A female Octopoteuthis deletron in the water column observed by the Monterey Bay Aquarium Research Institute's remotely operated vehicle on December 6th 2007. The photophores on the arm tips are visible. This animal was observed at 854 meters depth in Monterey Canyon near California. Spermatangia were present on the dorsal arms. They are visible as white dots.
Credit: ©2007 MBARI
BRISTOL: In the dark confines of the ocean, the males of a species of deep-sea squid appear to be rather indiscriminate when it comes to sexual orientation, new research suggests.
While same-sex sexual behaviour has been previously observed in many other species, including giant squid, this is the first time such a high proportion of same-sex mating has been reported.
Researchers from the U.S. used video footage taken with remotely operated vehicles to identify individuals of the species Octopoteuthis deletron.
The observations, described in the journal Biology Letters, show that males mate with same-sex partners about as frequently as they mate with females.
"This led to the hypothesis that they mate indiscriminately with any animal of the same species, which could be an advantage for animals with a solitary lifestyle living in a dark environment," said marine biologist and lead author Hendrik Hoving, from the Monterey Bay Aquarium Research Institute in California.
Opportunistic mating
Many species of squid gather in large groups for mating. They perform colourful displays and males become aggressive toward other males as they bid to attract female attention.
If all goes as planned, the male will insert a sperm packet into the muscular mantle cavity of the female using an arm known as the hectocotylus that has been modified to help induce fertilisation. The female will then store the sperm in a special pouch until she is ready to fertilise and lay her eggs.
However, O. deletron are found at depths of 400 to 800 metres. It's dark and encounters with the same species, let alone the opposite sex, are very rare.
From the video, researchers were able to differentiate the sex of 39 individuals and look for signs that they had recently mated by identifying sperm packages that get attached to various parts of the body during mating. Of the 19 females identified, 10 had been mated with, while nine of the 20 males observed had also been mated.
Doing it in the dark
Though the reproductive behaviour of deep-sea squid is largely unknown, there is evidence that it may be mechanically different from coastal squid.
Males lack a hectocotylus, however they have a penis that is thought to inject packages of sperm, known as spermatophores, into the skin of females. The spermatophores were found implanted in the head, mantle, arms, fins and tail and as many as 147 individual spermatophores were found on a single individual.
"It wasn't clear where the 'right' place for implanting was, but the distribution was similar between males and females," said Hoving.
Same-sex behaviour in animals
Same-sex sexual behaviours have been recorded in nearly 450 species. In some species, these behaviours are thought to be adaptive. Bottlenose dolphins, for example, will engage in same-sex behaviours to strengthen bonds and alliances within complex social structures.
In fruit flies, it has been explained as a means of practicing - a trial run, so to speak. Younger animals that have practiced courtship, mounting and other sexual behaviours with a same sex partner are more successful later
with heterosexual partners.
However, there are also many non-adaptive explanations. It may be a case of mistaken identity or simply too few encounters with females, both plausible explanations in the case of deep-sea squid.
Wasting sperm is costly
"These squid only reproduce once in their lifetime, so they are under a time constraint. They probably don't spend a lot of time trying to figure out what sex their partner is," said Hoving.
In the deep-sea environment it may be more costly to hesitate than to waste some spermatophores on the wrong sex.
"In cephalopods in general, there's not a lot of discrimination going on," commented Roy Caldwell, a professor of integrative biology at the University of California, Berkley, who works on cephalopod behaviour but was not involved with the study.
"Swimming around in a dark world there are two problems to overcome. The first is how often you find a mate and the second is that the channels of communication are pretty limited. It's a valid hypothesis."
