Physiological ecology
  Intertidal conditions for crabs, as for other marine invertebrates, are marked by extremes of temperature and drying, salinity fluctuations, and (obviously) lack of water for gas exchange. The topic of physiological ecology is divided into a section on chemoreception considered here, and sections on GAS EXCHANGE & METABOLISM, LOCOMOTION & TENACITY, DIEL SEASONAL & TIDAL RHYTHMS, and OSMOTIC REGULATION & SALINITY TOLERANCE, and THERMAL STRESSES considered elsewhere.
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Research study 1

photo composite showing arrangement of antennae on a hermit crab Pagurus armatus courtesy Ron Long, SFUdrawings showing how the aesthetasc hairs of a hermit-crab's antennule spread out during a flickChemosensory organs of crustaceans include antennae, antennules, and micro-pockets of sensory bristles scattered over the exoskeleton called aesthetascs.  The antennules (2nd antennae) of crustaceans function mainly for distance chemoreception. Each antennule consists of 3 segments, the most distal bearing an outer and inner flagellum (see photo inset and drawings on Right). The outer flagellum bears several rows of sensory asthetasc hairs. Each hair may have up to 400 sensory neurones associated with it. Four activities can be identified: flicking, wiping, pointing, and withdrawal.  In anomuran hermit crabs the antennules are relatively larger than in brachyuran crabs.

eddrawing showing fine structure of an aestetasc of a hermit crab Pagurus hirsutiusculusA research study on hermit crabs Pagurus alaskensis at Friday Harbor Laboratories, Washington employs high-speed cinematography to describe the 4 activities in detail.  First, flicking is non-rhythmic and flicks of left and right antennules are never synchronised. The crab flicks its antennules almost continuously, even when withdrawn into its shell.  Notice in the photograph the left antennule is extended, while the right one is partially withdrawn, possibly as part of a flick cycle. During the first 15msec of a flick (a flick lasts about 65msec), the aesthetasc hairs are splayed, thus maximising their surface area. Note in the sequence of drawings above Right how the outer flagellum bends to splay out the asthetasc hairs (these are in 7 rows).  The antennules can be rotated at their bases through about 180o.  Flicking is faster when stimulated by water currents, change in surroundings, or presence of food.  Flicking ceases with mechanical disturbance, and inhibition may occur with repeated application of the same stimulus.  The antennules are regularly wiped by the 3rd maxillipeds to remove debris caught up in the asthetasc hairs.  Depending upon the kinds of sensory information being received, the antennules may be extended (pointed), or withdrawn.  During withdrawal the antennules are flexed inwards and partially tucked away.  Snow 1973 J Exp Biol 58: 745; see also Snow 1974 J Morph 144: 195 for more details on morphology of the aesthetes and their hairs. Photo of Pagurus armatus, a similar-looking species to P. alaskensis, courtesy Ron Long, SFU, Burnaby.

NOTE  the terminology is a bit loose. In this account it seems that an aesthetasc is made up of a bunch of aesthetasc hairs, each hair connected by a dendrite to a cell body, and each of the cell bodies having a dark-staining nucleus (see drawing lower Right).  The axons run towards the central nervous system, forming the antennular nerve.  There are 300-500 neurones in each antennular aesthetasc of the hermit crab Pagurus hirsutiusculus Ghiradella et al. 1968 Protoplasma 66: 1; see also Ghiradella et al. 1968 Amer Zool 8: 603 and Ghiradella et al. 1970 Protoplasma 69: 145

NOTE  the author provides information on motor innervation and musculature in the antennules of hermit crabs Pagurus alaskensis in another article.  Snow 1973 J Exp Biol 58: 767.

Research study 2

graph showing the effect of taurine concentration on heart-rate in the kelp crab Pugettia productaphotograph of a kelp crab Pugettia producta hanging in some seaweedStudies on the effect of various nutrient chemicals including amino acids, fatty acids, and sugars on heart rate in the kelp crabs Pugettia producta collected around Santa Barbara, California, indicate that the branchial chamber is also a major chemoreceptive site (see graph).  A typical response curve to the amino acid taurine is shown in the accompanying graph.  The authors note that theirs is the first documentation of chemosensitivity to carbohydrates in a marine crustacean. Zimmer et al. 1979 J Exp Mar Biol Ecol 38: 135.

Kelp crab Pugettia producta clings to some
plastic plants in an aquarium 0.33X

Research study 3

photograph of spiny lobster Panulirus interruptusSpiny lobsters Panulirus interruptus in the Santa Barbara region of California eat both live and dead prey and are attracted by chemical odours of both types of foods when placed in traps as bait.  Tests of live vs. excised tissues of different potential prey species show that abalone (several species) and mackerel are most attractive to the lobsters, while angel-shark muscle and shrimp cephalothoraxes are non-attractive.  Analyses of chemicals emanating from excised tissues shows that primary amines (amino acids) likely do not contribute directly to attraction of lobsters.  Zimmer-Fausti 1982 Mar Behav Physiol 9: 35.

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CLICK HERE to see a video of a porcelain crab Petrolisthes eriomerus cleaning its antennae on the bristles of its 3rd maxillipeds. The smaller appendages working so busily are the 2nd maxillipeds. These seem to be collecting some edible bits from the antenna that are presumably then swallowed.

NOTE  the video replays automatically

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Research study 4

photograph of hermit crab Pagurus hirsutiusculus exploring a plaster shell-replica with its chelaeWhen investigating an empty shell for its suitability for habitation, hermit crabs Pagurus hirsutiusculus use their minor chelae to assess the shell’s physical and chemical characteristics.  An electron-microscopical and electrophysiological investigation in Oregon shows that setae and rows of teeth on the chela function as mechanoreceptors, and the setae also function as chemoreceptors, with particular sensitivity to calcium (see array of photographs below). If a crab is presented with a silicone-coated snail shell that lacks the calcium cue and a plaster block, it focuses its attention on the block, rather than the shell (see photograph upper Right). Note in the photograph that the crab uses the ventral side of its minor chela during its exploration of the shell. The author notes the presence on the chelar surface of pits and non-sensory tubercles, but can ascribe no function to them.  Chemosensitivity to calcium would likely enable the crabs to distinguish shells from pebbles and other objects, and to locate partially buried shells.  Mesce 1993 J Crust Biol 13: 95.

NOTE CaSO4.  Application of artificial seawater (ASW) containing normal concentration of Ca to sensory nerves induces discharge, but application of ASW with the calcium replaced by magnesium induces no discharge.  The author does not appear to test for possible neurophysiological activity to SO4

e-microscopical view of setae on the propodite of a chela of hermit crab Pagurus hirsutiusculus
Tufts of sensory setae on the propodite of the chela of a hermit crab Pagurus hirsutiusculus
e-microscopical view of sensory tubercules on the dorsal surfaces of the chela of a hermit crab Pagurus hirsutiusculus
Sensory tubercules on the dorsal surface of the chela of P. hirsutiusculus
e-microscopical view of the cutting teeth on the edge of the dactulus of a hermit crab Pagurus hirsutiusculus
Teeth on the cutting surface of the dactylus of P. hirsutiusculus
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Research study 5

side view of antennule of shore crab Hemigrapsus oregonensisAn interesting investigation related to that in Research Study 1 above on antennary flicking in hermit crabs concerns the ontogenetic scaling of flicking in shore crabs Hemigrapsus oregonensis done by a researcher at the University of California, Berkeley.  The 1st antennae (antennules) bear branches or arrays of hair-like sensory aesthetascs that are flicked through the water to sense odours (see photograph on Right).  On the quick downstroke the aesthetascs are laterally splayed, while on the slower upstroke they are clumped.  This clumping retains water momentarily and delivers a brief pulse of odour molecules to the sensory units borne on the aesthetascs.  During development, or photograph of 1st antenna of crab Hemigrapsus oregonensisontogeny, the antennae-aesthetasc arrays scale in size allometrically, but in the direction of becoming disproportionately smaller with body size.  Thus, the extent of splay is relatively larger in juveniles than in adults, which is contrary to what one would predict from normal surface-area to volume relationships during growth.  The author explains that if a juvenile’s antennules were to scale isometrically compared with an adult’s, they would be unable to force fluid within the array of aesthetascs during the flick owing to the much greater relative viscosity of seawater at small size.  Waldrop 2013 Chem Senses 38: 541.

NOTE  equivalent to “sniffing” in vertebrates.  The downward flick is about twice the speed of the upward return

This photograph may take a moment to figure out. It shows a front view of H. oregonensis
showing the left side only with 2 inset photos overlaying the left cheliped. The Left inset
shows the antennule in downstroke with the aesthetascs splayed out (white arrow),
while the Right one shows the antennule in upstroke with the aesthetascs clumped

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