Sexual Dimorphism in a Ceph Species

It's a tough one John. Whether the Tremoctopus male is as small (relative to the female) as popularised, or whether the male Ocythoe is even smaller than the enormous [and presposterously ugly] female could be debated (but who really cares; biggest/smallest; never any prize for being the second smallest).

In both genera, and the related Argonauta, the male's third right (or left) arm detaches and swims (????) to/into the female's mantle cavity. In Argonauta I think I recorded up to 6 (maybe 7) detached hecotlyii (whatever the plural of hectocotylus is).

A joke I often use in talks (serving also to wake up an audience) is to pretend to be the comparatively minute male, bumping into the female in the dark, and letting out a loud shriek (pretending to be the male, totally startled by the female's size and shear ugliness). In this moment of terror I jettison my reproductive arm (beats being eaten by the female doing it the 'conventional way'), and escape. Of course I have not encountered any male in collections that had jettisoned the arm (in all the arm was intact); kind-of makes you wonder why jettison it, if doing so is to escape being consumed, when death would appear to be imminent anyway (what's worst: death by female, death by bleeding, or death by sensescence? Nobody need answer.).

I don't tell the story often (it's a bit sad), but many years ago when trawling (bottom trawling, sadly) on scampi grounds in the Bay of Plenty (north-eastern NZ), a female Argonauta nodosa was retained in the trawl. Working in museums most of my (cough) adult life, and having spent an inordinate amount of time reconciling (or at least trying to) morphologies of squid and octopus fixed with different preservation histories, it was important to fix this animal without freezing it first (as all others that I'd handled had been frozen, then defrosted). Believing the female Argonauta to be dead, I placed it directly into a 5% formalin/seawater solution, injecting also into the viscera (freezing it would have lost valuable systematic characters). I could not believe what I saw - THREE detached hectocotylii (there's that difficult plural again) swam from the dead female's mantle and writhed around in the bottom of the bucket (even without a 'brain', and fully functioning circulatory ststem, and no, as far as can ascertain, alimentary system, these 'arms' live for some time).

Even when detached from the male (usually lodged around the base of the female's gill), the hectocotylus is still 'live'. Why the male ditches it (instead of 'gently transferring' genetic material via a conventional hectocotylus) is an unknown, especially if death is imminent. We really don't know a lot about these animals.

Why the male is smaller? Hmmmmm. The same seems to be true of ammonites (I believe), and seems to be the rule in cephalopods. Quite unfair I believe.
This is true of some other inverts as well (e.g. a lot of spiders, Steve's other favorite 8-leggers :wink: ). Is it possibly because those males that 'waste' energy in [excess] growth lose out reproductively to those that mature early and go about their business ASAP, and who therefore leave more offspring, causing eventual selection for individuals who mature earlier/at smaller sizes?
One would think, however, the tables might be balanced by higher mortality (predation?) of the [smaller] offspring of smaller males; or by female preference for larger (maybe fitter?) individuals as mates (though obviously this is either not always true, or just true within the scope of choosing the largest small male available).

Hmmm... no answers here.
First the size difference might be because small males can still produce a lot of sperm, however a large female can produce significantly more eggs than a small one. As far as predation on males they have adapted a defence stratigy in which they carry jellyfish tenticles on there arms, and when threatened they display them.
Also being a pelagic animal the male might not bump into the females often, and when a male incounters a female she might not have eggs developed yet so it is a way of storing sperm for a long time untill she is ready. Just some thoughts.
werid, swimming octo penis....

at least that's what my friends think of what the whole subject is, and it's also the fellow I quoted that gave me the offical name of 'squid-hugger' who lives in 'octo-topia'. :lol:
Pretty sensational stuff Kevin (and reasonable price too). Is determination of sexual dimorphism based on recuring co-association?
Steve O'Shea said:
Is determination of sexual dimorphism based on recuring co-association?
Yes, that, and the inner whorls of the macroconch (M) are very much like those of the smaller microconch (m). The microconchs usually have stonger ornamentation than the macroconchs but it is essentially the same. The trick is being able to tell when each form reaches maturity, usually some form of mature modification occurs at or near the aperture such as lappets, the septa form closer together, or there is a loss of ribbing (alot of others). They are usually found in the same bed and there is usually a higher proportion of M:m or m:M.
It has been suggested the reason for the larger female(M?) size was to hold large quantities of eggs.

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