why don't octopus and other cephlapods live very long?

In answer to the raised question — "how about vertebrates?" — there are a couple of interesting answers.

In one case, insects are compared to mammals, by a group that is seriously devoted to insects as food. I imagine that they were somewhat disappointed at this Q&A bit:
Doesn't it seem that, even with ECIs at the relatively low range of 10-15%, if the forest was properly managed for caterpillar (and termite) preservation (as has been recommended in several instances by researchers in Africa), it would be about as productive for animal agriculture as grassland? Is there a short answer for this complex question, or is the question not as complex as it seems?

Dr. Lindroth: On the surface the reasoning seems sound. But a number of complicating factors come to mind; the answer really is complex. For example, because grasslands have coevolved with large grazing mammals grasses can recover remarkably well from extensive grazing. Remove the same percentage of green foliage from a forest habitat and you'll not have the forest for long. And then there are the practical matters of harvest, etc. It is probably much easier to harvest 1000 lbs of large animal biomass from a grassland than an equivalent amount of insect biomass from a forest! This is not to say that management of forests for insect production should not be considered, just that the comparison with grassland systems is fraught with problems.
In short: "Why not raise and eat insects instead of beef?" and the answer is "Cattle conversion of grasslands are more efficient than you'd think, because the grasslands themselves are very productive and have evolved to deal with grazing."

Here's the article, with the Q&A toward the bottom. By the way, "the FIN" is the Food Insect Network. Yum!
FINL Vol. 6, No. 1 Insect Food Conversion Efficiencies

None of these references, by the way, come close to octopuses. And there's the additional complication of dry mass (grain) becoming wet mass (flesh, mostly water). Octopus growth is also sometimes measured the same way -- a paper up this thread calculated the dried mass of crabs involved.

I'm not getting any good comparable numbers for small mammals. I've learned that cavies generally break the rodent model by having a small number of very large babies (up to 18% of the parent's mass each!) and other interesting tidbits, but not closely relevant to our pursuits.

But it does seem that even on land, invertebrates (insects) are the champion individual converters of food to body mass.

A related term, assimilation efficiency, does not take into account the use of some of the assimilated material as energy. Here's one quote:
Natural History-Natural Cycles
Some approximate percentages of assimilation efficiency are shown in various diets below:

* 15% of the energy from decomposing material
* 30-40% of the energy from grasses
* 60-70% of the energy in young plant materials
* up to 80% from seeds.

***
As with primary producers and herbivores, energy is also lost by secondary consumers through respiration and organism death. The assimilation efficiencies of animal food by carnivores vary between 60% and 90%. And within those foods, vertebrate (species with spinal columns or backbones) prey are more efficiently digested than insect prey, as the indigestible insect exoskeletons make up a larger portion of the prey body than do scales, feathers, or hairs on vertebrates. Assimilation efficiencies of insectivores are between 70% and 80% and carnivores is 90%.
Omnivores are part of both the second and third trophic levels. Humans are omnivores because we are physiologically capable of eating animal flesh and vegetation. Some humans, however, choose not to eat animal protein.
Octopuses are capable of not only getting a lot out of their food, they're also quite efficient at conserving the energy of the result.

But not, alas, for a very long time. I still don't see an obvious relationship between lifespan and conversion efficiency.
 
Watching Tank go through the not eating part of senescence has me wondering about stored energy. He is the first octopus I have kept that continued much of his routine after eating stopped so I could observe the body changes (all others, male and female, have conserved energy and wasted away in their dens only to come out in the last day or two). Most notable was the loss of muscle in the arms followed by shrinkage of both the mantle length and arm girth. We know they go a long time without eating in the end and I wonder if the high rate of conversion is related to storage rather than usage.
 
It does seem that they are efficient both ways: converting sustenance to body mass, and body mass back to sustenance toward the end.

And this is for an animal with a metabolic rate higher than most invertebrates, as well as an energy-expensive brain. A single octopus's oxygen usage is as much as a tank-full of other marine creatures. At least, until he hunts them down and eats them.
 
Why don't cephalopods live long?

This is a tough question. Lets take a different approach and instead look at life history and game theory. Why don't all animals have long life spans? Or, to put it another way, what are the advantages of having a short semelparious (reproducing once and dying) life history? What are the disadvantages?

Game theory:
Lets say that it is a really bad year for copepods - the food for a model larval fish population and a model (para)larval squid population. This years offspring of both populations have few survivors due to lack of food. . .

Lets say the opposite happens, copepods are plentiful and finding food is easy. Larval fish and larval squid do well.

Now lets look at the reproductive output of our model fish and model squid. The fish could be best described as "slow and steady"; they divide their money between CDs, blue chip income stocks, real estate, precious metals and T-bills (from counties whose politicians can work together). Model fish take five years to mature. Once mature, they only put 5% of their body mass into reproduction. The squid, on the other hand, mature in a year and dump 50% of their body mass into reproduction. . . They put all their money in a single stock - the latest internet IPO. Squid ride their motorcycles in the rain.

In tough times, when there are few copepods, the fish have a huge competitive advantage. Sure most of their larva gets wiped out, just like the squids, but the adult fish continue to live and thrive and will be there next year to reproduce. The fishes population size may slightly decline but most likely this decline won't even be detectable.

The squid are decimated. They put all their eggs in one basked and things went very badly for them. The squid population crashes hard; they are economically extinct (meaning that fishermen can't catch enough to pay for gas).

In good times, when there is a lot of food for hatchlings, the squid do really well. As they put 50% of their body weight into reproduction and have a 10:1 advantage over the fish. Their advantage is even greater considering that it only takes one year for the squid to mature and reproduce, while the fish take many years to mature. A couple good years in a row and the larva fish still are not mature but all the squid are. . . In good times the squid population explodes. Squid's life history is dynamic and they can quickly take advantage of environmental change.

Both the fish and squid strategies are successful at times. Both work. Neither works all the time. What I just described was an Evolutionary stable strategy.

That explains why there are different strategies. Why do cephalopods have the "live fast, die young" strategy instead of the slow and steady strategy? This gets a bit speculative but the lifespans of other mollusks offers some clues. Those with large thick shells, like Tridacna clams and Queen conchs, tend to have longer life spans. Interestingly, another group of mollusks that lacks shells, the nudibranchs, tend to have shorter life spans. Within the cephalopods, the externally shelled Nautilus has one of the longest lifespans. By giving up the heavy protective armor of the shell, cephalopods gain faster growth rates, which gives them the potential to switch strategies from the fish model (I could have used tridacna clam or queen conchs) slow and steady model to the life fast and die young model.

Two additional points to ponder:

1) Intelligence has nothing to do with this explanation. We humans value it because it is the one trait that we happen to have. We tend to associate it with other animals like us, other mammals, most of which have long lifespans, parental care, etc. Evolution doesn't care.
2) What happens when we change the playing field, say by removing the large fish and mammals (predators of squid) and altering the environment (say by ocean acidification). Which group will benefit? Squid or fish?
 
Oh yeah, do forget that the climate and size of a specimen effects how long it can live, if you take a look at pygmy squid they don't even live for a full year. Then take a look at GPO, they live for 3 to 5 years and live in colder conditions than the squid I just mentioned earlier don't they?:-/

I love what Dr. Wood mentioned, the fast rate of sexual maturity and reproducing is a massive pro for cephalopods. Good thinking:biggrin2:!
 

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