Giant squids : The love life of legendary »octopuses«
A paternity test with a surprising outcome raises questions about the reproduction of the giant squid. Perhaps the role model of the mythical sea monster is lifelong & nbsp; true. by Bettina Wurche © DB Royal Society/dpa/dpaweb/picture alliance (excerpt)
A fisherman from Kyoto made a special catch in February 2020. In his gillnet he found a giant squid weighing over 100 kilos (Architeuthis dux) – the model of the legendary octopus in myths and pop culture. In the legends, he is a strange sea monster with slippery tentacles that pulls entire ships into the abyss. In fact, it is a mystery. The freshly caught specimen now sheds a little light on one of the greatest mysteries: how the sea giants have sex.
The fisherman reported the spectacular catch in his gillnet to the authorities, and the squid proved to be a godsend for science. A Japanese team of scientists led by Riho Murai and Noritaka Hirohashi from Shimane University found that it was a female & nbsp; – the animal apparently had an intimate encounter beforehand. The experts found 66 packets of seeds that had drilled deep into his muscles.
Information about these animals, especially their love life, is extremely rare. The giant squid lives in the depths of our oceans, which is why researchers usually only see caught or stranded specimens, dying or even decaying. Short film sequences of the mollusc juggernaut in its natural habitat are a rare stroke of luck. After all, we know that the tales of the octopus are far exaggerated.
Myth and reality
In real life, Architeuthis is rarely more than 18 meters long, says New Zealand giant squid expert Steve O'Shea. Most of them will not be taller than 13 meters. Of this, up to two and a half meters is accounted for by the coat & nbsp; – the sleeve-shaped front part on which fins and eyes sit & nbsp; – and the rest on the tentacles. And while an octopus has eight arms, the giant squid has ten.
Two of the ten arms are long tentacles with large suction cups with chitin-containing barbs at their club-like widened ends. Together with the giant squid Mesonychoteuthis hamiltoni, the giant squid is the largest mollusc on earth. The specimen caught off Japan was not yet fully grown with a coat length of 1.60 meters and a weight of 116 kilograms. Its ovaries with well-developed eggs indicate that the squid lady was probably already sexually mature.
How the pairing works for these giants is only partially known. But as the 66 packets of sperm drilled into the body show, they reproduce very differently than their smaller relatives. Especially less gentle. In most cephalopods, the males place their sperm packets, the spermatophores, on the female or directly in their coat with a specialized arm. The future mother often keeps the gift of love in a special seed bag for later. How exactly the sperm then reaches the eggs has not yet been clarified.
Sex from a safe distance
Architeuthis males, however, are more distant than their smaller relatives: they shoot their ten & nbsp; centimeters tall Sperm packets apparently with force and little targeted. Biologists have found the pointed spermatophores on the arms and fins, coat and head and even on the eyes of sexually mature females, but also on young females and even on males.
© Paulo de Oliveira/NHPA/Avalon/picture alliance (detail) Landed giant squid | An Architeuthis just a little over two meters long was caught by fishermen off Portugal in 2016.
The fact that the sperm is presented or fired in such packages opens up an additional source of information for experts about the love life of the mega-molluscs: paternity tests. In order to find out how many gentlemen deposited the 66 seed packets in the various body areas of their loved ones, the Japanese biologists first looked for genetic markers that could be used to distinguish individual individuals of the giant squid.
This is still easiest in Japan, because that is where most of the tissue samples of such animals can be found. On the long coasts of the archipelago, an unusually large number of giant squids wash up or end up in fishing nets. Apparently they particularly like the Japanese sea basin, with its currents that are warm even in winter. Therefore, the team was able to examine 42 Architheuthis specimens from Japanese collections and identified four meaningful microsatellite DNA sequences, which are known as SSRs & nbsp; – »simple sequence repeats«.
Searching for a partner in the deep sea
The experts working with Murai and Hirohashi got excited about the paternity analysis with their newly developed genetic instruments. The result was completely unexpected: All 66 sperm packets of her research object came from a single male. The experts hadn't expected that, because the habitat of the giant squid actually suggests a different reproductive strategy.
With the dark red camouflage color and the huge, light-sensitive eyes, Architeuthis is perfectly adapted to the dark sea depths of 400 to 1000 meters. Observations with unmanned diving robots show that deep-sea cuttlefish apparently have different reproduction cycles than their relatives from the upper ocean areas. Longevity, a slow metabolism and, above all, rare reproduction are also very typical for other animal groups in the low-nutrient and cool deep sea.
In particular, this also means that every opportunity must be seized. For example, the legendary deep-sea vampire Vampyroteuthis mates up to 38 times, but only lays a few eggs, concludes squid expert Henk-Jan Hoving from GEOMAR from an analysis of 47 large vampire females from the collection of the Santa Barbara Museum of Natural History in Los Angeles. Hoving thinks that these squid with the flamboyant red coat will live to be at least three to eight years old.
Are giant squids monogamous?
The Japanese biologists suspect that Architeuthis also rarely meet to reproduce: That is why females should seize each of the few opportunities to mate, ideally even with several partners. This is also supported by the fact that many cephalopods also complete another part of the reproductive cycle in groups. For example, a team of scientists led by Anne M. & nbsp; Hartwell from Akron University in Ohio in the Pacific had filmed around 1000 & nbsp; deep-sea octopus mothers at a depth of 3000 meters that were breeding there.
Perhaps, however, the surprising result from Japan now suggests, this picture is misleading. Because in the deep sea there is another mating strategy that meets the requirements of the habitat: lifelong fidelity. The deep-sea anglerfish has perfected this. As a young fish, the female catches one or more tiny males, who merge with her and are only limited to the function of a sperm donor.
Giant squids don't go that far – but perhaps the 66 seed packets from just one male are an indication that the cephalopods also mate for life. So far, however, this is only one of several hypotheses. Alternatively, the deep-sea squid that died in the fishing net might not have grown up for long and therefore had only mated once.
Another possible explanation would be that after copulation, a special mechanism prevents further mating, as is common in many animal groups. An indication of this could be the deep bite wounds of the examined squid, which could have come from the beak of another Architeuthis dux. Did the male deliberately injure the female in order to make her too weak for further mating? With that he would have secured the sole paternity for all descendants.
The distribution of the seed pacts all over the body shows that the male apparently did not fire his well-packaged genetic material specifically at the female, presumably from a greater distance. Perhaps, for its part, it feared ending up as a post-coital snack for the female. Cases of cannibalism in giant squids are documented & nbsp; – maybe this also happens during mating. The Japanese researchers emphasize that this has so far been pure speculation.
Hirohashi has long been intensively concerned with the reproduction of squid and the two possible reproduction strategies of deep-sea squids. Whether giant squids are monogamous or tend to be polyglot and what exactly happens before and after copulation cannot be clarified from this individual specimen, he explains. The next logical step: even more paternity tests. The experts now want to examine spermatophores from other Architeuthis specimens. Hirohashi maintains close contact with aquariums, research institutes and fishermen & nbsp; – so that the next giant squid lands fresh on his dissection table.