Behavior of fiddler crabs (Uca pugnax) in relation to sediment nutrition

Participants: Dr Judith Weis & Sona Mason, Federated Department of Biological Sciences Rutgers Newark.

Status: Began Summer 2009

The Problem:

Fiddler crabs, which are mainly detrital feeders, tend to apportion their behavorial time budgets among various behaviors, which include feeding on sediment, interacting with conspecifics and remaining below ground in their burrows.

Previous observations have noticed differences in feeding time budgets in fiddler crabs living on polluted versus cleaner sediments. In some cases, field observations noted that crabs living on sand (which has less nutrients than muddy sediment) spent more time feeding. However, other studies showed more feeding on richer sediments. There is also a body of literature indicating that contamination (such as metals) reduces feeding in a variety of animals. As the results from various studies differed in their conclusions, we hope to determine more conclusively whether fiddler crabs spend more time feeding on richer or poorer sediments. Also, we were interested to know whether these behaviors are limited to specimens living on their own sediment, or whether these behaviors remain the same when specimens are placed on sediment different than their home sediment.

Our working hypothesis states that sediments from contaminated sites probably carry a greater load of nutrients in addition to pollutants, thus abating the need for continuous feeding activity among fiddler crabs, allowing more time for other behaviors.


To determine how the feeding time budgets of fiddler crabs differ on relatively clean, vs polluted sediments. Also, nutrient analyses are to be carried out on fresh, partially and more fully depleted sediments to determine the nutrient quantity in each after being grazed on by fiddler crabs for a period of time.


Collection of fiddler crabs and sediment from polluted (Anderson Creek, Hackensack Meadowlands and Piles Creek, Linden) and reference site (Tuckerton), and maintaining them on their own and each other’s sediments in laboratory tanks. Observations of their subsequent feeding behaviors to examine what proportion of their time is spent feeding vs engaging in other activities.

Measurement of sediment nutrients (C, N, and chlorophyll, which is an indicator of benthic microalgae which are consumed by the crabs) initially and after one, three and six day’s feeding by crabs in each tank to determine any nutrient depletion of each sediment type. Polluted sediments are expected to sustain feeding behavior longer and be depleted of nutrients more slowly due to containing more nutrients.



Collect specimens from polluted and unpolluted sites, monitor feeding behavior of each population on their home sediment. Swap populations onto onto each other’s sediments, monitor any changes in behavior, mainly time spent feeding.


Laboratory analyses of carbon, nitrogen and chlorphyll a content in sediments from the three study sites, prior to and post fiddler grazing.


– Nutrient analyses of sediments before and after grazing by fiddler crabs.

– Quantification of feeding time budgets on polluted vs clean sediments by Uca pugnax.

– Final report of findings.

– Publication in a peer-reviewed journal and presentations.