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Sexing an Octopus - Photos of Hectocotylized Arm

DWhatley

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One of the first questions a new octopus keeper asks is for help identifying the sex of their new ward. Usually the easiest way to sex an octopus is by observing the third arm to the right (clockwise) on a sexually mature animal. This arm (hectocotylus) has a suckerless tip (ligula) and a channel for delivering spermataphores. The ligula is very difficult to identify while looking at a live animal but can be just be seen in the second (hummelincki) photograph. The channel is easier to see in photographs than trying to view it on a moving animal and is seen as a white line in the third (briareus) photograph. The easiest way to identifiy it is to see if the octopus keeps it rolled up protectively (all photos). SueNami (the briareus in photos 1 and 3) came with this arm removed up to the webbing. The assumption was that a male would be less likely to loose the arm and we assumed female. As the arm regrew we discovered that we have a boy named Sue and that the hectocotylized arm appears to grow back normally (functionality can only be assumed).

Video from @sedna that shows O. briareus hectocotylus
 

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DWhatley

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The specialized arm is common to most male octopuses (I recently read a paper that said one species drops a normal arm and grows out hectocotylized arm when it is sexually mature) but not every species roles up the arm. The arm is used (by some more than others) as an arm too but you will usually see it curled when not in direct use. My unknown species Macropus male only curls the tip (and then it is more just at right angles to the arm rather than curled) and it took me three month to see the specialized end (the ligula will not have suckers). Puddles moves his arms around a lot and is nocturnal so the limited lighting has made him difficult to sex.
 

DWhatley

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:smile:Certainly biologically interesting. It is not seen by the observer as it appears to happen only when they are depositing the spermataphores while the ligula is inside the mantle. The scientist first noticed the physical change during a failed mating attempt.
 

mucktopus

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neurobadger;154317 said:
Good crud, octopuses get erections too? (That appears to be what the latter article says.)

We have evidence for this in the wild too (Amphioctopus marginatus)- we hope the short little note, with frames of video, will be out this summer.
 

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