Are there more female octopuses than male?

[Octavarium]

I wonder, because it seems like so many people describe their octopuses laying eggs, it seems like the population is dominated by females. Is their any evidence of this in the progeny, or is it just a coincidence.

 

[Nancy]

Interesting question, but you also have to ask whether females are more likely to be captured as wild caught octopuses than males.

I remember reading a few years ago that female octopuses are more likely to crawl into pots, but I've also heard the opposite. I wonder whether any research has been done in this area.

Nancy

 

[robyn]

I've heard that female nautiluses supposedly comprise 75% of the population, can't remember where I heard it, but that could also be a trapping bias. Interesting question..

 

[Architeuthoceras]

In the Tropic Shale of Southern Utah you only find the female dimorph of Metoicoceras geslinianum, yet in New Mexico the ratio is about 1:1, biogeographical bias?

 

[gholland]

You also have to keep in mind that not many people will write to report about their octopus NOT laying eggs. Baited pots might be more likely to attract gravid females when they are in their food binging period right before laying.

 

[L8 2 RISE]

Could it have something to do with temperature? For example, in Caretta caretta, if the temperature of incubation is ~24-26 C the egg will most likely be male, if the temperature of incubation is ~32-34 C the egg will most likely be female, in between those 2 temperatures is neutral and usually results in a 1:1 ratio. Is this the case with octo eggs? If this is the case, could climate change have anything to do with a higher population of female octos?

 

[monty]

Could it have something to do with temperature? For example, in Caretta caretta, if the temperature of incubation is ~24-26 C the egg will most likely be male, if the temperature of incubation is ~32-34 C the egg will most likely be female, in between those 2 temperatures is neutral and usually results in a 1:1 ratio. Is this the case with octo eggs? If this is the case, could climate change have anything to do with a higher population of female octos?

I've been trying to figure out what drives sex determination in cephalopods, and as far as I've been able to tell, it's not known. There are no species of cephs that ever change sex or hermaphrodites, so a genetic determination seems likely, but hardly proven... temperature would certainly be a factor to look at in any studies. One more reason we need an octopus genome project (and a cuttlefish genome project, and a squid genome project, and a nautilus genome project, and especially vampyroteuthis/argonaut/spirula genome projects!)

 

[gjbarord]

Good discussion. Some of my thoughts.

I would guess that female octopus are just more likely to be caught than their male counterparts. Many are caught already at the end of their life and the pot would be an enticing den, while males would be more active at this point and probably fall victim to predation. I would think that the sex ratios in the wild are fairly balanced given that most octopus will die shortly after mating. There would be no advantage to having more males than females, or the reverse. Although, sperm competition has been noted in cuttlefish so perhaps more males exist in the wild??? I would not believe there to be more females based on anecdotal evidence.

The nautilus statement sounds intriguing. Perhaps in that case, the fact that nautilus have much slower maturity rates than other cephalopods and are able to reproduce more than once, having more females would allow the greatest chance for population expand.

Greg

 

[monty]

Great comments, but I'm confused by this part:

I would think that the sex ratios in the wild are fairly balanced given that most octopus will die shortly after mating. There would be no advantage to having more males than females, or the reverse. Although, sperm competition has been noted in cuttlefish so perhaps more males exist in the wild???

I think I'm missing something here... since most male octos (AFAIK all except argonauta, where the hectocotylus breaks off during mating, and I don't know if it is regrown, but even if it is, it's hight cost) can mate numerous times once they reach maturity, even with immature females that can store spermatophores for months, doesn't the "die shortly after mating" argument only apply if the male can't roam a lot looking for multiple mates?

It seems like, looking at large numbers of animals, most have a pretty even sex ratio, which certainly seems good for the whole population (if you believe in group selection) in that it keeps the gene pool shuffled. This would seem to apply to most cephs, and pretty much most animals, but there's certainly a number of exceptions... I know lion pride structure is one male and many females, but I don't know if that's maintained by an uneven birth sex ratio, or of males kill each other off, or there are just a lot of wandering unattached males. Similarly with deer and pinnipeds, IIRC, at least deer when it's OK for the locals to shoot bucks but not does... I wonder if anyone's done genetic diversity studies of deer populations comparing "hunting allowed" and "no hunting" (like national parks)?

What I'm getting at is that there seem to be more exceptions than rules in terms of looking at the diversity in animals in sex ratio's impact on reproductive strategy, so "there would be no advantage" doesn't seem to show obvious universal rules in animals that are easier to sex by observation...

I'd almost say sex ratio and reproductive strategy seems to have different selective advantage or disadvantage based on context, and is often neutral enough that animals have drifted into a diverse set of strategies. And it seems like cephs have a fairly wide variety of strategies, ranging from the single event schooling mating behavior followed by egg-laying and death common in shallow-water squids like Loligo and seen in Sepia apama to the more spread-over-time we see in captive bandensis to Nautilus laying eggs over years, to most octopus having a fairly long window for females to mate and store sperm, but then having a single brooding event just before senescence (except for chierchiae) to Steve's reconstruction of Architeuthis having infrequent encounters (squids passing in the night) needing extensive sperm storage rather like octopuses.

I'm not meaning this to be a counter-argument, more of a brain-dump of things I'm finding confusing, interesting, or both in this discussion...

 

[Octavarium]

Excellent input so far, it seems I learn more on these boards then i did in BIO 430 (marino bio)!

 

[Steve O'Shea]

There are no species of cephs that ever change sex or hermaphrodites....

Mark, Henk Hoving published a VERY interesting paper on hermaphrodite (I recall male and female structures, but not sure if fully functional) squid off South Africa. Brain is too exhausted to recall the details, but it came out last year in 'Reviews in Fish Biology and Fisheries' (I think the title was). Tomorrow I'll cite more, with literature at hand.

 

[monty]

Mark, Henk Hoving published a VERY interesting paper on hermaphrodite (I recall male and female structures, but not sure if fully functional) squid off South Africa. Brain is too exhausted to recall the details, but it came out last year in 'Reviews in Fish Biology and Fisheries' (I think the title was). Tomorrow I'll cite more, with literature at hand.

Fascinating! I suppose that's what I get for pretending there are any rules without exceptions in biology... I wonder if this is a reversion to an earlier evolutionary state (I know hermaphrodite non-ceph molluscs aren't too unusual) or something that developed on its own.

 

[Jean]

It's interesting that if we pot for an octopus (for the aquarium) 9 times out of 10 we'll get a male, if we want a female (not terribly likely, they're more strongly nocturnal than the males! Not good in a display animal) we have to dredge! this is for P. cordiformis.

J

 

[Steve O'Shea]

Here tiz

 

[monty]

fooey, I don't have full-text access to that one. It looks like the abstract pretty much says it all, though, right?

This actually is starting to sound familiar, I wonder if you've mentioned it before and I'd forgotten. It's certainly interesting, and I wonder what its evolutionary implications are... I could imagine it being just an activation of some useful sexual characteristics, in the "why do men have nipples?" sense, or a throwback to a Cambrian hermaphrodite mollusc's genes being re-enabled. Very curious indeed. And quite possibly a Rosetta stone for "how is sex determined on cephalopods" research!