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