Cephalopod Ageing and Gigantism

I was under the impression that just about every (non-Nautilus) cephalopod has a life history which involves growing at a blazing speed, reproducing, and then dying in about a year or two. The implications of such an ephemeral lifestyle on the growth of giant cephalopods is staggering, although it is possible that giants have a considerably longer lifespan than the norm. While the terminal "spawning once" strategy (formerly known as semelparity) is common amongst coleoids, intermittent terminal, multiple, and continuous spawnings are known to occur across various taxa (Rocha et al. 2001). An iteroparous strategy does not guarantee a longer lifespan, but they are generally thought of as longer lived (Rocha et al. 2001). With the abnormally long-lived and continuously spawning Nautilus excluded from data, degree-days to maturity and size are strongly correlated for coleoids (Wood and O'Dor 2000). The taxa used by Wood and O'Dor (2000) appeared to be simultaneous terminal and intermittent terminal spawning strategists; it could be potentially interesting to look at strategy and degree-days to maturity together, as nobody seems to have done it yet.


One of the "great mysteries of cephalopod biology" is the growth rate and lifespan of the giant squid Architeuthis (Grist and Jackson 2007). Architeuthis may no longer be considered the largest living cephalopod, but it is still a huge animal with average weights of 150 kg (330 lbs) for males and 275 kg (605 lbs) for females* (Grist and Jackson 2007). Cadmium concentrations in its digestive gland suggest that Architeuthis either feeds intensely while growing rapidly or is longer lived than most other cephalopods (Bustamante et al. 2008). ¹⁴C analysis of Architeuthis statoliths (inner-ear analogues) gave variable age estimates due to the assumed depths; depth calculations from ¹⁸O gave 95% confidence intervals of 0-37, 27-51, and 20-46 for 3 different specimens, depths estimated from captures gave figures of 0-33, 0-21, and 0-31 for the same specimens (Landman et al. 2004). The authors suggest that future studies should research the physical and chemical characteristics of the squids' environment and use more specimens in varying stages of growth (Landman et al. 2004). Steve O'Shea has an excellent article online on Architeuthis age determination from morphology; one squid with a 1.68 m mantle was estimated to be between ~1.5-4.8 years in age judging by daily(?)** growth rings on the statolith. O'Shea notes that layers on the eye lens and gladius both give ages close to 6 years, so clearly something here isn't being deposited on a daily basis.

* Wikipedia claims that these figures are maximums, but the source it cited has deleted these claims (figure 7).
** Deposits are known to be daily in at least some squids, but it is possible that others may do things differently. Landman et al. (2004) suggest that if their upper-bound estimates are correct, the rings could be formed from increased feeding in shallow water during a new or full moon.


Since none of the aforementioned methods provided a clear solution to this great mystery, Grist and Jackson (2007) applied growth models with energy balance taken into consideration. Cephalopod growth has two distinct stages, an exponential one for larvae and a linear one for adults (Grist and Jackson 2007). The various models gave a very wide estimate of lifespan (89 days to 753 years) but the authors used size-at-age data (from statoliths) in conjunction with this to estimate that females took 3 years to reach 275 kg and males took 6 years to reach 150 kg (Grist and Jackson 2007). In light of this, the authors suggest that there are behavioral (and presumably ecological) differences between the sexes (Grist and Jackson 2007).

This is certainly not the last we'll hear of this great mystery, more data will surely lead to a great deal of revision. If the average weights are found to be different, it could considerably alter the results of Grist and Jackson (2007), for instance. In all likelihood, it seems that Architeuthis does have a rather long lifespan for a cephalopod, but probably not one measured in decades. It would certainly help if at least juveniles could be raised in a lab, but perhaps I'm just partial to the idea of a captive Architeuthis...


So what about the other giant squids? If the "jumbo" ommastrephid Dosidicus gigas truly adds one layer a day to its gladius and statoliths, then it has an annual life-cycle in which it can reach a mantle length of 0.7 to 0.75 m (Nigmatullin et al. 2001). Particularly large specimens have been found up to 1.5-2 years in age with a mantle length of 1-1.2 meters (3'3"-3'11") and a weight of 30-50 kg (66-110 lbs) (Nigmatullin et al. 2001). Interestingly, the larger females show exponential growth while males have a slightly sigmoid growth pattern (Nigmatullin et al. 2001). D. gigas is of course an active, nektonic species with a monocyclic breeding strategy, so it probably isn't that comparable to Architeuthis.


I was originally going to talk about cirrate octopuses and the possible relation of their continuous spawning strategy and occasional gigantism, but I accidentally deleted it and realized it was rather nebulous at best. Rather than dote on it, I decided to at least get one thing posted this month.

References:

Grist, Eric P. M. and Jackson, George D. 2007. How long does it take to grow a giant squid? Rev Fish Biol Fisheries 17, 385-399

Landman, N. H. et al. 2004. Habitat and age of the giant squid (Architeuthis sanctipauli) inferred from isotope analysis. Marine Biology 144, 685-691.

Nigmatullin, Ch.M. et al. 2001. A review of the biology of the jumbo squid Dosidicus gigas (Cephalopoda: Ommastrephidae). Fisheries Research 54, 9-19

Rocha, Francisco et al. 2001. A review of reproductive strategies in cephalopods. Biol. Rev. 76, 291-304.

Wood, J. B and O'Dor R. K. 2000. Do larger cephalopods live longer? Effects of temperature and phylogeny on interspecific comparisons of age and size at maturity. Marine Biology 136, 91-99.