Marine Hermit Crab Response to Gastropod-Associated Stimuli Ashley Edwards, Erin Dempsey, Kyna Kruger BIOL November 20, 2014

Overview ●Marine hermit crabs use chemical cues to locate gastropod shells for shelter ●Tested four marine hermit crabs against different gastropod-associated stimuli ●Measures of proximity to stimulus recorded ●Responded most strongly to living marine gastropod ●Future studies should include dead marine gastropod as potential stimulus

Introduction ●Adult marine hermit crabs =vulnerable ●Shell-acquisition e.g. gastropod shells

Evolutionary Tool-Use ●Asymmetrical coiling ●“Gastropod shell habitat web” gastropod predation ∝ shell-acquisition

Location Strategies ●Chemical cues: 1)Predator’s digestion process 2)Hemolymph of decaying snail 3)Calcium release from shell ●Narrow time range

Proposal Gap in knowledge: Do marine hermit crabs have a preference for living marine gastropods? Hypothesis: Marine hermit crabs would prefer the living gastropod, followed by the marine gastropod empty shell, followed by the decaying terrestrial gastropod.

Materials and Methods ●Marine hermit crabs of unknown species ●Two testing sessions, 4 crabs observed for each condition o In total, N=8 for each condition ●Hermit crabs were removed from their shells prior to testing

Testing Apparatus ●Stimulus chamber blocked off by mesh ●Crabs prevented from interacting with each other ●Six gradations ●Sand and salt water provided in tank

Methodology ●Conditions: o Baseline (no stimulus) o Live marine gastropod o Decaying gastropod o Empty gastropod shell ●Position in tank noted in 1-minute intervals for a total of 10 minutes o Crabs placed at interval 3 (mid-point) to begin trial

Results: Decaying Gastropod ●Linear trendline is clearly skewed toward position 1 o Preference for proximity to the stimulus o Spike at position 4 is notable  Proximity to the starting position

Results: Empty shells ●Nearly identical to decaying gastropod response o Difference between trendline slopes is so small as to be negligible (7.0*10 -5 ) ●Preference for proximity to the stimulus

●Stronger preference for proximity o Greater skew toward position 1 and larger y-intercept than other conditions Results: Live gastropod

●Trendline does not match results from trials with stimulus present o As expected Results: Baseline condition

Results: Summary ●Clear preference for proximity to stimulus in all testing conditions o Near identical response to decaying gastropod and empty shell stimuli o Stronger reaction to live gastropod condition ●Baseline trial served its purpose

Findings ●Results supported our hypothesis ●Most attracted to live snails and were similarly attracted to the dead terrestrial snail and empty shell ●Scent of decaying marine snail attracts hermit crabs (Hazlett 1997). ●Opportunity of empty shell arises when decaying flesh is detected (Hazlett 1997).

Other studies ●Hazlett (1997) o Increase in locomotory behaviour when decaying snail flesh is detected. ●Laidre (2011) o Marine hermit crabs are unable to remove the snails from their shells. ●Thacker (1994) o Found that marine hermit crabs are not attracted to dead terrestrial snails.

Confounding Variables ●Desiccation ●Inaccurate portrayal of environment ●Crabs could have reacted with each other through barrier ●Possibility of injury ● Optimal response time

Future Studies ●Use decaying marine snail ●Measure locomotory behaviour ●Dominant visual or dominant chemical cues? ●Conduct data analysis