Download SHELL SELECTION BY THE HERMIT CRAB, PAGURUS HARTAE

SHELL SELECTION BY THE HERMIT CRAB, PAGURUS HARTAE
(MCLAUGHLIN & JENSEN, 1996) (DECAPODA, ANOMURA)
BY
MEI SATO1,3 ) and GREGORY C. JENSEN2,4 )
1 ) Tokyo University of Fisheries, Department of Aquatic Biosciences, Konan 4-5-7, Minato-ku,
Tokyo 108-8477, Japan
2 ) School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle,
Washington 98195, U.S.A.
INTRODUCTION
Most species of hermit crabs rely on gastropod shells to protect their uncalcified
abdomens, and these mobile shelters help guard against predation, desiccation,
and physical stresses (Hazlett, 1981). Shells are so essential that their availability
has been considered a limiting factor for populations (Vance, 1972; Spight, 1977).
The types of gastropod shells available also determine the size of hermit crabs
present in an area (Markham, 1968). In shell-limited environments crabs are often
forced to occupy inadequate shells, which in turn restricts not only their growth and
fecundity but also increases their risk of predation (Sripathi et al., 1977). There is
considerable evidence that hermit crabs do not enter gastropod shells at random,
but select shells according to shell species and their associated characteristics of
shape, epibionts, dimension, and weight (Grant & Ulmer, 1974). This differential
shell preference may lead to a form of habitat partitioning that increases the
likelihood of coexistence between similar hermit crabs (Vance, 1972).
Pagurus hartae (McLaughlin & Jensen, 1996) is a tiny species (maximum
carapace length 6 mm) found from the Queen Charlotte Islands, Canada to
the Mexican border. It occurs subtidally under rocks and in crevices at depths
of 6-635 m, and large assemblages can be found in parts of Barkley Sound
on Vancouver Island, British Columbia, Canada (Jensen, 1995). The present
experiment was undertaken to determine what types of shells this newly described
species occupies in natural surroundings and whether they exhibit any preference
for shell species.
3 ) Present address: Darling Marine Center, University of Maine, 193 Clark’s Cove Road, Walpole,
Maine 04573, U.S.A.
4 ) Corresponding author; e-mail: [email protected]
© Koninklijke Brill NV, Leiden, 2005
Also available online: www.brill.nl
Crustaceana 78 (6): 755-760
756
NOTES AND NEWS
MATERIALS AND METHODS
One hundred thirteen Pagurus hartae were captured on 15 and 23 August 2002
by dredge in Satellite Passage near the north shore of Helby Island (51◦ 01 N
125◦ 11 W) in Barkley Sound, British Columbia, Canada. The crabs were brought
back to the Bamfield Marine Sciences Centre where they were held in an open
circuit seawater system, and the species of shell used by each was identified.
To test for shell preference, 21 adult crabs were anaesthetized and extracted
from their shells. Crabs were anaesthetized using a drop of oil of cloves diluted
in 25 ml of seawater; after 10-15 minutes of immersion they were gently removed
from their shells. Visibly parasitized (e.g., rhizocephalan externae) and ovigerous
crabs were not used, nor were any that were damaged in the process. The naked
crabs were isolated in ice cube trays immersed in an ambient (10◦ C) open circuit
seawater system and allowed to recover for at least twelve hours.
Living and dead Homalopoma luridum (Dall, 1885), Calliostoma ligatum
(Gould, 1849), Amphissa columbiana Dall, 1916, Nassarius mendicus (Gould,
1850), and Alia carinata (Hinds, 1844) shells were collected from Barkley Sound
beaches. Shells were boiled in fresh water for 20 minutes to kill any associated
organisms and to aid in removing tissues. Hermits can recognize and select their
own previously inhabited shell by means of chemical cues; boiling destroys the
cues left behind by previous occupants (Benoit et al., 1996).
Shells were categorized into three types based on their shape: (a) Homalopoma
(short, globose structure), (b) Calliostoma (conical or top-shaped), or (c) Alia,
Amphissa, and Nassarius (elongated). Alia, Amphissa, and Nassarius were used
interchangeably, since they are very similar morphologically. Undamaged shells
with aperture measurements similar to the individual crab’s original shell were
used in the trials.
Naked crabs were placed singly into large glass preparation dishes (10 cm
in diameter, with a volume of 350 ml) provided with running seawater and a
substratum of washed sand and gravel. Each trial consisted of four shells each of
Homalopoma, Calliostoma, and a mix of Alia, Amphissa, and Nassarius, randomly
dispersed into the dish. Each trial ran for 12 hours and shell occupancy was noted
at the end of that time; each crab was used only once. Some shells were boiled and
reused in later trials. Shell choice was analysed using a Chi-square test.
RESULTS
The gastropod shells occupied by Pagurus hartae in the field consisted of 9 genera (fig. 1). Shells of Calliostoma ligatum were most frequently utilized (41.6%)
NOTES AND NEWS
757
Fig. 1. Proportions of shell types used by Pagurus hartae (McLaughlin & Jensen, 1996) in the field
(n = 113).
followed by Amphissa columbiana (26.5%), Alia carinata (9.7%), Nassarius mendicus (8.0%), and Homalopoma luridum (5.3%). Lacuna variegata Carpenter,
1864, Oenopota fidicula (Gould, 1849), Trichotropis cancellata Hinds, 1843, and
Trophonopsis spp. were each used by fewer than 3% of the crabs collected. Two
had severely worn and damaged shells that could not be identified.
Of the 21 P. hartae used in the shell selection experiment, 13 selected Homalopoma, five selected Alia, Amphissa, or Nassarius, and three selected Calliostoma
(fig. 2). Of those that chose Homalopoma, four were originally housed in Alia,
Amphissa, or Nassarius, six in Calliostoma, and three retained the same species
of shell. Of five crabs that chose one of Alia, Amphissa, or Nassarius, three were
originally housed in Calliostoma. The three crabs that selected Calliostoma were
originally housed in that species. P. hartae did not randomly choose their new
shells (χ 2 = 8.00, d.f. = 2, 0.01 < p < 0.025) inhabiting Homalopoma significantly more often than the other types of shells offered.
DISCUSSION
Most field-collected Pagurus hartae were found utilizing shells of Calliostoma
ligatum, yet this species was clearly not preferred when the crabs were given
758
NOTES AND NEWS
Fig. 2. Types of gastropod shell selected by Pagurus hartae (McLaughlin & Jensen, 1996) in the
laboratory.
a choice. In the laboratory, P. hartae selected Homalopoma luridum, a species
used by only 5.3% of the P. hartae collected in the field. This suggests that shell
utilization by P. hartae at that time and location depended on factors other than
preference, such as shell availability or interspecific competition. The latter seems
unlikely, however, given the very low numbers of other species that were present
in the catch.
Hermit crabs show a tendency to use the shells of the most abundant coexisting
gastropods, revealing the importance of shell availability in shaping their shell
utilization patterns (Reese, 1969). Given the high proportion of P. hartae in
C. ligatum shells, it can be inferred that this species was common in the P. hartae
habitat and indeed, it was by far the most abundant snail caught in the dredge along
with P. hartae. Elwood et al. (1979) found that Pagurus bernhardus (Linnaeus,
1758) with prior experience using a shell species were more likely to choose that
same species when given a choice. In our experiment, shell selection by P. hartae
was independent from the type of shell in which the crab was originally found.
Hermit crabs choose shells based on a wide variety of features, including shell
configuration, aperture size, and shell weight/crab weight and shell weight/shell
volume indices (Reese, 1963; Grant & Ulmer, 1974). From a physiological and
anti-predator standpoint, shell shape can have important implications for intertidal
and terrestrial hermits. In general, highly spiraled shells may hold water, thus
protecting hermit crabs from temperature extremes (due to the high specific heat of
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759
water), while low spiraled shells confer better protection from predation (Bertness,
1982). Since P. hartae are strictly subtidal, thermal considerations are not a factor
and protection from predators is probably the most important function of the shell.
The thick, globose shell of Homalopoma had the lowest spire of any of the species
tested and presumably offers the greatest protection.
Another intriguing possibility is that P. hartae could be selecting shells based in
part on their durability. Calliostoma shells decay at a remarkably fast rate following
the death of the snail, causing the shell to lose about half of its original strength in
compression tests after only three days (LaBarbera & Merz, 1992; Vermeij, 1993).
In contrast, dead Tegula shells withstood 12-15 months of use by intertidal hermits
before becoming too worn and damaged to use (Kuris et al., 1979). Such rapid
deterioration of Calliostoma shells would necessitate frequent replacement. Since
it has been demonstrated that some hermit crabs chemically recognize shells by
their calcium (Mesce, 1993a, b) it is conceivable that they could also recognize and
avoid shells that are dissolving at an excessive rate. This could be experimentally
tested using shells modified to slow their dissolution rate. Alternatively, choice
could be influenced by prior experience with rapidly dissolving shells; this would
require testing captive-reared crabs to see if it is a learned response.
ACKNOWLEDGEMENTS
We thank Dr. S. Zaklan for her help with this project, and the staff of the
Bamfield Marine Sciences Centre.
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First received 4 February 2005.
Final version accepted 17 March 2005.