Many organisms exhibit central place foraging--an individual or group of individuals disperses rhythmically away from a fixed site or refuge to forage, then subsequently returns to the refuge. For example, nesting birds leave the nest to forage and, when sufficient resources have been collected, return to the nest to feed their young. Pack animals forage as a group in the area surrounding their habitual resting site. Social insects forage in the area surrounding the nest, returning to the nest with the collected resources. In the winter, many species of birds forage from roosts. Daily the birds leave the roost to forage in the surrounding country, then return to the roost at night. Hamilton and Watt (1970) considered roosting birds to be an example of a central place system they called refuging (i.e., numerous adult individuals gather in the refuge from which they disperse to forage). They suggested that "Refuging systems containing large numbers of individuals may have highly complex communication systems and cooperative behavior patterns. An understanding of these more complex systems must include comparative and quantitative analyses of simpler systems." Roosting birds are a difficult system to investigate and few of the predictions from Hamilton’s model on refuging could be tested; the system was too complex. We propose to study a much simpler refuging system--clustering in the intertidal limpet Lottia digitalis--and using this system extensively, test the predictions from Hamilton’s refuging model. The study would not only provide a detailed test of this model, but will also--for the first time--apply central place foraging theory to a marine organism.

Limpets have great potential as model systems in which to study foraging and territoriality. Individuals are small (several cm in length), foraging ambits are short (usually < 0.3 m), their diet is simple (the algal film growing on hard substrates), they move slowly (< 3 mm/min), resource utilization is clear (radula scratch marks on the substrate) and they can be maintained in the lab in small enough areas to allow for replication. Through field and laboratory experimentation the proposed research would test whether the behaviors associated with central place foraging and refuging in Lottia digitalis match the model of refuging behavior developed by Hamilton and co-workers. Specifically, the research will test the following predictions from the model:

1 Resource depletion increases as the refuge is approached.
2 Resource exploitation (competition) decreases with distance from the refuge.
3 The energy intake experienced by an individual feeding at a distance from the refuge where resources are more abundant must balance the increased time and energy necessary to disperse this distance.
4 Aggression between individuals increases with either decreasing resource density or increasing number of individuals.
The proposed study would provide a comprehensive test of Hamilton’s detailed model of refuging behavior, a common behavior in both terrestrial and marine habitats. In addition, the work represents a rare attempt to use behavioral ecology and central place foraging theory to better understand an intertidal organism.

We investigated the role of nearshore topographically generated circulation on the cross-shelf dispersal of larvae. The study focused on sites along the southern Oregon coast. In summer, persistent convergences (‘fronts’) are found at the mouth of many small bays and coves. CTD transects across the fronts demonstrated that waters landward of the front were usually slightly warmer than offshore waters and, during upwelling, chlorophyll a concentrations were higher seaward of the front. These data suggest that some small bays are isolated from the coastal waters. Late stage barnacle nauplii were significantly more concentrated offshore of the front while the early stage nauplii tended to be more abundant within the bays. There were, however, no consistent differences in the concentration of barnacle cyprids across the front. These results suggest that fronts act as barriers to the shoreward dispersal of nauplii, but do not prevent the shoreward migration of cyprids. A range of larval stages of a phoronid and an anthozoan were both abundant in the bay waters, but were absent or very rare in waters seaward of the front suggesting that these larvae may not cross the front. This preliminary work suggests that secondary circulation affects the dispersal of larval invertebrates.

Along rocky shorelines, very nearshore oceanographic processes are virtually unknown, yet this is the first parcel of water larvae must face when they are spawned and the last parcel they must cross before they can settle at the shore. Nearshore currents could retain larvae near their release site causing high recruitment in the natal population generating relatively closed populations. Alternately, larvae could be flushed from the nearshore waters. This would probably lead to more extensive dispersal and populations that are more open. The many species go through their larval development well away from the coast. At the end of their pelagic phase these larvae must migrate back to the shore to settle. Flow patterns adjacent to the coast may prevent or aid this shoreward migration. By altering the pattern of larval dispersal and settlement, small-scale topographically generated circulation adjacent to the shore may significantly influence both the population and community ecology in intertidal and shallow subtidal habitats.

We are investigating the role of topographically generated fronts in the dispersal and settlement of larval invertebrates. Most of this work has focused on a front at Sunset Bay. The front was established only when winds were from the NW and waves were small. During the summer, the front was present 80% of the time and remained undisturbed for up to 3 weeks. The zooplankton community changed dramatically across the front. Larval settlement also was affected by the front. Mussel settlement was 10 X higher seaward of the front than landward mirroring their distribution in the plankton. Cyprids settled at higher numbers landward of the front (10 X), but pulses in settlement only occurred when the front broke down during downwelling events. Nearshore fronts strongly affect the dispersal and settlement of larval invertebrates. We have made steady progress in understanding this phenomenon, but we have much to learn. This proposal builds on our current understanding, replicates our observations in both time and space, and addresses new questions that our research has generated.

1. The data suggest three processes generate shore-parallel fronts: 1) alongshore flows, 2) boundary mixing, and 3) thermally driven estuarine circulation. This conclusion is based on too little data. Using standard physical (CTD and ADCP transects) and biological (vertical plankton tows) oceanographic techniques we would test if in geographically similar settings the characteristics of the fronts are similar suggesting a similar formation process.
2. Larval distributions are altered by the front and the distributions will be characteristic for each type of topographically generated front.
3. Whether the fronts play an important roll in larval dispersal and settlement depends on how long they are present. Presence/absence of fronts will be determined from thermistor moorings positioned across fronts at cove and open coastal sites at 3 locations in Oregon.
4. The effect of frontal circulation on larval dispersal depends on the depth at which larvae reside and flow at that depth. Using a propeller driven plankton sampler and a nueston net we will determine the vertical distribution of larvae around the fronts studied in #1. The vertical distribution of ocean currents will be determined from ADCP transects.
5. To test if fronts affect settlement of intertidal and shallow subtidal invertebrates settlement will be measured every other day in the intertidal and at moorings spanning fronts at cove and open coastal sites at Cape Arago and Port Orford. Concurrent CTD casts, vertical zooplankton tows, ADCP transects, and continuous temperature recordings will define the oceanographic setting.

Lottia gigantea, the owl limpet, defends territories, cleared areas in the intertidal upon which an algal film develops. A territory holder maintains and feeds on the algal film. Smaller, non-territory holders raid these "gardens" feeding upon the algal film. A territory holder must obtain an adequate ration without compromising the productivity of its garden. A raiding non-territory holder must obtain an adequate ration before it contacts the territory holder and is driven off. Twenty L. gigantea were maintained in the laboratory. Half were trained to behave territorially and half were trained as non-territory holders. The training mimicked natural encounters between territory and non-territory holding L. gigantea. Grazing limpets given territorial training left significantly (t = -4.92, df = 9, P = 0.00041) more algal cover behind then did limpets trained to be non-territorial, 71% vs. 50%, respectively. Territorial limpets seldom grazed the same area more than once (4% of the grazed area) while non-territorial limpets frequently foraged in areas more than once; of the area grazed, 20% had been visited more than once. Besides determining territorial behavior (fight or flee), previous agonistic experience also determines resource utilization strategies. Non-territorial limpets maximize immediate consumption; territorial limpets appear to maximize integrated consumption.