Prey selection, intake and, ultimately, the trophic impact of predators are determined by a succession of events that occur at the organismal level -- individual interactions among predators prey, and their environments. Furthermore, because the majority of predator-prey interactions occur in moving fluids, it is critical to observe and quantify predator-prey interactions within a hydrodynamic context. Successful predictions of trophic patterns in natural settings are limited by the ability to: 1) observe directly the effects of turbulence on feeding in pelagic organisms; 2) understand the mechanistic bases of animal-fluid interactions in turbulent environments; and 3) relate quantitative observations from still-water laboratory studies to nature. These limitations are pervasive in studies of trophic exchange within the larger scope of marine ecology.
Recent technological advances, and the combined expertise of the Co-PIs, enables meaningful studies of the influence of turbulence on feeding by the notoriously invasive lobate ctenophore, Mnemiopsis leidyi. Mnemiopsis is a delicate gelatinous predator which uses a laminar feeding current to entrain and capture prey. Using a remarkably effective feeding strategy, zooplankton standing stocks and overall zooplankton biodiversity are reduced, and standing stocks of phytoplankton are increased via a trophic cascade. Like many suspension feeders, however, the feeding current produced by Mnemiopsis may be vulnerable to hydrodynamic disruption by ambient flows. In fact, turbulent events may change the behavior, distribution and prey selection of lobate ctenophores such as Mnemiopsis. This species is an ideal model organism to determine the mechanisms by which turbulence affects trophic exchange patterns of ecologically influential planktonic suspension feeders.
Involving a combination of laboratory and in situ methods to quantify, at the organismal level, this study will determine effects of turbulent flows on the feeding mechanics and predator-prey interactions of Mnemiopsis. Understanding how these turbulent effects translate to the community level will be accomplished via in situ sampling techniques that relate natural turbulence levels to ingestion rates, prey selection and predatory impact of Mnemiopsis in the field. This approach extends beyond current laboratory and modeling studies, with the potential of establishing clear cause-and-effect relationships.
Intellectual Merit: This research will: 1) directly quantify turbulent effects on in situ predator-prey interactions; 2) provide mechanistic understanding of key variables influencing the ecological impact of an important invasive marine species; and 3) develop a novel approach for studying small-scale physical-biological interactions both in the laboratory and in the field.
Knowing how turbulence affects feeding in lobate ctenophores is valuable at the scale of the organism, as well as ecologically. The approach developed here also may be applied to a variety of other turbulence-dominated situations (e.g., mixing at fronts, animal-marine snow interactions) or to other organisms (other plankton, benthic-water column exchanges). In all cases, the outcomes depend upon small-scale physical-biological processes.
Broader Impacts: Undergraduates (5), the graduate student, and the Co-PIs will work as a team in both the field and the laboratory, providing all participants with experience in every aspect of the research. The participation of underrepresented undergraduates will be facilitated through a program at Caltech (Freshmen Summer Institute Research Program) aimed at providing research opportunities to minority students from campuses across the nation. The Caltech Co-PI will continue his role as faculty advisor to this program. An invasive species that can dramatically affect the food chain within semi-enclosed bodies of water, Mnemiopsis leidyi is the focus of broad international interest. Remediation has been the subject of ongoing discussions (and experiments); therefore, results of this research will be communicated to the international scientific community in a timely fashion. In addition, contacts with media (e.g., PBS Shape of Life series, Fantastic Jellies exhibit at the New England Aquarium) involved in scientific education of the general public will be used to convey new findings.
The major activities of this project included laboratory-scale tests of turbulence effects on Mnemiopsis, a voracious predator with significant trophic impacts; real-time measurements of prey capture; and development of technologies for field measurements of the salient biophysical processes. The specific objective of the laboratory studies was to determine the behavioral response of Mnemiopsis to increasing levels of turbulence. This was accomplished with a new turbulence generator designed for this project. The turbulence generator created a spatial gradient of turbulence, with maximum levels at the top of the tank and lower levels at depth. This provided a more realistic refuge from turbulenence as compared to previous studies which did not facilitate refuge behaviors. A specific objective of the field studies was to develop a new technology to enable daytime particle image velocimetry measurements of the animal-fluid interactions. This was accomplished using a novel optical filtering technique to enhance a recently developed self-contained underwater velocimetry apparatus (SCUVA). We observed behavioral responses of Mnemiopsis to realistic turbulence in the lab for the first time. Companion measurements of clearance rates linked those behaviors to the trophic impact. In particular, the animals are found to adjust their swimming pattern to compensate for prey loss due to turbulence. We also successfully demonstrated the daytime SCUVA technology on deployments at the Marine Biological Labs in Woods Hole, MA. The key outcomes of this project are the elucidation of the mechanism whereby this invasive predator is able to adjust to environmental factors such as turbulence to affect the populations of its prey. The observed limits to the ability to Mnemiopsis to make such adjustments suggests corresponding limits on its environmental impact. The development of the daytime SCUVA technology will feed into ongoing efforts to develop lab-in-the-ocean technologies to observe a broad range of processes relevant to biological oceanography.
