Alvin

The Deep Submergence Vehicle Alvin is the best known marine research vessel. It was commissioned by the United States Navy in 1964, but it calls Woods Hole Oceanographic Institution home. For its 49th birthday, it's received a refit that will increase its dive range by two kilometers and give the people inside more room and greater vision. Unfortunately, due to the limitations of its batteries, Alvin won't be able to reach its rated depth for another few years. Lithium-ion batteries are considered to be too great a fire risk at the moment.

Alvin returning to the surface carrying samples (photo Wikipedia)

Alvin sprang to fame in 1977 when scientists inside it made the first observations of hydrothermal vent communities off the Galapagos Islands. These were the first communities of multicellular organisms ever discovered that were able to survive in isolation from the sun. To marine biologists, the discovery of hydrothermal vent communities was more exciting than the moon landings less than a decade earlier. And it was Alvin, like Apollo 11, that made it possible. Unlike Apollo 11, Alvin continues to make discoveries and has contributed more to marine science than any other vehicle.

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

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

A stepping stone of rotting wood

Many of the animals living at hydrothermal vents and cold seeps carry chemosynthetic bacterial symbionts in their body, which convert methane or hydrogen sulfide into food. Some have lost the ability to feed on anything other than what the bacteria living inside their tissues provide them. Almost all cannot survive without a sufficient supply of methane or hydrogen sulfide. One hypothesis is that decomposing organic matter that has sunk from the surface, like whale carcasses, seaweed, and wood could serve as a food source, providing stepping stones between vents or seeps.

A field of mussels at a cold seep (photo Wikipedia)

Animals more typically found at vents and seeps are known to colonise the remains of whales in the deep sea. Smith et al. (1989), were the first to report vent animals colonising whale skeletons. They also provided a conservative estimate of the distribution of whale carcasses on the ocean floor and suggested that they would provide a persistent and abundant habitat for cold seep and hydrothermal vent animals. Adding sunken wood and seaweed to the list only increases the amount of available habitat.

A whale skeleton in the deep sea with patrolling hagfish (photo Wikipedia)

A gap in our understanding, though, is how enough methane or hydrogen sulfide is produced to support populations of chemosynthetic animals by decaying wood. The deep sea is a cold place, which is not conducive to the rapid breakdown of organic material. A large amount of wood would be required to provide the surface area necessary to produce enough gas. One hypothesis though, is that the surface area of wood might be increased by larger organisms, such as wood-boring bivalves, breaking it up first.

Researchers tested this idea by depositing wood logs on the Eastern Mediterranean seafloor at 1700 meters down and returned a year later to examine the animals and bacteria the had colonised the wood. They also measured the chemicals in the water released by the bacteria breaking down the wood. Using underwater robots, they observed that wood-boring bivalves had indeed broken the wood into smaller pieces, which were further broken down by other organisms. 

The activity of the organisms digesting the wood reduced the amount of dissolved oxygen, resulting in anoxic conditions that allowed sulfate-reducing bacteria to move in and produce hydrogen sulfide. The hydrogen sulfide then attracted a species of mussel, which usually found at cold seeps where it gains energy from symbiotic chemosynthetic bacteria. The mussels seemed to preferentially colonise cavities under the bark of the wood, presumably because sulfide levels were higher there.

The chemosynthetic mussel Idas modiolaeformis was found in the sunken wood piles (photo from Bienhold et al. 2013)

So it appears that wood boring organisms are able to pave the way for chemosynthetic organisms to colonise sunken wood in the deep sea. Their burrows, feces and the chips of wood that they produce all increase the surface area of material available for hydrogen sulfide producing bacteria to digest. Moreover, their activity and the activity of other organisms produce the anoxic conditions required for sulfate reduction, which is necessary to support chemosynthetic life.

A hypothetical succession of animals on submerged wood in the deep sea over a year. Initially wood-boring bivalves move in, followed by predators and detritus feeders (e.g. polychaetes and sipunculids). The respiration of the colonisers creates anoxic niches that allow the chemosynthetic mussels to move in (diagram from Bienhold et al. 2013).

Smith, C., Kukert, H., Wheatcroft, R., Jumars, P., & Deming, J. (1989). Vent fauna on whale remains Nature, 341 (6237), 27-28 DOI: 10.1038/341027a0  

Bienhold, C., Pop Ristova, P., Wenzhöfer, F., Dittmar, T., & Boetius, A. (2013). How Deep-Sea Wood Falls Sustain Chemosynthetic Life PLoS ONE, 8 (1) DOI: 10.1371/journal.pone.0053590

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!...

Everyone loves bacon

Researchers at the VENUS Observatory put a pig carcass in 300 meters of water and watched to see what came to eat it. Mostly amphipod crustaceans, it would seem.

Results vary only slightly when the carcass is not in a cage. The wounds that appear early in the video are caused by sharks, I'm pretty sure.

It's Yoda, but not as you know him

A new species of acorn worm has been named after Jedi Master Yoda, the best character in the Star Wars trilogy*. Acorn worms are not true worms. They are more closely related to echinoderms (starfish, sea urchins, sea cucumbers, etc.) than they are to worms. They were once placed as a subphylum of the chordata (i.e. our own phylum), but are now placed within their own phylum, the hemichordata.

Yoda purpurata, the newly described species of acorn worm

The paper described three new species of deep-sea acorn worms in the family Torquaratoridae. Two of which, Allapasus isidis and Tergivelum cinnabarinum, were from previously known genera. But, Yoda purpurata is a new genus and species. It's named after Yoda because the appendages at the head end of the animal are reminiscent of Yoda's ears. All three species were found at about 2.5 kilometers deep on the mid-Atlantic ridge.

*To count as a true Star Wars film, it can't just carry the name. You also have to be able to sit through it without wanting to punch George Lucas. This caveat leaves just three films that can be considered part of the Star Wars canon. And these three films are the originals, not the remakes. 

Priede, I G, Osborn, K J, Gebruk, A V, Jones, D, Shale, D, Rogacheva, A, & Holland, N D (2012). Observations on torquaratorid acorn worms (Hemichordata, Enteropneusta) from the North Atlantic with descriptions of a new genus and three new species Invertebrate Biology, 131 (3), 244-257 DOI: 10.1111/j.1744-7410.2012.00266.x

Vampire squid

Ed Yong is a science writer who's work I often enjoy reading. He has an interesting piece about vampire squid, Vampyroteuthis infernalis, that he has written for his blog "Not Exactly Rocket Science". Vampire squid are not actually squid, although they are related to squid and octopus. They are classified in their own order of cephalopods, the Vampyromorphida, in which they are the only known extant species.

An unusual crustacean meets its parents

Many animals living in the ocean have complex life histories where the young look nothing like the adults and often occupy different habitats. Frogs, with their early tadpole stage, are classic examples of animals with complex life histories. But, tadpoles look far more like frogs than the larvae of other animals resemble their adult forms. Different species of distantly related crustacean larvae, for instance, can look far more like each other than they resemble the adults of their own species.

The nuaplius larvae stage (left) of a cylopoid copepod (top) and penaeid shrimp (bottom) and their adult forms (right). These distantly related crustaceans appear similar as larvae, but not as adults (images Wikipedia)

Because larvae look so different from the adult form, identifying the species that larvae belong to can be tricky. Often it requires the larvae to be carefully reared in the laboratory to see what they eventually turn into, but this isn't always possible. In some cases, it is possible to place larvae within a species using genetic techniques, but this requires a DNA sequence from the adult to compare to.

One type of crustacean larva that has been difficult to assign to an adult form are the Cerataspids. There are three known species that have been placed in two genera, Cerataspis monstrosa, C. petiti and Cerataspides longiremus. Like many unusual crustacean larvae, the first to be discovered (C. monstrosa in 1828) was thought to be an adult of the crustacean order Leptostraca. However, it later became apparent that they were probably larvae of shrimp within the Penaeoid superfamily, possibly from the family Aristeidae.

The crustacean larva Cerataspis monstrosa (image from Bracken-Grissom et al. 2012)

Through a combination of skill and luck, Bracken-Grissom et al. were able to resolve the adult identity of C. monstrosa. The luck involved getting their hands on a specimen of the larva that was suitable for DNA analysis. Almost all we know about C. monstrosa comes from examining specimens in the gut contents of its predators, like skipjack tuna. But, Bracken-Grissom et al. unexpectedly obtained a single specimen from a trawl at a depth of 420 meters in the Gulf of Mexico.

Bracken-Grissom et al. collected DNA sequence data from the specimen and compared it to sequences of crustaceans available from a database of genetic sequences. Because C. monstrosa had been liked with Penaeoid shrimp in the family Aristeidae, they concentrated their analysis within those taxonomic groups. And they hit pay dirt. The DNA from the C. monstrosa specimen was a near perfect match with a deep-sea Aristeid shrimp Plesiopenaeus armatus.

The Aristeid shrimp Plesiopenaeus armatus (image from Bracken-Grissom et al. 2012)

Plesiopenaeus armatus has a similar geographic distribution to C. monstrosa, but it is known from deeper water. The contrast between the larval habitat and  adult habitat is not unusual for organisms with complex life histories. Indeed, many complex life histories involve much more dramatic habitat shifts. However, the transition from mid-water pelagic larvae to abyssal adults is not known in many species.

The findings of Bracken-Grissom et al. have implications for the other species of Cerataspis larvae that haven't been linked to their adult form. They suggest that C. petiti is the larva of the only other known species in the genus Plesiopenaeus, P. coruscans. Further they suggest that Cerataspides longiremus is the laval stage of a closely related Aristeid shrimp, possibly an unidentified representative of the same genus.

Bracken-Grissom HD, Felder DL, Vollmer NL, Martin JW, & Crandall KA (2012). Phylogenetics links monster larva to deep-sea shrimp Ecology and Evolution DOI: 10.1002/ece3.347

Playing Gaia

Phytoplankton doesn't need much to keep it happy. Just some sunlight and a few important macronutrients, such as nitrate, phosphate and silicic acid. However, in some areas of the ocean the abundance of phytoplankton is substantially lower than would be expected from the plentiful supply of nutrients. Such areas are known as "high-nutrient, low-cholorophyll" (HNLC) regions and make up 20% of the ocean surface.

Diatoms make up a large part of the phytoplankton (photo Wikipedia)

A feature often associated with HNLC regions is the low bioavailability of iron. Iron is critical for phytoplankton as it is required to make chlorophyll and important enzymes like nitrate reductase. It has, therefore, been suggested that iron bioavailability is the most important factor limiting phytoplankton population size in HNLC regions. But, zooplankton grazing and deep mixing of surface water that reduces the time that phytoplankton has access to light have also been hypothesised.

Progression of the largest recorded natural phytoplankton bloom in the Southern Ocean, which occurred earlier this year. It was thought to be caused by the addition of iron blown into the sea by strong offshore winds. At its peak, it measured 100 km north-south and 200 km east-west (images Jan Lieser & NASA Terra Modis).

A study, published in Nature, investigated the effect of adding iron to the water on phytoplankton abundance. It was not the first experiment to do so, there have been several. But, unlike the others, this experiment showed that much of the dead phytoplankton was transported into the deep sea. Which is important for our understanding of the role that the ocean plays in regulating the climate.

A map showing the location of iron fertilization experiments. The experiment published in Nature is the EIFEX study, which is the second from the right (image taken from Baar et al. 2004).

During photosynthesis, phytoplankton consumes carbonic acid, which is formed when carbon dioxide dissolves into water. If sufficient quantities of the carbon that the phytoplankton take up can be transported into the deep ocean, away from the atmosphere, it can effectively reduce the amount of carbon dioxide in the atmosphere, cooling the planet. Indeed, phytoplankton blooms may have played a role in triggering past ice ages.

The study recorded phytoplankton abundance and particles falling beneath the bloom at various depths for 37 days, capturing the rise and fall of the bloom. The bloom peaked after 24 days; chlorophyll concentrations was almost four times those at control sites and the bloom covered nearly 800 square kilometers. As the bloom collapsed, at least half of the biomass sank below 1,000 meters, most of which is likely to have reached the seafloor.

Such evidence indicates that at larger scales and over longer periods, algal blooms could transport climatically significant amounts of carbon into the deep sea. In surface waters, the carbon could be returned to the atmosphere in a few months. But, in deep sea sediments the carbon might not reach the atmosphere for centuries. However, more work needs to be done to confirm the effect of phytoplankton blooms on climate.

A surprising finding of the study was that light availability did not seem to effect the plankton bloom as much as might of been expected. Chlorophyll concentrations remained high down to 100 meters where light availability should limit phytoplankton abundance. Indeed, the results showed that chlorophyll concentrations were similar or higher than those recorded at shallower depths in previous iron fertilization experiments.

Reference:

Victor Smetacek, Christine Klaas, Volker H. Strass, Philipp Assmy, Marina Montresor, Boris Cisewski, Nicolas Savoye, Adrian Webb, Francesco d’Ovidio, Jesus M. Arrieta, Ulrich Bathmann, Richard Bellerby, Gry Mine Berg, Peter Croot, Santiago Gonzalez, Joachim Henjes, Gerhard J. Herndl, Linn J.Hoffmann, Harry Leach, Martin Losch, Matthew M. Mills, Craig Neill, Ilka Peeken, Rudiger Rottgers, Oliver Sachs, Eberhard Sauter, Maike M. Schmidt, Jill Schwarz, Anja Terbruggen, & Dieter Wolf-Gladrow (2012). Deep carbon export from a Southern Ocean iron-fertilized diatom bloom Nature, 487 (7407), 313-319 DOI: 10.1038/nature11229

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

Tweets in the deep

When the intrepid adventurers in the Lord of the Rings reached Moria they heard drums in the deep. Now James Cameron has reached the oceans deepest point, the Challenger Deep in the Marianna Trench off Guam. I'm not sure if he heard drums, I suspect he didn't, but he's sent a tweet:

Just arrived at the ocean's deepest pt. Hitting bottom never felt so good. Can't wait to share what I'm seeing w/ you.

I'll bet he can't wait because he's going to charge you money to "share" what he's seeing with you. He has plans to release two documentaries with the footage he collects.


It's the first time since 1960 that anyone has been to the Challenger Deep. At 10, 898 meters below the surface, it's an impressive feat. Nice work James. And nice work Australian engineering team who built the submersible he piloted. Now how about lending me the keys?...

They came from the deep

Polychaetes are annelid worms that are mostly found in marine environments. Earth worms are annelids too, but they're oligochaetes. The oligochaetes are mostly found in freshwater and terrestrial systems.

The major differences between polychaetes and oligochaetes that can be used to distinguish them are the parapodia and chaetae. Polychaetes have them and oligochaetes don't. However, some aquatic oligochaetes have structures that look very similar to parapodia, and the parapodia and chaetae are hard to find on some polychaetes. There are exceptions to every rule!

Cross section of a polychaete showing a typical body plan.

Parapodia are fleshy outgrowths that are generally used for locomotion. In polychaetes the parapodia divide into upper and lower lobes. The upper lobe is called the notopod (pl. notopodia) and the lower lobe is called the neuropod (pl. neuropodia). Out of the lobes grow the chaetae, which are bristle-like in appearance. It's the many chaetae that give polychaetes their name.

Recently, the Telegraph newspaper published some electron micrograph images of some polychaetes that were collected from a hydrothermal vent. They make the already alien appearance of the worms even more strange. The false colour (EM images are black and white) does not help things.

Portrait of a polychaete. I'm not sure, but I think it's a Nereid polychaete (rag worm).

Another polychaete. This one is a Polynoid polychaete (scale worm). I wonder if it's predatory?

The reporting of these images has been pretty poor. Stories I have seen make it sound like these worms are only found in the deep-sea, which is not true. And that they eat the bacteria at hydrothermal vents, which is also not true. The images are of worms collected from deep hydrothermal vents, but there are many shallow water examples of both rag worms and scale worms. And both groups are generally predatory.

Rag worms and scale worms feed by rapidly everting their pharynx, which has some hardened mandibles attached. Prey that are caught in the jaws are dragged back into the mouth. In the upper image the rag worm still has its jaws hidden on the inside, while the scale worm in the lower image has partially exposed its jaws.

The everted jaws of Perinereis cultrifera, a Nereid polychaete (photo estran 22)

The other thing that was reported badly in some places involves the rag worm image. The structures that are coloured in a glowing pink are not eyes, they're sensory palps. They function as organs of smell, not sight. They eyes are on the upper surface of the animal and the image is of the underside. 

The head of a polychaete. The eyes are the four black spots at the left-hand end. The pharynx can be seen on the inside of the transparent body (photo Wim van Egmond).