Chemoreceptors

Some species of hermit crabs can locate chemically predation sites where snails are consumed and subsequently obtain their shells. This study addressed four questions: (1) Is chemotaxis to snail odors prevalent among hermit crabs? (2) Do members of hermit crab lineages respond similarly to common snail odors? (3) Do hermit crabs respond more acutely to snails whose shells they most frequently occupy? and (4) Does phylogeny of snails influence responses by hermit crabs? Two sets of congeners (Clibanarius vittatuslC. tricolor and Dardanus venosuslD. fucosus) in the family Diogenidae, and three congeners (Pagurus pollicaris, P. longicarpus, and P. annulipes) in the family Paguridae were tested. Fifteen species of snails from 11 families served as test odors. Hermit crab response was measured by the fondling display, where one hermit crab investigates the shell of a neighboring crab. The diogenids discriminated odors more readily than did the pagurids. Correlations between responses and shells most frequently occupied existed for C. vittatus and D. venosus. Clibanarius tricolor was the only crab to respond to confamilial test odors.

Precise measurement of receptor thresholds is important in understanding
the role a sense organ plays in the biology and the sensory information
processing of an organism. Previous crustacean chemoreceptor thresholds
are too high because of the adaping effects of high stimulus intensities
and the order effects of stimulus presentation sequence used in threshold
experiments now in the literature. The present study uses an experimental
design geared to produce the lowest adaptation possible and shows
that single unit chemoreceptor activity occurs at lower concentrations
than any previously documented. The differences between lateral and
medial chemoreceptor and the variability of the antennular chemoreceptor
population are also treated in the present study. It is concluded that
threshold differences do not account for the differential behavioral
effect of lateral vs medial antennal filament ablation, and that the
observed response variability is inherent in receptor response and
important to receptor function.

Periodic movements of the olfactory organs in the spiny lobster known
as antennular "flicking" temporally enhance the response of the olfactory
receptors to changes in the odor environment. The temporally
enhanced response onsets as a series of transient discharges phase
locked with the flick. This response pattern results from superimposed
increases in stimulus concentration at the receptor sites due
to flicking modulated permeability of a chemical diffusion barrier
surrounding the receptors, presumably created by the densely packed
nature of the receptor hair tuft. It is concluded that flicking
provides the lobster with a physiological mechanism to compensate
for the indiscrete temporal nature of chemical stimumi.

Antennular chemoreceptors of P. argus were surveyed electro-physiologically
to determine their relative sensitivity to low molecular
weight components of the shrimp, Penaeus duorarum. An ultrafiltrate
(less than 1,000 molecular weight) of shrimp extract did not differ
significantly in stimulus value from the total extract itself. A
mixture of the 19 amino acids present in the ultrafiltrate was slightly
less stimulatory than the total extract. Taurine alone approximated
the stimulus value of the amino acid mixture at its component concentration.
Taurine was also the most stimulatory of the 19 amino acids when
compared at the homogeneous 10^-3 M concentration. Taurine is the major
individual stimulant of shrimp extract but a contribution is also made
from other non-amino acid low molecular weight compounds.

The physiological effect of antennular flicking was analyzed
behaviorally and electrophysiologically in the spiny
lobster, Panulirus argus. Behavioral studies indicate
that glycine and L-glutamic acid cause a transient
concentration-dependent increase in flick rate. Electro-physiological
analysis of primary chemosensory afferents
indicates that flicking modulates ongoing chemically-elicited
activity in approximately half of the units
observed by causing either a brief increase or decrease
in the frequency of impulses. Responses of remaining units
were not altered by flicking. Antennular flicking is
discussed as a mechanism to prolong the address of the
peripheral chemoreceptors to the central nervous system
and allow the lobster to prolong its awareness of the
chemical environment.

The Sargassum community consists of a unique and idverse assemblage of fauna critical to pelagic food chains. Associated organisms presumably have adaptations to assist in finding Sargassum. This study investigated cues used for habitat location and selection by the Sargassum crab, Portunus sayi. Chemical detection trials were conducted with a two-chamber choice apparatus with Sargassum spp. and Thalassia testudinum as source odors. Visual detection trials (devoid of chemical cues) and habitat selection trials were conducted in which crabs were given a choice of habitats. Results showed that P. sayi respoded to chemical odors from Sargassum spp. Crabs visually located habitats but did not visually distinguish between different habitats. In habitat selection trials, crabs selected Sargassum spp. over artificial Sargassum and T. testudinum. These results suggest that crabs isolated from Sargassum likely use chemoreception from longer distances ; within visual proximity of a potential patch, crabs use both chemical and visual information.