One awesome mollusc deserves another, so let's follow up last weeks octopus post with one on that group's close relatives the cuttlefish.

Cuttlefish are relatively small (the largest grow to 50cm) squid-like cephalopods that present a nice soft and digestible meal to predatory fish and marine mammals. Having lost the shell that most molluscs use to protect themselves cuttlefish have had to develop other defences. Most strikingly, cuttlefish are masters of camouflage


Just this week, researchers have reported evidence for a other trick that cuttlefish can pull off. When males of the Austrian Mourning Cuttlefish (Sepia plangon) see a female they put on a show, producing striped patterns that evidently impress the female. But these animals form male-dominated groups, and rival males often interrupt would-be woo-ers  in mid-display. So, when they spy a receptive female, males want to put on their flamboyant show for her to judge, but also want to make sure they don't attract the attention of rival males that might want to spoil the party. The male Mourning Cuttlefish's answer to this problem? Using only half of his body to put on the female-impressing show, and throwing would-be spoilers off the scent by mimicking a female with the other half.




This gender-splitting  tactic seems to be pretty common. In aquarium experiments about 40% of males would  attempt the deceptive signal when they were displaying in the presence of a rival. Just as the cuttlefish camouflage response requires information from the physical environment, the gender-splitting trick is influenced by what the male can learn of the social environment. If more than one female is available the male will display to both  without bothering to hide his intentions for observers (probably because working out an angle from which he could excite two females while staying under the radar is just not possible). Likewise, if more than one rival male is about that don't bother with the deception - since it wouldn't be possible to maintain the illusion for two rivals viewing from different positions.

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Brown, Garwood & Williamson (In press) It pays to cheat: tactical deception in a cephalopod social signalling system. Biology Letters. http://dx.doi.org/10.1098/rsbl.2012.0435w

Labels: cephalopods, molluscs, sci-blogs, science, sunday spinelessness

I don't want to talk too much more about the purpose of the argonaut shell, partly because it has already been well covered. Ed Yong wrote a predicably clear and interesting post on the research which uncovered it (which also produced an interesting comments thread) and the lead researcher, Julian Finn from Museum Victoria in Australia, also discussed his work in a really great video.


Instead, I want to talk about the origin of the argonaut shell. Octopuses are molluscs, part of a group of soft-bodied animals that includes clams and mussels and snails. Most molluscs have shells. In fact, despite being arugably the most morphologically diverse of the 35 animal phyla, only a few small groups of molluscs don't contain at least some species that produce shells. The easiest way to explain the presence of shells in so many different molluscan groups is to hypothesize that the last common ancestor of all molluscs had a shell, and most of that ur-mollsuc's descendants have retained this organ. 

In evolutionary biology we call traits that are shared between organisms as a result of their shared evolutionary history "homologies". Homologous traits are often compared with "analgous" ones, parts of organisms that are similar as the result of independent innovations in different evolutionary lineages. We can illustrate the concept using a bat's wing as an example. The forelimbs of bats and whale are made up of the same bones, despite the fact that whales swim and bats fly. That's because bats and whales are both mammals, and they inherited their forelimb bones from a common ancestor before each group radically repurposed their limbs. On the other hand, despite the fact that both bats and stoneflies fly, the insect wing and the bat wing are separate evolutionary inventions and not something the two groups share as a result of shared evolutionary history:

Labels: argonauts, cephalopods, molluscs, sci-blogs, sunday spinelessness

The title contains everything i have to say on this video:

Labels: cephalopods, octopus, sci-blogs, sunday spinelessness

Stuff.co.nz has a very cool video that was shot from an underwater video camera as it was grabbed by a lurking octopus:

It's amazing to see the octopus's suckered arm shoot out from a reef and grab at the diver, and to see the octopus enjoying a ride on the blunt side of the diver's spear a little later. But for me, as someone who spends his days studying landsnails, it really set me to thinking about the remarkable diversity of form in the phylum Mollusca.

This series has focused on the other great animal phylum, the Arthropoda, and with good reason. Most animals are arthropods. But that group has achieved its dominance by finding a good basic body plan, an exoskeleton and set of jointed limbs, and making a million variations on the theme. By comparison, evolution has molded the soft mollusc body into thousands of forms. Cephalopods like the Octopus maorum featured above are perhaps the most amazing result. They are intelligent, free swimming predators with three hearts and amazing powers of disguise. Which is why I still find it amazing that cephalopods likely evolved from snail-like ancestors.

I really shouldn't be so surprised. There are plenty of snails around today that cast off that group's reputation for a sedentary lifestyle to swim the seas as predators. One example that might be familiar to readers is the violet snail, Janthina sp., whose pretty purple shells are occasionally washed up on New Zealand beaches. The violet snail can't actively swim as an adult, instead it builds a raft of air bubbles trapped in chitin and floats on the surface where it eats swimming jellyfish.

Violet snails shell from Flickr user quadprop and washed up animal with raft from wikipedian Rez242

The violet snails are pretty cool, but they aren't a massive divergence from the gastropod body plan. The snails that used to be known as pteropods are much more radically adapted to swimming. These snails are now recognised as two distinct groups, the "sea butterflies" (Thecosomata) which have retained their shells and the "sea angels" (Gymnosomata) which have lost them. As you might have guessed form their names, both of these groups have developed "wings" (derived from the foot muscle) they use to flap their way through the water column.

Sea angel from USGS and sea butterfly from NOAA

There is one group of swimming snails that are even more stunning than the sea angels and the sea butterflies. The nudibranchs (commonly referred to as "sea slugs") are pretty weird bunch of gastropods to start with. They come in a bewildering range of colours and some of them are even solar powered and there are a couple of species that have become adapted to swimming, or at least floating, on the ocean surface. Two species from the family Glaucidae, Glaucus atlanticus and Glaucilla marginata have developed swim bladders and long limb like protuberances that make the look, to me at least, a little like fish:

Glaucus atlanticus licensed CC 2.0 by flickr user tarotastic

Labels: cephalopods, environment and ecology, molluscs, octopus, photos, sci-blogs, snails, sunday spinelessness