Life of a squid

The heathen at IIDB are talking about squid—it’s infectious, I tell you, and the godless seem especially susceptible—and in particular about this interesting paper on squid fisheries. Squid are on the rise, and are impressively numerous.

We can get an idea of the abundance of squid in the world’s ocean by considering the consumption of cephalopods (mainly squid) from just one cephalopod predator the sperm whale. Sperm whales alone are estimated to consume in excess of 100 million tonnes of cephalopods a year. This is equivalent to the total world fishery catch and probably exceeds half the total biomass of mankind on the earth (Clarke 1983). It is therefore highly likely that the standing biomass of squids within the world’s oceans probably exceeds the total weight of humankind on the earth. Given such importance squid have generally not been given the attention they deserve or have not been incorporated to the degree they need to in ecosystem models. Future research needs to rectify this.

Squid are creatures of speed: they grow fast and die young. Teleosts and cephalopods follow rather different life strategies.

The form of growth of squid is also unique and interesting. Squid just keep growing. They do not show the distinctive flattening in their growth curve shown by their fish competitors. Many species growth can be modeled with exponential or linear curves. The interesting thing is they continue growing even during their maturation phase until they die or are eaten. They seem to achieve this because of a number of unique qualities, (1) they have a protein based metabolism with efficient digestion so food is converted to growth rather than stored, (2) they are efficient feeders, using their suckered arms and beak they can remove only the highly digestible parts of prey and ‘spit out the bones’ and (3) they can grow by continually increasing the number of their muscle fibres (hyperplasia) a feature not shared by their fish counterparts. While juvenile fish recruit new muscle fibres by hyperplasia they reach a point where growth only occurs by increasing the size of existing muscle fibres (hypertrophy). This probably contributes to their flattening growth curve. Alternatively, squid show both hyperplasia and hypertrophy throughout their life span, thus they continue to recruit new fibres as well as increase the size of existing fibres (Figure 1). Such a strategy might account for their continuous growth. All of the above features contribute to the unique form of growth and the ability of squid to grow fast and fill available niches. Their life is very much life-in-the-fast-lane. They are the ‘weeds’ of the sea.

Live fast, die young…and leave a really decrepit corpse, it seems. Here’s a description of a species that really knows how to have a good time.

Much of my Southern Ocean research has focused on the warty squid Moroteuthis ingens. Up until recent years this species was poorly understood and delegated to obscurity due to lack of biological information. However, this species is regularly caught in both fishing and research trawls and my research has focused on New Zealand, The Falkland Islands and more recently Australia’s sub-Antarctic island regions. The biological understanding of this species is now perhaps the best of any sub-Antarctic squid. It is a large squid growing to over 500mm in mantle length and females achieve a much larger size than males. While M. ingens is epipelagic during its juvenile stage it undergoes an ontogentic descent to take up a demersal existence (Jackson 1993). This species has a biologically unusual and interesting reproductive strategy referred to as terminal spawning (Jackson & Mladenov 1994). Although it is a muscular squid, females (and to a lesser extent males) undergo a dramatic change associated with reproduction. Females produce a huge ovary that can reach the size of a rugby ball and weigh as much as a kilogram. In fact the ovary can weigh more than the total body weight of the male. In association with the development of the ovary the female undergoes a dramatic tissue breakdown in its body wall. This process results in a total loss of muscle fibres that transforms the muscular female into something more analogous to a jellyfish and death is associated with spawning. Moroteuthis ingens and other onychoteuthids are important prey for a number of vertebrate predators (at least four mammals, 17 birds, 13 fish, Jackson et al 1998). It is suspected that this tissue breakdown may result in dead individuals floating to the surface where they are accessible to mammals and birds.

Cool stuff…read the whole paper!