In honor of PZ’s elaborate, unusual and at times distressing love of all things tentacled, I hereby devote today’s entry to the overlap between my favorite theme (the biology of aging) and two of his (cephalopods and evolution, though mentioning the latter may be redundant, per Dobzhansky: “Nothing in biology makes sense except in the light of evolution”).
In order to find an article about decrepitude among the Cephalopoda, I had to go back a few years. Octopuses and squids, despite their many admirable qualities, are apparently not considered convenient model organisms for the study of aging.
Personally, I think this should change. I’ve always considered cephalopods (specifically, octopuses) fascinating from a neuroscience perspective, ever since I found out that they could get depressed. The last common ancestor of humans and octopuses very likely had nothing more than a ganglion or two, and certainly nothing resembling the large brains of the extant modern species, so we have an example of analogous structures whose specifics evolved independently (ditto for the eyes of vertebrates and cephalopods). Hence octopus brains allow us to study comparative evolution of brains: What is required for the function of a brain? What inherent similarities and differences exist between independently evolved large brains, and do the similarities explain common syndromes like depression? Back to my favorite subject: Do cephalopod brains suffer from neurodegenerative diseases as a function of aging, and do they share with mammalian brains the greater sensitivity of neurons to protein aggregation?
But I digress. I promised you a paper about cephalopod aging, and I shall deliver. In 2002, Anderson et al. penned a manuscript that fits the bill (I should say, “the beak”): Octopus senescence: The beginning of the end:
Senescence is a normal stage of an octopus’s life cycle that often occurs before death. Some of the following symptoms typify it: lack of feeding, retraction of skin around the eyes, uncoordinated movement, increased undirected activity, and white unhealing lesions on the body. There is inter- and intraspecific variability. Senescence is not a disease or a result of disease, although diseases can also be a symptom of it. Both males and females go through a senescent stage before dying-the males after mating, the females while brooding eggs and after the eggs hatch. There are many aspects of octopus senescence that have not yet been studied. This study discusses the ecological implications of senescence.
The story of octopus senescence is a textbook example of the “disposable soma” theory of aging, which holds that aging results from a tradeoff between maintenance of the non-reproductive part of the body (the soma) and the reproductive part (the germ line). It’s less important to live forever as an individual than to reproduce, so the resources devoted to the former will always be less than that needed to keep the somatic tissue in perfect working order for all time. Another corollary of the theory is that deleterious traits that affect the soma only after reproduction will be masked from selection, allowing maladaptive traits to be expressed at the end of life.
In several special cases, the decline following reproduction is quite rapid: Among the cephalopods, many species of squid have massive die-offs after mating. Closer to most of our dinner tables, salmon make a heroic journey back to their birthplaces only to squirt, undergo drastic morphological changes, and then die. Mammals generally don’t kick off immediately after mating (especially females, which need to stick around to nurse and raise their young), but there is nonetheless evidence of age-related change in mammals that can be best explained by the disposable-soma theory (see this example in ungulates). Many symptoms of human aging can also be rationalized as the delayed expression of deleterious traits in late life.
Note a subtlety that helps test the rule: Female octopuses stick around longer, to tend to their eggs. Even though this behavior is manifest after the simple act of reproduction, it’s essential to the survival of the offspring (or to put it another way, the fitness of the parent), and is hence subject to positive selection.
There you have it: a treatment of cephalopod aging, an homage in honor of Pharyngula’s award. Congratulations once again, PZ, and may you have many happy years of blogging ahead of you.