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THE DISTRIBUTION, ABUNDANCE AND SHELL SELECTION BEHAVIOUR OF THREE SPECIES OF HERMIT CRABS
Introduction
Hermit
crabs of the family Anomorans live in empty gastropod shells, which serve as
mobile burrows offering protection from desiccation and predation. Hermits are
physically well adapted to their life-style; their abdominal region being soft
and extremely flexible as well as exhibiting torsion allowing the crab to fit
into the twisted internal space of a gastropod shell. The fifth and sixth pairs
of limbs are adapted (pereopods) for gripping the inside of the shell to
maintain position and the second and third pairs of limbs are used for walking.
Hermits have one major Cheliped and one smaller. Both are used to feed but the
larger of the two is employed as a protective ‘door’ when the hermit
retracts itself into the shell.
The
criteria used by hermit crabs in their shell selection behaviour have been well
studied. The purpose of this work is to assess if there is shell species
preference shown among the subjects and to look for a correlation between hermit
size and chosen shell size. The diversity and abundance of shells used by
hermits was also explored to assess which gastropod species were most abundant
and look at the availability of empty shells to see how they affect resource
(shell) availability. Three sites of study were chosen on Hoga; a sandy beach
for the terrestrial species, a seagrass bed (the boat bay) and the mangrove
habitat near Furake village. The three species of hermit crab involved in the
study were the aquatic Dordanus
scentellatus from the boat bay and Clibanarius
longitarsus from the mangroves, and the land hermit Coenobitta sp.
To
understand the abundance and shell selection crietrtia utilised by intertidal
hermit crabs of the Wakatobi Marine National Park, Southeast Sulawesi
·
Identify the
3 chosen species of hermit crab and the shells they occupy.
·
Identify the
species of gastropod mollusc in each study area.
·
Assess the
relative abundance of each species of hermit crab, gastropod mollusc and empty
shell at each site using random quadrat techniques in 4 areas of each of 3
sites.
·
Perform
sufficient replicates to accurately reflect the true population
·
To determine
if there is a pattern in the distribution seen in the separate study sites.
·
To
investigate the size choice made by hermits and find if there is a correlation
between shell size and hermit size.
·
Assess
whether a shell species preference is seen in the hermit crabs.
·
To observe
any aggressive interactions occurring during shell selection.
Size
and species preference;
Hermit
crabs were collected from each of the three study sites in separate experiments.
In the lab, the hermits were removed from their shells. This was either by the
tickling method; where the crab is teased out of its shell by poking a thin wire
behind its cephalothorax and tickling its pereopods to remove it or by the
heating method; placing the apex of the shell into boiling water to evict the
hermit. Neither method is particularly damaging.
The
hermit crabs were placed naked into a tank (46cmx61cm) with a selection of empty
shells to choose from. In the species choice experiments, various shells of
different type were offered and in the size preference tests, the same species
of shell of different sizes were given. The hermits were tested in batches of
10, as previous studies suggest that hermits naturally display hierarchical
behaviour during shell selection and would behave differently if tested
individually without this competition. 30 empty shells were offered in each
experiment. Each experiment was left to run for a 72-hour period as previous
studies indicate that hermits stop changing shell after this amount of time. The
hermits were then removed from their chosen shells and measurements taken of the
shell volume, length, aperture length and width. The species of shell was noted.
The hermit carapace length & width, cheliper length & width were also
measured with Vernier Callipers. Each individual was used only once for
experimentation.
Distribution
and abundance
At
each of the sites, four areas were chosen to perform random Quadrat sampling.
Ten quadrats were placed down, and the number of each species of gastropod,
empty shell and hermit-inhabited shell was recorded. Five replicates in total
were taken of the seagrass and mangrove site quadrats. The sandy beach was
sampled along a 10m transect along the shore beginning at the high tide mark.
Three sites where hermit clusters occurred were looked at and the quadrats were
taken at low tide every 2m moving down the shore.
Behavioural
observations
Aggressive
and shell inspection behaviour was observed both in the field and lab. Tests
were performed in the lab to see if hermits exhibited direct swaps with each
other for more suitable shells. Buried shells were offered in the lab to
determine what depths in the sediment shells were still available to
hermits.Empty shells from Strombus gibberulus were marked and left in the Furake
sites to see if the hermits were so shell limited they would accept shells they
rarely encountered.Damaged shells were offered also and were accepted.
From
the investigation of distribution and abundance of hermits, gastropods and empty
shells it was found that certain species of shell were only found in certain
areas i.e. Terebrallia palustris,
Terebrallia sulcata and Cerithium
coralium were only found in the mangrove area. Also in the mangroves there seemed to be a general pattern
that sites 1 and 2 have a larger number of gastropods which are generally found
clustered in rock pools and sites 3 and 4 have a higher number of hermit crabs.
This could be explained by the fact that sites 1 and 2 are further
upshore than sites 3 and 4, which are more seaward.
This suggests that the gastropods are more tolerant to sustained exposure
and desiccation and therefore the hermits congregate around areas closer to the
waters edge .i.e the edge of the creek that runs through the mangroves where
sites 3 and 4 are situated.
In
the seagrass area, gastropods were the most abundant in all the sites and mainly
occupied the shell Strombus gibberulus.
The hermit crabs were found to occupy a variety of shell species, mainly Rhinoclavis
aspera, Rhinoclavis vertagus and some Strombus
gibberulus. This may suggest
that there is a reduced predation of gastropods that live in the Strombus
shells due to their wide abundance perhaps because they are more advantageous
for protection. There are a number
of hermits found in this shell, which may have become available due to natural
causes of death of the gastropod but also due to their high abundance there are
fewer empty shells for the hermits. Therefore
many are found in a variety of different species of shell.
On
the sandy beach site of Hoga, cluster formations of the land hermit Coenobita sp. were mainly found. These large gatherings are thought
to be geared around shell exchange. The
clusters occurred on the highest local ground, generally occurring around broken
tree trunks. The clusters examined
varied in size from 106 – 57 but were constantly changing with time and tide
and hence difficult to compare. The
clusters were mainly formed of hermits occupying a variety of shell identities
but the most common tended to be Rhinoclavis
aspera and Strombus gibberulus.
The hermits in the clusters were generally of similar size.
Shell
selection observed in hermit crabs indicates that hermit crabs vary their
preference for shells of a certain weight and volume, depending upon the species
of shell. In this experiment 4
measurements of crab size and 5 of shell size were taken and correlation will be
found out from these results. A
variety of sizes of shells were found in each area and the main pattern found
was that the crabs were mainly smaller than the shells they chose, showing that
due to the lack of choice of empty shells the crabs make do with what they find.
Selective pressures i.e. predation operate to determine the most
advantageous response in shell selectivity.
It is more advantageous for a hermit subjected to predation by being
pulled out of its shell, to live in a large shell with a high volume index
providing a deep recess. Also if a
shell larger than necessary is picked, it may be an advantageous response
extending the period of time before they outgrow their shells.
When shells are scarce, hermit crabs are often forced to occupy shells
that are abundant which sometimes may be too small and this can have serious
consequences not only because of an increased risk of predation but also because
of lower growth rates. If food were
a limiting factor and predators were lacking, then selective pressure would be
in the direction of reducing energy expenditures, which would favour crabs
selecting and inhabiting smaller and lighter shells.
A
final year dissertation report entitled The Distribution, Abundance and Shell
Selection Behaviour of three species of Hermit Crab will be produced by
Angie Gooderham, University of Bangor and Lindsay Randall, University of Hull by May 2003.