Kim, C. Shell adequacy of hermit crabs...
Abstract
Pagurus samuelis is found consistently higher than Pagurus granosimanus in the
rocky intertidal of China Point in Monterey Bay . The adequacy and availibility of Tegula
shells to P. samuelis and P. granosimanus was studied to investigate the possibility of
competition for shells as a mechanism of maintaining tidal height segregation. The
investigation was done in three portions: a mark and release study of live snails to
directly find the rate of supply of new shells, creating a shell adequacy index using crab
weight and shell diameter, and a comparison of the shell adequacy of P. samuelis and P.
granosimanus where they co-occur and occur separately. Although the mark and release
study yeilded no mortality after six weeks, data from the shell adequacy index suggests
that large P. samuelis and P. granosimanus are not shell limited. The interspecifc
competition for shells was not significantly different from intraspecific competition,
and therefore does not play a major role in maintaining tidal segregation.
—
Kim, C. Shell adequacy of hermit crabs...
Introduction
Pagurus samuelis and Pagurus granosimanus are hermit crabs found
along the western coast of North America. Bollay (1964) reported that P.
samuelis was found consistently at higher tidal height than P. granosimanus.
More than 80% of the animals of both species of reproductive size are found in
shells of species of Tegula (Bollay 1964). Because both species of hermit crab
utilize Tegula spp. shells and Bollay reported that almost no empty inhabitable
shells were in the intertidal, it was hypothesized that the distribution of P.
samuelis and P. granosimanus is being maintained by interspecies
competition.
Competition can determine the way, where and how organisms live.
Vance (1972a) defined competition as "when two or more organisms seek a
common, necessary resource which occurs in insufficient supply to meet their
combined needs," and demonstrated that shell competition between Pagurus
granosimanus, Pagurus hirsiuticulus and Pagurus beringanus on the
Washington coast species affects the composition of a community and can be a
major limiting resource
A constant supply of shells are a necessary resource for hermit crabs.
The basic function of hermit crab’s shells is protection (Vance 1972b), but
inadequate shell size can decrease resistance to desiccation (Taylor 1981) and
predation (Vance 1972b), decrease fecundity (Bertness 1981b), and slow down
growth rate (Markham 1968). A constant supply of shells is needed so that
hermit crabs can move into adequate shells as they mature.
Although there was not enough time to investigate, all components of
competition, an investigation of shell competition and its potential influence
on intertidal segregation of P. samuelis and P. granosimanus was done in the
Kim, C. Shell adequacy of hermit crabs...
Monterey Bay in May and April. Three aspects of shell resources were
examined: the rate of supply of shells, adequacy of hermit crab shells in
overlap and non-overlap zones and comparisons of shell adequacy in
different wave exposure sites.
Methods and Materials
Mark and Release
A mark and release study of live T. funebralis was done to determine the
supply rate of new shells into the hermit crab population. Specimens were
collected from a 0.25 m2. The T. funebralis shells were wiped with a paper
towel and allowed to dry. Then, a spot of enamel model paint (Testor) was
painted onto the shell, and after enamel dried, the snails were released into the
field.
Population studies
To determine the relationship between Tegula sizes and the shells that
hermit crabs occupy, collections were made from three locations in the rocky
intertidal zone off of China Point in the Monterey Bay in the months of May
and April (Fig. 1). Three locations were chosen for their varied wave action;
they were rocky areas by Agassiz Beach (protected), Hewett Transect (semi¬
protected), and West Beach (exposed). All three locations had large algae
covered rocks surrounded by a loose substrate and sea grass.
At each site, quadrats were sampled every two meters along a transect
line running perpendicular to shore from -O.5 m (MLLW) to determine
vertical distribution of hermit crab species in each area. At the protected and
semi-protected areas, T. funebralis were collected from within a 0.25 m2 area,
and hermit crabs were collected for up to half an hour or until 50 hermit crabs
were collected. At the exposed area, the first 50 Tegula shells - indiscriminate
of whether it was a snail or a hermit crab - were collected.
Kim, C. Shell adequacy of hermit crabs...
In lab, maximum basal diameter of shells of the crabs and snails were
measured using venier calipers. Äfter blotting dry naked hermit crabs with a
paper towel, live body weight was measured.
A shell adequacy index was generated as described in Vance (1972a). In
a glass tank 9.5" x 12" x 17.5" with sand, Pelvetia, and constant circulation of
cold sea water, 50 P. samuelis were given five days to select an preferred sized
shell from 274 Tegula spp. shell. Then, live body weight of the crab and
maximum basal diameter of shells were measured. A linear regression was fit
to the logarithms of body weight and maximum basal diameter. The same
procedure using 56 P. granosimanus and 194 Tegula spp. shells was used to
determine a shell adequacy index for P. granosimanus.
The shell adequacy ratio was defined as maximum basal diameter of a
hermit crab’s shell in the field divided by the maximum basal diameter of the
calculated preferred size as estimated by the shell adequacy index. Shell
adequacy ratios less than one indicate that the shell a crab occupied in the
field was smaller than its preferred shell, and a shell adequacy ratio greater
than one indicate a crab occupied a shell greater than its preferred shell.
Differences between population size distribution were analyzed by non¬
parametric Kolmogorov-smirnov tests (Sokal & Rohlf, 1981), and Pearson chi¬
square tests were used to analyze shell adequacy ratios.
Interspecific competition
To investigate interspecies competition, hermit crab data from the semi¬
protected and protected areas were combined and resorted into areas where P.
samuelis and P. granosimanus co-occur and areas where they occur
separately. The exposed area data was excluded because of the difference of
collection method. The combined shell adequacy of P. samuelis and P.
Kim, C. Shell adequacy of hermit crabs...
granosimanus in overlap and non-overlap areas were analyzed by Z-factor
ANOVA t-tests assuming unequal variances.
Results
Mark and Release Study
Äfter six weeks, the mark and release study yielded no mortality in
marked T. funebralis. Ten days after released, only 50 of the original 191
marked snails found after searching for an hour, and no marked hermit crabs
were found.
Between site comparisons
The population distribution of the crab body weight show that the P.
samuelis is slightly smaller than P. granosimanus at each area (Fig. 2-4). The
modal weights of the crabs are the same in the protected and semi-protected
areas, but in the exposed area the modal weight of the P. samuelis is larger
than P. granosimanus.
In the protected area, shell utilization patterns of both species of hermit
crab were skewed to larger shells than Tegula spp. and were significantly
different (Fig. 5). In the semi-protected area, the shell utilization patterns of
both P. samuelis and P. granosimanus were skewed to larger shells than Tegula
spp. and were also significantly different (Fig. 6). In the exposed area, shell
distributions of all three species were not significantly different (Fig. 7).
The equations from the shell adequacy indices are In crab weight -
3.1088 + 0.2020 x In shell width (p«0.001, r2-0.85) for P. granosimanus and In
crab weight = 3.1437 + 0.3159 x In shell width (p20.001, r2-0.89) for P. samuelis.
These slopes were significantly different (F= 18.87, p20.001).
In the protected area, P. samuelis and P. granosimanus have the same
modal shell adequacy ratio (Fig. 8), but the proportion of P. granosimanus in
Kim, C. Shell adequacy of hermit crabs...
the protected area with adequacy indices less than one is significantly more
than the proportion of P. samuelis with adequacy indices less than one
(p=0.026, chi-square). In both the semi-protected and exposed areas, the
proportion of P. granosimanus and P. samuelis with adequacy indices less
than one were not significantly different (p-0.557 chi-square, p-0.620 chi¬
square respectively) (Fig. 9-10).
In a shell adequacy ratio versus maximum basal diameter of all data
combined, both P. samuelis and P. granosimanus were in the 0.8-1.2 range (Fig.
11) In both hermit crab species the animals with shell adequacies greater than
1.2 are in shells larger than 15 mm, and the hermit crabs with shell adequacies
less than .8 are in shells smaller than 15 mm.
Interspecific comparisons
Mean shell adequacy ratios of P. samuelis and P. granosimanus in areas
where they co-occur and where they occur alone are not significantly
different (p-O.O8ANOVA, p=0.06 ANOVA respectively) (Fig. 12-13).
Discussion
The shell adequacy of a hermit crab’s shell affects its resistance to
desiccation, protection against predation, its fecundity, and its growth rate,
making shells a common and necessary resource for P. samuelis and P.
granosimanus. Although further exploration is needed, but data suggests that
large P. samuelis and P. granosimanus are not shell limited. Shell adequacy
ratios imply that Interspecific competition between P. granosimanus and P.
samuelis around China point in the Monterey Bay area is not a major
contributor to the segregation observed in the field, and that wave action may
affect shell availability and adequacy of P. granosimanus and P. samuelis.
Kim, C. Shell adequacy of hermit crabs...
The introduction of new shells to the hermit crab population is very
slow and could not be measured in six weeks. Wilber and Herrnkind (1984)
marked and released snails in the salt marshes of Florida and found the rate of
new snail shells into the hermit crab population, but the mortality rate of the
T. funebralis is too low to measure directly in six weeks. Further study with a
larger number of tagged snails and a longer time period are needed to
determine the actual rate of introduction of new shells.
The shell distributions of the protected and semi-protected areas
suggested that larger hermit crabs are shell limited because hermit crab shell
distributions were skewed to larger shells than Tegula spp. distributions (Fig.
5-6), but the shell adequacy ratio versus shell diameter plot (Fig. 11) indicates
that the crabs needing smaller shells are more shell stressed than crabs
needing larger shells. Bertness’s (1981b) studies indicated that larger crabs
are more successful in fights for shells than smaller crabs, so the presence of
small crabs in the 15-20 mm shells in this study are indicative of an abundance
of large shells. If there were a shortage of large shells, the smaller crabs
would have been evicted by larger crabs.
The distribution of Tegula spp. seems to contradict the findings of the
shell adequacy vs. shell diameter plot, but the number of hermit crabs and
Tegula spp. collected differ by one order magnitude, so even a very slow
mortality rate of the Tegula spp. could keep the P. samuelis and P.
granosimanus well supplied with shells.
Another possible explanation for the discrepancy is that predators may
prefer certain sizes of Tegula spp. skewing the availability of one size over the
others. Fawcett(1984) noted that the Pisaster giganteus preferentially eats
larger snails. The crab, Cancer is known to break off pieces of shell in
Kim, C. Shell adequacy of hermit crabs...
preying on shelled animals, and this type of predation if selective would
reduce the relative availability of a shell size class.
Interspecific competition
Comparisons of mean shell adequacy ratios of P. samuelis and P.
granosimanus in areas of overlap and non-overlap indicate that the strength
of interspecies competition for shells is not significantly different from the
strength of intraspecies competition for shells (Fig. 12-13). If interspecific
competition were a major factor of segregation of P. samuelis and P.
granosimanus, the shell adequacy ratios in areas where the two species co¬
occur would be lower than shell adequacy ratios in areas where the two occur
separately, but that trend is not in the data. Although the mean adequacy ratio
of both species is not significantly different, the mean shell adequacy ratio of
both P. samuelis and P. granosimanus actually increases when found together
indicating that shell adequacy improves in the zone in which both species co-
occur.
Wave action affects on shell adequacy
The proportion of hermit crabs with shell adequacies greater than one
in the exposed and semi-protected areas are not significantly different, but a
greater proportion of the P. granosimanus of the protected area have shell
adequacy ratios less than one than P. samuelis. The number one was used as
the arbitrary cut off point at which to compare those shells that were in
"poor" shells and "good" shells, but their was no method for checking the
exactness of the shell adequacy index.
Acknowledgements
My deepest thanks goes to Jim Watanabe for patiently walking me through the
process of choosing a topic, changing topics, finding a question to answer,
Kim, C. Shell adequacy of hermit crabs...
helping me answer the question and introducing me to the nitty gritty details
of research and writing scientific papers..
I would like to thank Edwardo Martinez for getting up for early morning tides
and making collecting entertaining, and for making the days "exciting"
Thank you to Alan Lam and Jill Snively for continual moral support
throughout the quarter, and making it one of the most memorable.
Kim, C. Shell adequacy of hermit crabs...
Literature cited
Bertness, M. D. 1981a. Pattern and plasticity is tropical hermit crab growth and
reproduction. American Naturalist 117: 754-773.
Bertness, M. D. 1981b. The influence of shell-type on hermit crab growth rate
and clutch size (Decapoda, anomura). Crustaceana 40 (2): 197-205.
Bollay, M. 1964. Distribution and utilization of gastropod shells by the hermit
crabs Pagurus samuelis, Pagurus granosimanus and Pagurus
hirsiuticulus at Pacific Grove, California. Veliger 6 (suppl.): 71-76.
Markham, J. C. 1968. Notes on growth-patterns and shell-utilization of the
hermit crab Pagurus bernhardus (L.). Ophelia 5: 189-205.
Sokal, R. R. and F. J. Rohlf. 1981. Biometry. Freeman, San Francisco, CA. 859 pp.
2nd edition.
Taylor, P.R. 1981. Hermit crab fitness: the effect of shell condition and
behavioral adaptations on environmental resistance. Journal of
Experimental Marine Biology and Ecology 52: 205-218.
Vance, R. R. 1972. Competition and mechanism of coexistence in three
sympatric species of intertidal hermit crabs. Ecology 53: 1062-1074.
70
Vance, R.R. 197z. The role of shell adequacy in behavioral interactions
involving hermit crabs. Ecology 53: 1075-1085.
Wilber, Jr. and Hernkind. 1984. Shell acquisistion in Salt Marshes. Journal of
Crustacean Biology 2(4): 588-592.
10
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Figure legend
Figure 1. Map of China point and locations of sites. Protected= Agassiz,
Semiprotected= Hewett. Exposed site=West.
Figure 2. Crab weight distribution in protected area of P. samuelis and P.
granosimanus.
Figure 3. Crab weight distribution in semi-protected area of P. samuelis and P.
granosimanus.
Figure 4. Crab weight distribution in exposed area of P. samuelis and P.
granosimanus.
Figure 5. Shell size distribution in protected area of Tegula spp., P. samuelis,
and P. granosimanus. Kolmogorov-Smirnov (K-S) Test of Tegula to P. samuelis
distribution, pæ0.001. K-S test of Tegula to P. granosimanus distribution,
p#0.001. K-S test of P. samuelis to P. granosimanus distribution, p-0.042.
Figure 6. Shell size distribution in semi-protected area of Tegula spp., P.
samuelis, and P. granosimanus. Kolmogorov-Smirnov (K-S) Test of Tegula to P.
samuelis distribution, p-0.039. K-S test of Tegula to P. granosimanus
distribution, p=0.012. K-S test of P. samuelis to P. granosimanus distribution.
P=.374.
Figure 7. Shell size distribution in exposed area of Tegula spp., P. samuelis, and
P. granosimanus. K-S test of Tegula to P. samuelis distribution, p-0.602. K-S
test of Tegula to P. granosimanus distribution, p= 0.541. K-S test of P. samuelis
to P. granosimanus distribution, p= O.993.
Figure 8. Shell adequacy distribution in protected area of P. samuelis and P.
granosimanus.
Figure 9. Shell adequacy distribution in semi-protected area of P. samuelis and
P. granosimanus
Figure 10. Shell adequacy distribution in semi-protected area of P. samuelis
and P. granosimanus
Figure 11. Shell adequacy of P. samuelis in absence of P.granosimanus in the
protected and semi-protected areas combined vs. Shell adequacy of P. samuelis
in presence of protected and semi-protected areas combined.
Figure 12. Shell adequacy of P. granosimanus in absence of P. samuelis in
protected and semi-protected areas combined vs Shell adequacy of P.
granosmimanus in presence of P. samuelis in protected and semi-protected
areas combined.
Figure 13. Shell adequacy vs. Maximum basal diameter of P. samuelis and P.
granosimanus
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