Giant Squid Mystery Cracked Via DNA Technology, Study Reveals Results
Giant Squid
Demographic history analyses of the genetic data suggest that there has been a recent population expansion or selective sweep, which may explain the low level of genetic diversity, according to the study results.
In this study, researchers have characterized the mitochondrial genome (mitogenome) diversity of 43 Architeuthis samples collected from across the range of the species, in order to use DNA (genetic information) to provide new and otherwise difficult to obtain insights into the life of the giant animal.
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Introduction
The giant squid, Architeuthi s spp., is one of the largest invertebrates. Described primarily from remains in sperm whale stomachs, from carcasses of dead or moribund animals found floating on the ocean surface or washed up on beaches, and, rarely, from fresh specimens caught by deep-sea trawling activity, it was not until 2004 that a live specimen was observed in its natural habitat and this year that the first video footage was published. Because of their huge size and elusive nature, many myths and legendary sea monsters have been based on them, including the fabled sinker of ships, the Kraken. They have also been of considerable interest and speculation both in the scientific community and in popular literature, inspiring authors such as Jules Verne and Herman Melville. However, despite this, the scarcity of preserved specimens means knowledge about their biology, distribution and taxonomy is very limited, and, indeed, often confused with fiction. For example, while claims have been made of individuals measuring up to 50 m in total length, a more realistic estimate is a maximum total length of 18 m for females, with males reaching slightly smaller sizes. What is more certain is that, with the exception of the polar regions, Architeuthis are globally distributed. Furthermore, they are probably voracious carnivores, preying upon fish, but also smaller cephalopods, including other giant squid. Carbon and nitrogen isotope profiles obtained from analysing upper beaks suggest Architeuthis undergoes an ontogenetic diet shift early in life, abandoning smaller prey of relatively low trophic status in favour of larger prey of higher status. Isotope profiles also indicate that adult giant squid inhabit relatively small, well-defined and productive areas, where food resources have a constant carbon isotope composition. It is also clear that juvenile giant squid are hunted by many other animals, including dolphins, fish and sea birds, and the adults are consumed in large quantities by whales, especially the sperm whale, Physeter macrocephalus. As such it is likely (although unproved) that the giant squid population must be large, in order to sustain such levels of predation by whales. A number of other characters are more uncertain.
There has been debate, for example, about the activity level and metabolism of Architeuthis. Although many scientists have concluded that giant squid must be sluggish ambush-predators, which wait quietly in the water column for their prey to stumble into them, more recent data based on a comparison of citrate synthase activity between cephalopods appears to support quite the opposite, consistent with recent photographic evidence; the giant squid portrayed in these images seems to be a highly active predator with considerable strength.
The taxonomy of the giant squid remains controversial. Since the first description as Architeuthis dux by the Danish naturalist Japetus Steenstrup in 1857, as many as 21 nominal species of Architeuthis have been described. Given the fact that many of these were described based exclusively on location, and in some cases on incomplete remains such as single beaks, suckers or arms regurgitated by sperm whales, the majority of these have been considered likely to be synonymous, and today different opinions suggest there may be as many as eight or as few as one species (with three subspecies). In addition to this uncertainty, the life cycle and life history of the giant squid also remain enigmatic, as do growth rate, time of sexual maturity and longevity, with the many estimates of maximum age ranging from 1 to 46 years.
Given the obvious difficulty of providing answers to some of the earlier-mentioned questions using conventional monitoring/observational techniques, alternative approaches are required if more is to be understood about the giant squid. Recent advances in DNA sequencing techniques have made the sequencing of large stretches of DNA (e.g. mitochondrial genomes) both quick and economical, and mitochondrial genome-based analyses are playing an increasingly important role in phylogenetic and population genetic studies.
Article Reference:
Inger Winkelmann, Paula F. Campos, Jan Strugnell, Yves Cherel, Peter J. Smith, Tsunemi Kubodera, Louise Allcock, Marie-Louise Kampmann, Hannes Schroeder, Angel Guerra, Mark Norman, Julian Finn, Debra Ingrao, Malcolm Clarke, and M. Thomas P. Gilbert. 2013. Mitochondrial genome diversity and population structure of the giant squid Architeuthis: genetics sheds new light on one of the most enigmatic marine species, Proc. R. Soc. B. 2013 280 20130273; doi:10.1098/rspb.2013.0273.
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