only one predator, the total biomass of cephalopods in the world’s oceans must greatly exceed that of humans. Thus, a better knowledge of their growth and dynamics will help to assist us in understanding an important component of the world marine ecosystem. This special issue provides broad-scale review and discussion papers as well as specific data papers, all related to issues of cephalopod growth. The majority of work in cephalopod age and growth has been focused on both the near-shore myopsid and offshore oegopsid squid. Hence, Jackson (2004, myopsid squid) andArkhipkin (2004, oegopsid squid) present timely reviews in this issue. These reviews argue that there is considerable scope for further research on statolith ageing techniques, as many species have not yet been studied in any detail. They also suggest that stringency in statolith ageing techniques needs to be maintained and call for an expansion of increment periodicity studies. There are greater challenges studying age and growth of octopus because there are no standard ageing techniques. Semmens et al. (2004) review the history and challenges in studying octopus growth, and highlight the need to develop and expand novel techniques for ageing octopus. Cuttlefish are also an important, widespread and challenging group to study in terms of age and growth, and deserve further attention. Moltschaniwskyj (2004) takes a novel approach in reference to the history and overview of cephalopod growth at a variety of biological scales, including relative growth of organs, muscle fibres, proximal composition and protein synthesis. Examining these processes has helped explain how cephalopods fuel their rapid growth and life histories, although different measurements will provide information on different time scales. Moltschaniwskyj (2004) points out that protein synthesis can provide information on the