There is an interesting blog over on discovermagazine.com about the way sea walnuts (or ctenophores, or Mnemiopsis leidyi) feed (in addition to a cool video, which is posted below). Apparently, these organisms use their cilia to create almost undetectable currents, and they are then capable of catching unsuspecting prey with great efficiency. Due to their incredible ability to feed stealthily and efficiently, they have been particularly devastating invaders in European water bodies. When these comb jellies showed up in the Black Sea, they contributed to a food web collapse by consuming many of the fish larvae that would typically serve as the base of the food chain. In fact, gelatinous zooplankton are often considered productivity dead-ends; they consume productivity in the forms of other plankton, however, they offer little food value to other species. So the productivity is not transferred to other trophic levels, and food webs collapse. This is also becoming a problem in human impacted systems.
This blog made me remember some research some colleagues at the School of Marine and Atmospheric Science conducted. PhD student Marianne McNamara, under the tutelage of Darcy Lonsdale, investigated the impact of high abundances of ctenophores on larval bivalve mortality.
In their article “Shifting abundance of the ctenophore Mnemiopsis leidyi and the implications for larval bivalve mortality,” published earlier this year in Marine Biology, McNamara et al investigated how ctenophore abundance has changed, their digestion rates, and finally, their ability to control bivalve larvae. The data from this article is of particular importance for the hard clam restoration and management effort in Great South Bay, NY (their field sites), since the comb jellies may exert a strong predation pressure on hard clam larvae.
They conducted field surveys to investigate the abundance of ctenophores and other zooplankton. They enumerated and took volumetric measurements of the comb jellies, then looked at their gut contents. Finally, they conducted lab feeding experiments, and then used equations to calculate their ability to control bivalve larvae.
McNamara et al found high densities of ctenophores in the early summer, and larger ctenophores in the late summer, and when compared to the literature, densities were considerably higher than in previous decades. This is of particular importance, since bivalve veligers made up approximately 63% of the ctenophores’ gut contents, indicating this is a particularly valuable food source for the jellies. In addition, using their equations from densities and feeding rates, they predicted that at peak abundances, the ctenophores could consume over 94% of the bivalve veligers in Great South Bay. This is a particularly alarming figure. In addition, the peak abundances of ctenophores occurs earlier in the year (early summer) now than it did decades ago (in the fall), putting peak abundances of comb jellies in the water column at the same time as the bivalve larvae.
Clearly, this study illustrates the potential ecosystem impacts of increasing gelatinous zooplankton. While they have already been shown to be particularly harmful as invaders, it is now apparent that they can have impacts where they are native as well. It is likely that increasing human impacts leading to pelagic dominated production will lead to more ctenophores in coastal systems, which can prevent benthos from reestablishing in these areas. This might be the case in Great South Bay, where the hard clam populations are struggling to recover despite the Nature Conservancy’s efforts at replenishing them. Now I don’t know about their high end estimates, as one could imagine if ctenophores were capable of consuming essentially all of the bivalve veligers, then veligers and comb jellies wouldn’t be collected together in plankton tows. However, it is clear that ctenophores can possibly have a major impact on a local ecosystem.
McNamara, M., Lonsdale, D., & Cerrato, R. (2009). Shifting abundance of the ctenophore Mnemiopsis leidyi and the implications for larval bivalve mortality Marine Biology, 157 (2), 401-412 DOI: 10.1007/s00227-009-1327-6