Grouping
Behavior in Rock-Boring Sea Urchins (Echinometra
lucunter)

Abstract:

Echinometra
lucunter,
or Rock-Boring Sea Urchins, are a species of tropical echinoderm
which are found in abundance in the Atlantic Ocean throughout the
West Indies (Plazas, 2012). Common within rocky intertidal regions
(McPherson, 1969), the urchins burrow into rocks to create their own
shelters, which are continually enlarged as the urchins grow (McLean,
1967), supported largely by a diet of various algal species including
Sargassum
(Plazas,
2012). While swimming in Discovery Bay, Jamaica, we noticed that
these urchins apparently cluster on rocks, with some areas covered
with urchins and others containing few or no urchins, leading us to
ask the following questions: 1. Do E.
lucunter exhibit
grouping behavior? 2. If so, do factors such as abundance of food
and habitat size and availability influence grouping behavior? We
hypothesized that the urchins do exhibit grouping behavior, and that
if faced with the choice, they will cluster around larger habitats
rather than food or smaller habitats. In order to test these
hypotheses, we monitored changes in the spatial distribution of ten
E.
lucunter individuals,
which we placed in an aquarium with a roughly uniform distribution
pattern and then photographed over a specified number of time
intervals under three experimental conditions: no food or shelter,
shelter with no food, and food with no shelter. We then further
tested the urchins' preferences for food vs. shelter and for small
vs. large shelters, in order to determine potential drivers behind
the suggested grouping behavior. We then measured distances between
each urchin and its nearest neighbor, and will analyze this data by
applying the Nearest Neighbor Analysis ("Nearest neighbor
analysis") to determine if the urchins are exhibiting random,
uniform, or clustered distributions. From a preliminary examination
of histograms of our results, it appears possible that Echinometra
lucunter
individuals do exhibit grouping behavior, but there is no discernible
preference for food vs. habitat or for smaller vs. larger habitats.
Statistical analysis is required to determine whether or not these
apparent trends are significant.

The initial setup for our first experiment, testing the distribution
of Echinometra
lucunter
in an environment with no food or shelter.