Squid family planning

Female southern bottletail squid may be able to influence the paternity and quality of their offspring by eating the sperm of males. This behaviour is likely to be the result of the conflict that arises between males and females because of their competing evolutionary interests in reproduction. Both sexes use a variety of tactics to manipulate the outcome of mating into their favour. In southern bottletail squid, Sepiadarium austrinum, males use aggression to coerce females into copulations that they might otherwise avoid. 

A new paper from Ben Wegener, Devi Stuart-Fox, Mark Norman and Bob Wong shows that males don’t have it all their own way. Mating occurs head-to-head and is initiated by the male who lunges at the female and grasps her in his tentacles. The male then transfers packets of sperm, called spermatophores, to the female by sticking them into a cavity just below her mouth where they can survive for up to three weeks. But, the spermatophores often have shorter lives because the females will remove and eat them, sometimes before the male has finished copulating with her.

The authors also determined how females were using the nutrients gained from eating the spermatophores. They fed a group of spermatophore-depleted males on a diet laced with a radioactive marker, which was incorporated into new spermatophores as they produced them. Once the females had eaten the radiolabeled spermatophores it was possible to find where the nutrients were being used by assaying for the marker in tissue samples. 

Levels of the marker were elevated in a number of tissues, including the eggs and reproductive glands. Females, therefore, benefit from consuming spermatophores by gaining some additional nutrition that can be allocated to producing offspring. It’s also possible that spermatophore consumption is a form of cryptic female choice, where the spermatophores of low quality males are eaten preferentially. But, this remains to be demonstrated.

This story is also published on the Australasian Evolution Society website in the Research Highlights section.

Reference

Wegener, B. J., Stuart-Fox, D., Norman, M. D., & Wong, B. B. M. (2013). Spermatophore consumption in a cephalopod Biology Letters, 9 (4) DOI: 10.1098/rsbl.2013.0192

Giant squid have a giant distribution

As many as twenty one species of giant squid have been identified, but most of these were controversial. The general consensus was that there could be one with three subspecies or up to eight distinct species. Now, research shows that there is only one species with no subspecies. This is remarkable given that giant squid are found in nearly every part of the deep sea and their populations are probably large.

Winkelmann et al. (2013) sequenced the mitochondrial genome of 43 giant squid that covered individuals from all of the most widely accepted of the proposed species. They found extremely low genetic diversity and almost no genetic structure between squid from different locations. Only basking sharks, which have long generation times, small population sizes and are recovering from a recently small population size, have lower genetic diversity.

The most likely explanation for the low genetic diversity is that sometime during the last ice-age the population of giant squid declined to a very small size. Small populations are associated with low genetic diversity because random genetic drift affects gene frequency more strongly than it does in large populations. Changes in the numbers of predators or competitors may have caused the decline, but that's just speculation.

The lack of genetic structure is interesting. It suggests that giant squid are incredibly mobile. It is unlikely to be the adults that are moving long distances as other studies show that they stay in relatively contained patches of the deep sea. That argues for long distance dispersal in the eggs and larvae of the giant squid. It is common for marine species to have larvae that disperse long distances, but dispersal distance in the giant squid is extreme.

The study only looked at mitochondrial genes, which are only inherited from the mother. The vast majority of genes are in the nuclear genome and the researchers didn't publish those results in this paper. It will be interesting to see what their results are when they come to analyse them. Sometimes the information gained from looking at the nuclear genes can be at odds with that of the mitochondrial genes because of the differences in the way the genes are inherited.

Winkelmann, I., Campos, P., Strugnell, J., Cherel, Y., Smith, P., Kubodera, T., Allcock, L., Kampmann, M., Schroeder, H., Guerra, A., Norman, M., Finn, J., Ingrao, D., Clarke, M., & Gilbert, M. (2013). Mitochondrial genome diversity and population structure of the giant squid Architeuthis: genetics sheds new light on one of the most enigmatic marine species Proceedings of the Royal Society B: Biological Sciences, 280 (1759), 20130273-20130273 DOI: 10.1098/rspb.2013.0273

Physics versus biology

When I was in high school, my physics teacher drew a massive rectangle that took up almost the entire black board and proclaimed, "this is physics". He then drew a little rectangle inside the first one and proclaimed, "this is biology". The he made a tiny little smudge of chalk on one of the sides of the 'biology' rectangle and said, "this is chemistry, so you all should study physics because all the other major division of science are just sub-disciplines of physics".

Amusing as his performance was, there are many aspects of biology that cannot be directly or indirectly inferred from our understanding of physics. Notably, we would have never formulated the theory of evolution, which underpins our modern understanding of biology, if we had to rely on progress in physics alone. Clearly though, physics is important in shaping the evolution of particular traits. I've written many times about physics in biology, such as swimming in sharks (here), flight in albatross (here), the hammer strike of mantis shrimp (here) and the visual capability of giant and colossal squid (here and revisited here).

Almost all organisms that detect and use light do so in the same part of the spectrum, which is pretty much the same part of the spectrum we see in. Although many use slightly shorter wavelengths in the ultraviolet or slighter longer wavelengths in the infrared, no organisms that we know of use the huge parts of the spectrum in the radio, x-ray and gamma ray wavelengths. I've wondered why this is before, but Mathew Cobb wondered it out loud and got some interesting answers.

It's allometric, my dear Watson

Giant and colossal squid have the largest eyes of any living animals. Eyes are expensive organs to build and maintain, which led some researchers to suggest that there must be a strong evolutionary advantage for large eyes in giant squid. Using a mathematical model they found that giant squid eyes were best suited for detecting large dimly lit objects. They argued that the only stimulus that was both large enough and important enough for giant and colossal squid to detect was the light produced by bioluminescent organisms disturbed by hunting sperm whales.

A giant squid, Architeuthis dux (top), and a colossal squid, Mesonychoteuthis hamiltoni (bottom), being hauled up from the depths (images from National Geographic here and here respectively).

When I wrote about the paper, one of the criticisms I had was that the authors had failed to consider allometric scaling. Although the authors made comparisons of eye size with fish and extinct marine reptiles of similar size, they had not looked at eye size in other squid. Giant and colossal squid are the largest of all squid and their eyes could simply be large because they scaled up with their body size. I did a very crude analysis by conducting a literature search for papers that reported both eye size and body size in squid. From that I concluded that eye size was not disproportionately large relative to body size in giant and colossal squid.

Now, fortunately, nobody needs to rely on my poor-man's analysis. Schmitz et al. have published in BMC Evolutionary Biology that examines data from 87 different squid species and concludes that when allometric scaling is taken into account eye size in giant and colossal squids is not exceptional. In fact, it's pretty much exactly what you would expect if you scaled up another squid species to the same size. Indeed, there were a couple of groups, such as the bobtail squid, that had larger eyes relative to body size than the giant squid.

A regression of eye diameter on mantle length for 87 species of squid. Points for individual measurements in giant (yellow) and colossal (red) squid are shown for comparison (taken from Schmitz et al. 2013).

Further, Schmitz et al. also argue that many of the parameter values used in the original study are inappropriate. The original study based all of their optical performance calculations on the largest recorded giant squid eye diameter of 27 centimeters. But, this is problematic because the optical ability of such a large eye is likely to apply mainly to very large old squid, who are likely to have already reproduced. Eyes that only provide an advantage late in life are unlikely to contribute much to individual fitness. The original paper also probably set the values for the density and amount of light emitted from bioluminescent organisms in the deep sea too high.

When Schmitz et al. used more realistic values in the model they found that there was no unique advantage of large eyes for detecting large luminous objects, such as foraging sperm whales. Pupil sizes ranging from 2 centimeters up to the 15 centimeters used in the original model performed roughly equally well at detecting point sources and large luminous objects. Moreover, as eye size increased there was a slightly greater advantage for detecting point sources of light rather than large luminous objects. Thus, with more realistic parameter values, the conclusions of the original paper are essentially reversed.

References

Schmitz, L., Motani, R., Oufiero, C., Martin, C., McGee, M., Gamarra, A., Lee, J., & Wainwright, P. (2013). Allometry indicates giant eyes of giant squid are not exceptional BMC Evolutionary Biology, 13 (1) DOI: 10.1186/1471-2148-13-45  

Nilsson, D., Warrant, E., Johnsen, S., Hanlon, R., & Shashar, N. (2012). A Unique Advantage for Giant Eyes in Giant Squid Current Biology, 22 (8), 683-688 DOI: 10.1016/j.cub.2012.02.031

Flying squid really fly

Many pelagic squid are able to launch themselves into the air using jets of water expelled through a funnel beneath their head. There are a number of photos online that show squid out of the water and holding their fins and tentacles in a gliding position. But it has been unclear whether the squid where using simple gliding, like a paper plane, or actively controlling the flight.

The neon flying squid, Ommastrephes bartramii, holding its fins and tentacles for flight (photo Geoff Jones).

Now researchers have taken photographic sequences for two schools of the neon flying squid, Ommastrephes bartramii, in flight. The sequences captured the entire flight process, from exiting the water to reentry. Their analysis of the photographs provides the first compelling evidence that flying squid are performing true biomechanical flight.

Neon flying squid in flight with a red footed booby looking on (photo from Muramatsu et al. 2013)

The researchers identified four phases of the flight; launching, jetting, gliding and diving. During the launch phase the squid's fins and tentacles are held in a streamlined position and the squid propels itself out of the water. The squid then spreads its fins and tentacles jetting through the air. Once the water within the mantle cavity is expended the squid continues to glide. Finally, the squid folds its fins and tentacles back into a streamlined position, changes is pitch and dives into the water with barely a splash.

The phases of squid flight; a) launching, b) jetting, c) gliding and d) diving (from Muramatsu et al. 2013)

Using birds within some images the researchers were able to estimate the length of the airborne squid and therefore their speed and distance covered. During the jetting phase the squid travelled at between 8.8 and 11.2 meters per second, which is about human sprinting speed. The squid covered up to 33.5 meters in flight, substantially less than a flying fish (~400 meters***), but better than previous estimates for flying squid (~10 meters).

*** Correction - While flying fish have been documented to travel over 400 meters in a single jump, their typical jumps are about 50 meters.

Muramatsu, K., Yamamoto, J., Abe, T., Sekiguchi, K., Hoshi, N., & Sakurai, Y. (2013). Oceanic squid do fly Marine Biology DOI: 10.1007/s00227-013-2169-9

A first look at the giant squid footage

A fleeting glimpse of the footage of a live giant squid in its typical deep sea habitat, captured by filmmakers in Japan. You might want to watch it with the sound off...

Giant squid in full HD!!

A week ago the Discovery Channel announced that it had footage of a giant squid that it would be releasing in January 2013! It could be the first video footage ever captured of a live giant squid in its natural habitat. Previous footage of a live giant squid has been taken at the surface after the squid was caught. In 2005 and Japanese team lead by Tsunemi Kubodera and Kyoichi Mori took the first still images of a giant squid attacking a baited line. The next year a team lead bu Kubodera caught a small female squid using a similar method and filmed it as it was brought into their boat.

A giant squid attacking a baited line (photo Kubodera & Mori)

I'm guessing that Kubodera is involved again, but other researchers may have used similar methods to get this latest footage. Basically, look for where sperm whales are diving, drop a baited line down a few hundred meters with a camera on it and wait. Easy!...

Top five on Friday

On his website "Why Evolution is True", Jerry Coyne has posted pictures of a pretty spectacular looking snail, Blaesospira echinus. So, I thought that today I would post my top five favourite mollusks. There were two mollusks in last week's list of my top five favourite marine animals. To give the other amazing species in phylum Mollusca a chance, I'll leave them out.

1. Elysia chlorotica

It was hard to keep this one out of last week's list. Like Glaucus atlanticus (who was number 1 last week), it steals cells from the organisms it eats and uses them for its own ends. In Elysia's case, it steals chloroplasts from the seaweed Vaucheria litorea. It's able to keep these alive for up to 9 or 10 months, which indicates that it has acquired genes for this task, probably by horizontal gene transfer.

The beautiful and fascinating Elysia chlorotica (Photo Wikipedia)

2. Giant squid, Archieteuthis dux

It's the second biggest invertebrate, after the colossal squid. And like the colossal squid, it has the biggest eyes of any animal. Eight species of giant squid have been named, but it's almost certain that there are fewer species and there may be only one, A. dux. 

The first live giant squid ever to be photographed in its natural habitat, the deep sea (photo Kubodera and Mori)

3. Dorytethis opalescens (formerly Loligo opalescens)

Like most cephalopod mollusks, D. opalescens is an amazing colour changer. But, unlike many other cephalopods it uses two different cell types to change colour (see here for an amazing bit of science communication explaining it all). It lives close to the surface and one of the reasons for its colour changing skill is thought to be that it avoids predators by countershading the light-dark patterns of wave lensing.

Doryteuthis opalescens (photo Wikipedia)

Wave lensing pattern on a sandy seafloor (Photo National Geographic)

4. Giant clam, Tridacna gigas

It's big, it's beautiful. Like corals, all of its colouration comes from symbiotic algae living inside its body.

The giant clam, Tridacna gigas (photo Wikipedia)

5. Janthina janthina

A bubble rafting snail that lives in the open ocean. The bubble raft is likely to have evolved from an ancestral egg coat.

The bubble rafting snail Janthina janthina (photo Denis Riek)

All the better to see you with

Giant and colossal squid have bigger eyes than any other living animals. A paper published in Current Biology last month asks why it is that they do when other animals get by with smaller eyes. Intriguingly, they suggest that it might be sperm whale predation that has driven both body and eye size in these massive invertebrates. Unfortunately, I think the authors tackle the problem from and adaptationist perspective and do not give alternative hypotheses due consideration.

A kraken fights with a leviathan in a diorama at the Museum of Natural History (image Wikimedia Commons)

Eyes are metabolically expensive organs to build and to maintain. So it's interesting that they are so prevalent in the deep sea where sunlight is weak or never reaches. But, in the deep, animals and other organisms make their own light. Indeed, it has been estimated that 80 - 90% of creatures in the deep sea are bioluminescent. Clearly then, making and detecting light are important for life below the sun's influence. But, most animals get away with eyes much smaller than those of giant and colossal squid.

Generally speaking, big eyes are more sensitive and provide better spatial resolution. But, the amount of light reaching the retina is dependent on the ratio of focal length (the distance between lens and retina) and pupil diameter (camera mavens will recognise this ratio as the f/stop value). In some cases smaller eyes can be more sensitive because they have a shorter focal length. With longer focal length, larger eyes have greater acuity and can increase sensitivity by increasing pupil size.

The paper examines just how much visual performance in the sea improves with increasing eye size and what visual strategies giant eyes are best suited for. They take into account the focal length and pupil size plus a raft of other factors such as the transmission of light in water and background illumination. They find that giant eyes are best suited to detecting the distant shapes of large moving objects illuminated by small bioluminescent organisms disturbed by the object's passage.

The authors argue that the only objects that are both large enough and important enough for giant and colossal squid to detect at distance are hunting sperm whales. The bioluminesence stimulated by moving whales would allow giant and colossal squid to see them at about 120 meters away. Unfortunately for the squid, this is inside the distance at which the whales would detect them with their sonar. The authors argue, therefore, that the squid must use their ability to detect whales at this distance to prepare for a coordinated escape response.

The authors, I'm sure, would acknowledge that their study is speculative and important questions remain. For instance, the maximum recorded sprint speed of hunting whales is about  9 ms^-1, which would give a giant squid more than 10 seconds of advanced warning of a whale closing at speed. Giant and colossal squid are unlikely to be strong enough swimmers to flee beyond sonar range, leaving them the only option of evasive maneuvers. Surely smaller eyes and shorter detection distances would still leave them ample time to prepare to outmaneuver the whale.

The authors rule out the possibility that giant and colossal squid use their eyes to detect prey because huge eyes offer only marginally better performance than much smaller eyes. However, they only consider individual prey items and looking at the published studies on the diet of giant squid, many of their prey species are schooling fish (e.g. Macruronus novaezelandiae, Micromesistius poutassou and Trachurus trachurus) and squid (e.g. Nototodarus sloanii, Ommastrephes bartramii and Todarodes sagittatus). Although detecting individuals of these species might not favour the evolution of giant eyes, schools of prey could easily reach sperm whale size and would also trigger bioluminescence as they move. 

On moonless nights fishermen at the surface are able to use the pattern of bioluminesence stimulated by schools of fish to distinguish among several species. It's possible that giant and colossal squid could use patterns of bioluminescence to determine whether it's being stimulated by prey or non-prey species or, of course, a hunting whale. Longer prey detection distances would seem to be a highly advantageous trait for fueling the fast growth rate of giant squid (reaching 150 - 250 kg in about 5 years).

In making their argument that the eye size of giant and colossal squid is unusually large, the authors contrast them with several other extant whales and fish. None of these extant species, though, are visual predators that hunt at great depth. The authors do also compare to the extinct ichthyosaurs, which had eyes of similar size and were probably visual predators that hunted at depth. They suggest that this is because ichthyosaurs had a similar need to detect the bioluminesence stimulated by large moving objects, perhaps pliosaurs or other ichthyosaurs.

None of these comparisons are truly fair, even the ecologically similar ichthyosaurs, because they don't take into account allometric scaling effects. When making trait comparisons among lineages you should always examine the trait within lineages. It is entirely plausible that the eyes of giant and colossal squid are large simply because they're scaled up versions of those in species with smaller body size. The authors claim that giant and colossal squid eyes are unusually large even for squid, but the paper they cite in support of this point only examines changes in eye size in a single species as it grows.

The problem with looking at the relationship between eye size and body size within a single species of cephalopod is that eye size as a proportion of body size decreases as they age. This is almost certainly not e case when you examine the relationship between body size and eye size among species. As in cephalopods, eye size in vertebrates generally decreases as a proportion of body size as individuals grow. But, eye size increases as you move from species with smaller body size to species with larger body size. Unfortunately, no such among species relationship has been published for cephalopods.

In a crude attempt to get an idea of the scaling relationship in squid, I searched the literature for reports of eye size that could be linked to a length measurement. I was able to find data in three species (Dosidicus gigas, Loligo opalescens and Illex illecebrosus), but eye size as a proportion of mantel length in giant and colossal squid was within the range of these species. Although my "analysis" should be taken with some salt (e.g. some values were estimated from graphs), it seems that giant and colossal squid eyes are not unusually large when their body size is taken into account.

Although I have strong doubts that the eyes of giant and colossal squid were selected for detecting hunting sperm whales, this study does provide some interesting information about the performance of eyes in the sea. Because deep-sea squid and their predators are so hard to observe, we really only have recourse to mathematical models to determine the selective pressures on predators and prey. But, we should never ever start with the assumption that the trait of interest is adaptive and then look for explanations.

Nilsson, D., Warrant, E., Johnsen, S., Hanlon, R., & Shashar, N. (2012). A Unique Advantage for Giant Eyes in Giant Squid Current Biology, 22 (8), 683-688 

DOI: 10.1016/j.cub.2012.02.031

Winning at hide and seek in the mesopelagic

A paper published a few months ago in Current Biology serves to highlight just how amazing cephalopods (squid, cuttlefish, octopus and their kin) are. The paper concerns how two species of cephalopods (Japetella heathi an octopus and Onychoteuthis banksii a squid) have evolved to avoid predators in the dynamic light environment of the mesopelagic layer of the open ocean.

The mesopelagic layer of the ocean extends from 200 meters to 1000 meters deep. Sunlight in this zone transitions from present, but too dim for photosynthesis at the top, to totally absent at the bottom. This light transition poses a tricky problem for animals living there that want to avoid being eaten.

In the shallower parts of the mesopelagic, predators are able to detect the shadows of prey swimming overhead. Here, the best strategy for small animals to reduce the risk of being eaten is for them to be translucent. Tissues that allow light to pass through cast a weaker shadow, which means that predators have to get closer to their prey in order to see them.

In the deep dark depths, translucence is not such a good strategy. There's not enough sunlight for predators to hunt for shadows. Some predators, though, bring their own light in the form of light organs near their eyes. In the directed light of bioluminescence small imperfections in light transmission cause the light to be scattered making translucent animals much brighter than the background and easy prey.

Where some predators, like the headlight fish (Diaphus effulgens), hunt by producing their own light, it pays to be a colour that absorbs light at the same wavelengths. The vast majority of bioluminescent organisms produce light in the blue wavelengths, which is best absorbed by reds and blacks. Unsurprisingly then, most animals that occur where sunlight is absent are red or black to reduce the risk that predators will find them using biologically produced light.

But, some animals, like our two species of cephalopod, range over most of the mesopelagic and encounter a wide variety of light environments. High on the wish list for such animals would be the ability to become translucent in diffuse sunlight and red in directed bioluminescent light. And it turns out that this is exactly what J. heathi and O. banksii can do.

Like most cephalopods, J. heathi and O. banksii have pigment containing cells in their epidermis, known as chromatophores. Cephalopods are able to rapidly change colour by expanding the size of the chromatophores. When muscles attached to a chromatophore contract, the pigment containing sac inside stretches out into a flat disc, which increases the visible area of pigment by about 50 times.

A diagram showing the structure of a cephalopod chromatophore and the arrangement of the associated muscle and nerve cells (image: Richard Young)

Most of the time J. heathi and O. banksii are translucent, but when they are exposed to blue light, like that produced by bioluminescent organs, they rapidly change colour to red. Neither strategy is complete; the chromatophores reduce translucence even when contracted and they are not numerous enough to allow them to become totally red. Thus, I would guess that these cephalopods are at a disadvantage relative to organisms that specialize in being red or translucent. But, in contrast to the these cephalopods, such specialists would be limited in the light environments that they could use.

The translucent and red forms of J. heathi (left) and O. banksii (right). Note that neither the translucent nor the red strategy are complete (image constructed from figures in the paper)

Interestingly, the images seem to suggest that J. heathi, which has the deeper distribution, is more able to produce the red colouration that is advantageous where predators hunt using bioluminescence. The authors, however, do not test or discuss this possibility. In the article they do briefly mention that older J. heathi are more common at deeper depths and have greater chromatophore coverage. So, the apparent difference in chromatophore coverage between the two species could be a consequence of either age or depth distribution differences, or perhaps both.

Zylinski, S., & Johnsen, S. (2011). Mesopelagic cephalopods switch between transparency and pigmentation to optimize camouflage in the deep Current Biology, 21 (22), 1937-1941 DOI: 10.1016/j.cub.2011.10.014

Flight is the new black

There are several kinds of animals that leave the water by jumping. A few have adaptations that allow them to increase the distance that they travel in a jump. For instance, flying fish and flying squid use their fins (and tentacles in the squid) to help them glide above the surface.

A flying fish in flight. Note the enlarged pectoral and pelvic fins to assist in gliding (photo Danielandmelora).

It's long been thought that jumping and gliding are an adaptations to aid in predator avoidance.  By leaving the water it is harder for predators to track their prey. And, prey that glide can change direction in the air to make it even harder for predators, because it becomes more difficult to determine where the prey will re-enter the water. 

A squid (probably Ommastrephes bartramii) in flight. Note the way the fins and tentacles (with membranes between the arms) are held for gliding (photo Geoff Jones). 

Recently, though, some researchers have suggested that squid might use aerial gliding to reduce the energetic cost of migration. They were able to collect data on the acceleration and velocity of squid in air from analysing photographs of jumping squid taken in rapid sequence. They found that travel in air was five times as fast as any measurements of squid movement in water. I'm a little skeptical of this claim. Mostly because speed is not a good way to measure the energetics of movement. 

Squid have two ways of moving. They can use their fins or they can use a jet propulsion system. The jet propulsion system is the primary form of movement in most squid, but often it's used in concert with the fins. And it's the jet propulsion system that squid use to exit the water. To move this way, the squid takes up water into its mantle cavity and then forces it out a narrow funnel beneath its head.  

The funnel of Illex illecebrosus, which is used in jet propulsion. The arrows in the image point to the lateral adductor muscles that support the attachment of the funnel to the head (photo M. Vecchione).

To help direct the water through the funnel, rather than back out the mantle opening, squid have a specialised apparatus to 'lock' the mantle shut. Cartilaginous pegs (one on each side) on the  inside rim of the of the mantle slot in to clips on the upper margin of the funnel. These lock the mantle closed as it contracts, which forces the water to squirt out through the funnel in a jet.

The funnel-locking apparatus of a flying squid, O. bartramii. The peg on the right is found on the inside rim of the mantle and fits inside the clip on the left, which is found at the top of the funnel. They clip together to lock the mantle opening shut and help to direct water through the funnel. The tissue in the image has been stained blue (photo R. Young).

Squid must achieve quite high speeds when using their jet propulsion system to exit the water. And the squid continue to shoot water out through the funnel to rocket through the air. Travelling at this speed is likely to have a metabolic cost because oxygen demands increase exponentially with increasing swimming speeds¹. I suspect that, although jetting through the air is more efficient than putting the same effort into swimming, it’ll be more energetically efficient to travel at speeds that are too slow for flight. The effort required for flight may explain why squid are rarely seen jumping and have never been observed making repeated jumps. 

Jet propelled squid (probably Sthenoteuthis pteropus). Note the trails of water behind the squid, squirted from their funnels (photo Bob Hulse).

Another paper² that's just come out shows a whole new group of swimmers are able to fly too. The animal part of the plankton, or zooplankton, is made up in large part by tiny crustaceans called copepods. Copepods swim by beating elongated antennae like oars, which moves them through the water in a typically jerky fashion. With just a single swimming stroke some copepods are able to exit the water and travel up to 17 centimetres through the air. Not bad for an animal that's only a couple of millimetres long. 

Copepod crustaceans make up a large part of the zooplankton. They swim by beating their elongated antennae producing a typically jerky swimming motion.

Just like flying squid and flying fish, copepods can travel further through the air for the same effort than they can travel through the water. But, the swimming speeds required to exit the water are extremely high. Indeed, the study reports that before they leave the water the velocity of the copepods was higher than the previously documented maximum swimming speeds of similar sized copepods.

The really neat thing about this paper is that the authors were able to document why the copepods were jumping. And it was not because it’s a more efficient way of travelling. It’s a way of avoiding predators. The authors observed the jumping copepods in the wild and found that they jumped in response to approaching predatory fish. And it seems to be a pretty successful escape mechanism too. Of the 89 escape jumps they observed only one individual was eaten.

You can read more about the jumping copepods and see a video of them jumping on the ScienceNow website.

References:

1. Webber, D. M. & D'Or, R. K. (1986) Monitoring the metabolic rate and activity of free-swimming squid with telemetered jet pressure. Journal of Experimental Biology 126, 205 - 224

2. Gemmell, B. J., Jiang, H., Strickler, R. J., & Buskey, E. J. (2012) Plankton reach new heights in effort to avoid predators. Proceedings of the Royal Society B: Biological Sciences doi:10.1098/rspb.2012.0163