EPIFAUNAL INTERACTIONS AND SETTLEMENT PATTERNS ACROSS A DEPTH GRADIENT OVER SIX YEARS

The study of community structure in ancient fossil assemblages is often fraught with difficulties. Taphonomic bias removes organisms with soft parts and organisms can be removed and added via transport. In contrast, encrusting communities are not as easily biased by transport, and have the added advantage of preserving spatial positioning of individuals and colonies on a substrate. Behavioral interactions can be studied when growth brings individuals into contact, or larval settlement occurs on top of, or adjacent to, an established individual. The community complexity, overall abundance, and intensity of interactions of encrusting organisms on hard surfaces may reflect aspects of the local environment such as energy, light availability, and nutrients.

There is a previously noted difference between encrusting community structure on small ephemeral substrates (shells) and the more typically studied hardgrounds. Ephemeral substrates as a group present a unique environment for encrusters. In the fossil record, encrusting communities on smaller macrofossils are a common occurrence. A better understanding of the behavioral aspects of space-limited encrusters on modern small shell substrates will improve our ability to interpret paleocommunities of the past.

Two shell species were deployed along a depth gradient in the Bahamas and were left at the sediment-water interface for 1, 2, and 6 years. Space utilization and interactions between groups of encrusters were quantified for all preservable encrusters. Results show that photosynthetic species or species with photosymbionts, where they appear, are dominant space competitors at the shallowest site in the study (15m). In the deeper photic zone (70m), diversity of encrusters rises as does between-guild interaction. However, less surface area was occupied on shells at this depth. Below the photic zone, unoccupied space dominates the shells and interactions decline.

The examination of behavioral aspects of hard-substrate communities reflects food source, larval abundance, larval site selection, growth rates, and often the ability to out-compete other encrusters for space. These characteristics can reveal information about local environment or paleoenvironment that is difficult to extract from macrofossil assemblages.