Studies of the aggregative
behavior of zooplankton

The object of this study is to understand the adaptive value of copepod swarming behavior and mysid schooling behavior, and to investigate the roles of various sensory systems in mediating these behaviors. Most of the field work for this project will be carried out at the Smithsonian Institution’s field laboratory on Carrie Bow Cay in Belize, Central America. 

Smithsonian Institution's field station at Carrie Bow Cay.

Studies to date have focused on the swarm-forming copepod, Dioithona oculata. 

Adult female Dioithona oculata with egg sacs.

These copepods form dense swarms between the prop roots of the red mangroves growing on the edges of cays on the barrier reef of Belize. 

Red mangroves on Twin Cays, Belize. 

Below water, the mangrove prop roots form a complex ecosystem inhabited by a diverse community of algae, invertebrates and fish. Swarms of Dioithona form in shafts of sunlight that penetrate the otherwise shaded habitat around the mangrove prop roots. Densities within these swarms may be as high as 90 copepods·mL-1.

Dioithona oculata swarm next to encrusted prop root.

Predatory fish on the fringes of the mangrove prop root habitat keep schools of planktivorous fish away from the swarms between the prop roots.

Barracuda on mangrove habitat fringe.

We use underwater video techniques to film the behavior of swarming copepods in nature...

Underwater video system for studies of copepod swarms.

…and perform laboratory studies of their behavior at the field station on Carrie Bow Cay.

Experimental setup for laboratory studies of zooplankton behavior.

Animals are observed and recorded under controlled conditions in the laboratory at Carrie Bow Cay. Video tapes are analyzed back at the Marine Science Institute in Port Aransas, TX where swimming behavior of these zooplankton is quantified using a video-computer system for motion analysis. 

 
Mangrove encrusting sponges and other invertebrates of Carrie Bow Cay, Belize. 

Zooplankton researchers at Carrie Bow Cay (left to right: Dr. Julie Ambler, Millersville University; Matt Kracht, undergraduate-Millersville University; Steve Hays, Smithsonian Institution; Jay Peterson, graduate student-University of Texas; Dr. Ed Buskey, University of Texas).

Information regarding admissions and other information on the graduate program at The University of Texas Marine Science Institute can be found on the Graduate Degree Program section of this web site. 

A more detailed summary of the research project follows.

NSF PROPOSAL PROJECT SUMMARY 

Zooplankton aggregations such as swarms and schools occur on scales ranging from centimeters to meters (and greater) and have important implications for trophic dynamics in the sea. The formation and maintenance of these aggregations has a strong behavioral component, although hydrodynamics also play an important role in the location and nature of these aggregations. In mangrove prop root habitats there are two excellent examples of aggregative behavior in zooplankton: the swarming behavior of the copepod Dioithona oculata and the schooling behavior of the mysid Mysidium columbiae. Previous studies have examined the swarming behavior of D. oculata in situ, studied the role of photoreception in swarm formation and maintenance and examined the metabolic costs of swarming behavior. This study will focus on several unanswered questions regarding copepod swarms and begin studies of schooling behavior of mysids. 

The swarming behavior of D. oculata has a strong diel component; swarms form at dawn and disperse at dusk. During the night, currents disperse the copepods into adjacent channels and bays, up to tens of meters from the mangroves. One objective of this study is to determine the sensory information used by D. oculata to find its way back to the mangrove habitat at dawn. It is hypothesized that the copepods (whose eyes are not capable of image formation) use the angular distribution of light underwater to locate the horizon, and orient their swimming toward it. Another objective is to carefully investigate the value of swarming behavior as an anti-predation adaptation; this will be done for both visual predators (planktivorous fish) and non-visual predators (cubozoan medusae). D. oculata swarms are also found near coral reefs, where environmental and optical conditions are quite different from mangrove habitats. Swarms near coral reefs will be studied in situ, and the factors affecting swarm formation and maintenance in these environments will be quantified. It is thought that light reflected off the bottom is used as a swarm marker for copepods on coral reefs, but that hydrodynamics also plays an important role in swarm location, since swarms often form in the lee of coral heads. 

Mysidium columbiae is a holoplanktonic mysid that schools in shaded areas near mangroves. It's well developed compound eyes are thought to have limited visual acuity but highly developed movement perception; visual cues are thought to play an important role in schooling behavior. The role of vision in the schooling behavior of this species will be investigated by determining the visual threshold and visual acuity of this mysid. The adaptive value of schooling as an anti-predation device and metabolic costs of schooling behavior will also be investigated. 

Zooplankton aggregations such as copepod swarms and mysid schools represent one end of a continuum of plankton patchiness from those controlled mainly by the behavior of the organisms to those controlled mainly by physical concentrating mechanisms. By achieving a better understanding of the factors influencing behaviorally controlled zooplankton aggregations, the range of potential contributions of behavior to other forms of patchiness will become clearer. By understanding the adaptive value of swarming and schooling behavior in zooplankton, the potential effects of patchiness on plankton trophic dynamics will be revealed.