By Matt Walker
Editor, Earth News
Male O. ingens with erect penis and ejaculated spermatophores on table (penis is white tubular structure in lower half of the picture)
The mating habits of deep-sea squid have been revealed for the first time, after the discovery of a male squid with a huge elongated and erect penis.
The male squid's sexual organ is almost as long as its whole body, including the squid's mantle, head and arms.
That shows how male deep-sea squid inseminate females; they use their huge penis to shoot out packages of sperm, injecting them into the female's body.
The discovery may also help explain how giant squid mate in the ocean depths.
Deep-water fisheries expert Dr Alexander Arkhipkin of the Falkland Islands Government Fisheries Department, based in Stanley, explains how he and his colleagues made the discovery, details of which are published in the Journal of Molluscan Studies.
"The mature male squid was caught during a deep-water research cruise on the Patagonian slope. We took the animal from the catch, and it was moribund with arms and tentacles still moving, and chromatophores on the skin contracting and expanding," he told the BBC.
"When the mantle of the squid was opened for maturity assessment, we witnessed an unusual event.
"The penis of the squid, which had extended only slightly over the mantle margin, suddenly started to erect, and elongated quickly to 67cm total length, almost the same length as the whole animal."
A mature O. ingens deep-sea squid
The squid's sexual agitation caught the researchers by surprise.
However, its occurrence has helped solve a mystery of how deep-sea squid mate.
Biologists know much more about the mating habits of shallow water cephalopods, the group containing octopus, squid and cuttlefish.
All cephalopods are hindered by their body shape, which comprises a closed hood-type structure called a mantle, which forms most of what appear to be a cephalopod's body and head.
The animals use this mantle to move via jet propulsion, they must ventilate it to breathe, and they must also hide their excretory and sexual organs within its structure.
That poses a challenge to male cephalopods: how do they get their sperm past this mantle, and how does the sperm stay there when water is being forcibly passed through the mantle cavity so females can move and breathe?
Shallow water cephalopods have evolved a special arm to do the job.
They have short penises which produce packets of sperm, called spermatophores, then one of their eight limbs is modified to transfer this sperm to special receptacles on the female.
These receptacles are located either on their skin, or internally.
Deep-water male squid are known to use a more primitive method, which involves somehow injecting their sperm into the female's body.
However, it remained a mystery as to how they did it, as they lack a modified arm.
"In deep-water squid we could only guess how this happens," says Dr Arkhipkin.
But the catching of the male deep-sea squid, of the species Onykia ingens, has revealed the answer.
Male O. ingens with non-erect penis (white tubular structure in lower half of the picture)
"Obviously a strongly elongated penis is the solution," says Dr Arkhipkin.
The squid uses its lengthy organ to reach into the body of the female, and it then injects the sperm directly to prevent it being washed away.
How the sperm injected into a female's body then reaches her reproductive organs remains a mystery.
It may circulate in the cephalopod's blood, just as it does in the bodies of gastropods, which are snail-like molluscs that are distantly related to cephalopods, which are also molluscs.
But it does suggest that one more outlandish theory about how giant squid reproduce may now be ruled out.
So few giant squid (Architeuthis dux), or its even larger relative the colossal squid (Mesonychoteuthis hamiltoni), have been sighted or brought ashore that scientists have had to speculate about their mating habits.
Dead specimens have been found, but that made it impossible to see that they too likely have extremely long penises, which are tucked under their mantle, before being extended to reach the female, says Dr Arkhipkin.
"So some authors even hypothesised that the giant squid 'shoot' their spermatophores hydraulically from distance to the females," he says.
"Obviously our findings show that the reproductive habits of giant squid may be bizarre, but not to that extent."
Last month, the same researchers revealed research showing the discovery that some deep-sea fish and squid species have mysteriously migrated across the world, from the north Pacific Ocean to the southwest Atlantic.