New Discovery Links Larval-Adult Forms of a Marine Crustacean
To be honest, Cerataspis monstrosa looks like a reject from a B-level alien movie, and the history of this larval crustacean is concordantly undignified on several levels. The name itself speaks volumes—C. monstrosa will never compete with the panda for its position as a top iconic charismatic species. It is strikingly asymmetrical and bears a thick horny carapace that is incongruously fringed by delicate looking tentacles. Secondly, C. monstrosa bears the dubious distinction of being first discovered within the entrails of dolphin corpses. Finally, despite being recorded over 180 years ago, until very recently scientists had no idea what an adult form of this animal actually looked like. The larval forms occasionally showed up inside dead fish or dolphins, but because of their rarity and submarine lifestyle, no one had ever actually been able to determine what kind of animal such a “monster larva” could possible become.
It is not uncommon for invertebrates, both terrestrial and marine, to have dramatically different appearances across successive life stages. If you didn’t know any better, would you see a grub-like caterpillar and think “Oh, a baby butterfly”? Unlikely. Despite such dramatic changes in appearance, most species have been sufficiently studied—or are sufficiently similar to related, better-studied species—that we have a fairly good idea of how their metamorphosis proceeds, and what the “before” and “after” pictures look like.
Not so for the enigmatic C. monstrosa, until this year. After nearly two centuries of mystery, a research team led by George Washington University’s Dr. Keith Crandall has finally deduced what becomes of C. monstrosa when it escapes predation by marine predators and actually achieves maturity.The discovery fill s a significant gap in our knowledge of this organism, and their results were reported in a recent issue of the journal Ecology & Evolution.
The team was well-equipped to solve the C. monstrosa mystery: they have compiled an extensive library of crustacean DNA and had been lucky enough to score a rare specimen of the larva during a collecting trip in the Gulf of Mexico. They also had a lead: previous studies had shown that C. monstrosa may be related to “shrimp-like decapods,” and speculation that the larva may be genetically allied with penaeoid shrimp has circulated for years.
The team sequenced 4,136 base pairs covering five different genes from their prized C. monstrosa specimen and compared these to corresponding sequences from penaeoid shrimp in their database. The results were clear: genes from their mystery organism aligned perfectly with Plesiopenaeus armatus, a marine shrimp that is also known to overlap with C. monstrosa geographically. The adult form of this animal had been under our metaphorical noses for centuries, but its appearance is so distinct from its larvae that it took molecular techniques to link the two. The “monstrous” larva finally has a family. Not only that, but two other categories of Cerataspis, C. petiti and Cerataspides (formerly classified as Cerataspis) longiremus were tentatively linked to other species of Plesiopenaeius, suggesting that what were considered to be six enigmatic species have now become three species for which full life cycles are known.
Why all the excitement, beyond solving a centuries-long puzzle? Linking the larval and adult forms of an animal carries significant ecological and evolutionary implications. Mapping the entire life cycle of an organism allows for a better understanding of its function in the food web, distribution patterns, evolutionary history and population dynamics in response to both biotic and abiotic components of its ecosystem. Resulting data can be useful for predicting future distribution shifts, which is important for a species that is apparently a food source for many fishes and some marine mammals.
One preliminary insight from this case is that while larval forms appear to exist in pelagic zones, the adults are abyssal, spending their time in some of the deepest depths of the ocean. That means that this species shifts between distinctly different habitats and ecological communities as it matures and could be a link facilitating feedback responses to conditions from one zone to the next. With oceanic temperature and chemistry currently in danger of severe alterations due to global climate change, tracking such “link” species may provide critical data regarding the health and function of marine communities.
Anne-Marie Hodge, EGN’s Species Science Advisor, is currently a PhD student at the University of Wyoming in Ecology, and holds a Bachelor of Science in Zoology from Auburn University and a Master of Science in Biology from the University of North Carolina-Wilmington. While at Auburn, she established and served as president of Alabama’s first chapter of the Society for Conservation Biology. Anne-Marie’s research interests include community ecology, predator-prey dynamics, and conservation, with a focus on tropical carnivores. She has research experience in Mexico, Belize, Ecuador, and Kenya.