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

Waterfall climbing fish

Diadromous fish are those that live part of their lives at sea and part of their lives if freshwater. Some of these fish reproduce in the upper parts of rivers above barriers like waterfalls, which they must scale in order to make it to the breeding sites. A newly published paper looks at how the Nopili goby, Sicyopterus stimpsoni, manages to climb waterfalls. The researchers found that the way the goby feeds and the way it climbs are very similar.

The Nopili goby, Sicyopterus stimpsoni (photo Takashi Maie)

During feeding the Nopili goby extends its upper jaw out much further and its lower jaw much less than other gobies. In climbing the basic motion is the same except the upper jaw maintains closer contact with the rock. Climbing is also assisted by pelvic fins fused into a sucker, a feature of all gobies. Because no other goby feeds in the same way, it's unclear whether the feeding or climbing movements evolved first.

The climbing galaxias, Galaxias brevipinnis (photo Robert McCormack)

There are many other fish that have a diadromous life-history, eels and salmon being the classic examples. There are fewer fish that climb waterfalls. However, in southern Australia and New Zealand there is a fish close to my heart that has a very similar life-history to the Nopili goby, but it climbs waterfalls in a different way. The climbing galaxias, Galaxias brevipinnis, climbs using its broad pectoral and pelvic fins and wiggling upwards.

Cullen J. A., Maie T., Schoenfuss H. L., & Blob R. W. (2013). Evolutionary Novelty versus Exaptation: Oral Kinematics in Feeding versus Climbing in the Waterfall-Climbing Hawaiian Goby Sicyopterus stimpsoni PLOS One, 8 (1) DOI: 10.1371/journal.pone.0053274

Rapid speciation in starfish

Australian waters are extremely rich in starfish species. Indeed, a little over 15% of all known species of starfish occur in Australia. For at least two of these starfish, speciation occurred extraordinarily fast. At most, they became separated about 22 thousand years ago, but the best estimate for the timing of the split is about 6 thousand years ago.

We know that evolution can be very rapid (e.g. sticklebacks) and that sometimes this leads to speciation (e.g. cichlids). But, in these cases selection is probably acting on a small number of alleles that are already present in the population. What makes the starfish study so breathtaking is that there has been profound changes to life history in the two species, which likely involved selection on many morphological and physiological traits.

Puritz et al. looked at Cryptasterina pentagona and its sister species C. hystria. Like most starfish, C. pentagona has separate sexes and reproduces by 'broadcasting' sperm and eggs into the water column where fertilisation occurs. In stark contrast, C. hystria produces both sperm and eggs simultaneously, and it exclusively self-fertilises within its own body cavity. The embryos of C. pentagona develop in the plankton, while C. hystria broods its offspring within the gonad until they are ready to emerge as small starfish.

It takes an expert to distinguish Cryptasterina hystria (top) and C. pentagona (bottom) in the wild. In fact I've seen the bottom picture shown as C. hystria and C. pentagona, but I think I got it right (photo Jon Puritz).

Puritz et al. speculate that water temperature may have provided the selective pressure that favoured the evolution of the C. hystria life history. Viviparity, like that seen in C. hystria, has been documented in a number of other starfish species. And it is consistently associated with species that occur in colder water. The two Cryptasterina species are separated by about 375 kilometers, with C. pentagona in the warmer north and C. hystria to the cooler south.

The authors also argue that small population size may have selected for self-fertilisation. If there are so few individuals in the population that your gametes are unlikely to meet another individual's, it's better to fertilise your own than to not reproduce at all. It's expected that genetic variation in a population that self-fertilises should be very low. But, genetic variation in C. hystria is so low it suggests the whole species derived from very few individuals, perhaps just a single one.


The transition from broadcast spawning with planktonic larval development to self-fertilisation with larvae brooded within the gonad has occurred in another Cryptasterina species, C. pacifica. In the closely related genus Parvulastra, a similar transition has occurred too, but probably over 500 thousand years. This suggests that the genetic variation required for the dramatic shift in life history is widely present in the group of starfish to which the genera Cryptasterina and Parvulastra belong. But, the speed at which evolution has occurred is truly astonishing.

Parvulastra exigua, note its similarity to the Cryptasterina species (photo Museum Victoria).

Puritz JB, Keever CC, Addison JA, Byrne M, Hart MW, Grosberg RK, & Toonen RJ (2012). Extraordinarily rapid life-history divergence between Cryptasterina sea star species. Proceedings. Biological sciences / The Royal Society, 279 (1744), 3914-3922 PMID: 22810427

Hermit crab migration

On Tuesday I wrote about a crustacean with an unusual larval form. The larvae occur in mid-water while the adults occur in the deep sea. Habitat shifts associated with changes in life history stage are not uncommon and some of them are even more dramatic than changing depth. Take the hermit crabs in the Virgin Islands. The adults are terrestrial, but the larvae develop in the sea. The adults must migrate to the waters edge in order to reproduce and release their eggs into the water.

Hermit Crab Migration from Steve Simonsen on Vimeo.

Hermit crabs are not true crabs. True crabs are in the crustacean infraorder Brachyura, while hermit crabs are in the infraorder Anomura. There are Brachyurans that perform similar mass migrations to release their eggs into the sea. One of the best known is the migration of the Christmas Island crab.

If the eggs and larvae survive out in the ocean, tiny juveniles climb back onto land and migrate into the forests where they grow into adults. In most years, only a very small number will make it back to land. In some rare years, the numbers that arrive on the shore are mind-boggling.

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