The effects of history on the shell preference of the hermit crab Pagurus longicarpus (Say)

225

J. Exp. Mar. Biol. Ecol., 1984, Vol. 81, pp. 225-234

Elsevier

JEb% 329

THE EFFECTS OF HISTORY HERMIT

ON THE SHELL PRJZFERENCE OF THE

CRAB PAGURUS

L0NGICiiRPU.Y

(Say)

NEIL W. BLACKSTONE Department of Biology, Yale University, New Haven. CT 0651 I ( U. S.A

Abstract: Two aspects of the effects of history on the preference of a hermit crab for a snail shell were investigated using male Pagunts Iongicarpus (Say): (1) does a shell’s history influence a hermit crab’s preference for that shell, and (2) does a hermit crab’s shell utilization history inffuence its future shell preference? First, tests of the same Pagtms longicarpus individuals were done in Ilyannssa obsoleta (Say) shells of two treatments (beach-collected and freshly killed). These tests indicated that shell history is of little consequence to shell preference in this hermit crab species. Secondly Pagurus longicarpus individuals were entrained to shells of particular snail species by enforced utilization. Preference tests with these individuals yielded results similar to those from non-entrained crabs, indicating that, in this hermit crab species, shell preference is not affected by shell use and is an endogenous characteristic. Key words: Pagurus; hermit crab; shell preference; behavioral evolution

Hermit crabs are anomuran crustaceans which usually inhabit empty gastropod shells. Such shells are often found to be in limiting supply (Provenzano, 1960; Hazlett, 1970; Vance, 1972; Kellogg, 1976; Spight, 1977; Abrams, 1980; Bertness, 1980). Since shells can be easily measured and manipulated, hermit crabs have often been studied to elucidate principals of population and co~unity ecology (e.g. Childress, 1972; Vance, 1972; Abrams, 1980; Bertness, 1981a). Such studies rely on hermit crab shell utilization and preference data to formulate their conclusions. In assessing the evolutionary implications of such work, the question of the plasticity of shell preference (and therefore utilization) arises. Is shell preference easily influenced by shell utilization? Does inhabiting a certain species of shell predispose a hermit crab to prefer shells of that species? Here the effects of history on shell preference were investigated using Pagurus longiunpus (Say) individuals from Long Island Sound at Guilford, Connecticut.

METHODS

In any test of shell preference in hermit crabs, it is necessary to consider the types of shells that will be used. Often, these tests are done with shells of freshly killed 0022-0981/84/$03.00

0

1984 Elsevier Science Publishers B.V.

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NEIL W. BLACKSTONE

gastropods (“mint” shells). Such shells have not been used by hermit crabs and so have a similar history with respect to hermit crab use. With Pagums Zongicarpus,however, one of the most commonly utilized shells, Zlyanassa obsoleta (Say), retains much organic material, particularly the periostracum, after the mint shell treatment. This organic material might bias preference of Pagurus longicarpus against mint Ilyanassa obsoleta shells because of the additional weight or because this material disguises the calcareous nature of the shell (cf. Mesce, 1982). A possible alternative to mint shells are beach-collected (“weathered”) shells. Weathered shells are the remains of gastropods which died and were cleaned by the actions of sun, wind, and waves. Collections of weathered shells likely contain some shells which have been inhabited by hermit crabs and some shells which have not. Shell history with respect to hermit crab use and perhaps other parameters is therefore not standardized for these shells. Hence, some initial tests were done to compare the preference of Pagurus Iongicarpus individuals when tested between mint shells and weathered shells. To prepare mint shells, live IZyanassa obsoleta were collected and frozen. After thawing, the soft body parts were removed and the shells were soaked in sea water for 4 wk. Weathered shells of Ilyanassa obsoleta were collected from the intertidal zone; heavily worn, damaged, or encrusted shells were discarded. Preference tests were carried out on a group of 21 male Pagurus Zongicarpus (mean anterior shield length = 2.9 mm) which were held in isolation prior to the tests until they molted. Several days after molting, each individual was offered five weathered shells. These shells formed a series of increasing size, each shell being zz 5 % larger in width and length than the last. This series was gauged so that the test crab would prefer an intermediate-size shell. Over the next 48 h occupancy was checked several times by measuring the occupied shell with a hand caliper. Once the test crab exhibited a preference for one or two shells adjacent in size, five additional shells were added. These shells formed a series of increasing size, each shell being GZ2% larger in width and length than the last; this series bracketed the occupied shell(s) ofthe first five shells. Occupancy was then measured until the test crab occupied the same shell for five consecutive observations over a 48-h period. The crab was then removed from its preferred weathered shell by briefly immersing it in warm sea water (35 “C). The anterior shield length (hereafter ASL; the length of the hard part of the carapace and a convenient measure of body size), carapace length, and weight of the crab were then measured. (Note that ASL is highly correlated with carapace length, R2 = 0.99, and log ASL is highly correlated with log weight, R2 = 0.97; hence, these size measures can be used interchangeably.) Employing the same procedure, this crab was then offered mint shells. Using the same criteria, its preferred mint shell was determined, and the crab was removed from its shell and its size measurements taken again. Data from the two shell treatments were displayed using regressions of preferred shell size on crab size; differences between shell treatments were assessed using t-tests for paired comparisons. To determine if there is a preference between shells of the two shell treatments, 10 of the above crabs (mean ASL = 3.1 mm) were offered a choice between their preferred

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IN HERMIT CRABS

227

weathered and mint Ilyanassa obsokta shells. Each individual was offered the two preferred shells and 15 observations of shell occupancy were taken over a 7-day period. This was done because shell preference in Pagums ~on~~a~us is decidedly probabilistic (see Table IV). At any given time an individual might be in either shell offered it and actual preference is best determined by compiling a distribution of shell preference based on multiple observations. Based on the results of these shell history preference tests, weathered shells were used in the crab history preference tests. Three such tests were done. In the first test, intermediate-size, adult male crabs (N = 12; mean ASL =I 3.0 mm) were collected in shells of Urosa(pinx cinerea (Say) and held in shells of this species for two molts (2-3 months). During that time, crabs were isolated and fed fresh or frozen mussels every 3 days; larger shells were offered after each molt. After two molts (mean ASL = 3.9 mm) shell preference was tested between shells of ~ros~~inx cinerea and Littorina Iittorea using a methodology similar to that used when testing preference between mint and weathered shells. In the second test, seven male Pagurus longicarpus were collected as very small juvenile adults (mean ASL < 1.5 mm) inhabiting high-spired Nassarius trivittatus(Say) shells. Thesecrabs were raised in a similar manner as the above 12 males. High-spired ~~anassa obsoleta shells were offered after each molt until these crabs outgrew shells of this species ( NN1 yr), whereupon they were given shells of the similarly shaped but larger Urosalpinx cinerea. At the time of the preference tests these crabs had inhabited Urosalpinx cinerea shells continuously for > 1 yr. Preference of these individuals was compared between Urosalpinx cinerea and L~ttorina~~ttoreashells. Because these crabs were of a size (mean ASL = 5.2 mm at time of tests) at which their preferred shells were larger than the largest shells of both these species, a different methodology had to be employed here. Data from the previous test showed the following relationship: (preferred shell width of Littorina littorea) = 1.14 (preferred shell width of Urosalpinx cinema) - 0.47. Hence a large Urosalpinx cinerea shell was selected for each crab and the equiv~ent-wide Littor~~ff~ittorea shell was calculated. These pairs of shells were offered to their respective test crabs and occupancy was observed twice daily for the next week (15 observations per individual). These tests are similar to the equivalence tests of Elwood et al. (1979). In the third test, 10 male Pagurus Iongicarpus were collected as small juvenile adults (mean ASL = 2.3 mm) and their preference was tested between shells of Nass~~~s t~v~ttatusand L~tto~na littorea. The crabs were then held in isolation and given shells of h’yanassa obsolete. Crabs were raised in a similar manner as the above males; abundant food was given and larger shells were offered with each molt. After six to eight molts (x 9 months) of occupying Ilyanassa obsoleta, these crabs (mean ASL = 3.7 mm) were tested for preference between shells of Ziyanassa obsoleta and Littorina Iittorea. A similar methodolo~ as that outlined above for weathered and mint shells was employed in both these between shell species comparisons. The decision to use IWZ_WZ~~US trivittatus shells when testing the small crabs was made because Nussarius trivittatusand llyanassa

NEIL W. BLACKSTONE

228

obsoletu shells change shape with size, becoming relatively narrower and higher-spired

as they become larger. Nassarius trivittatus and Ilyanassa obsoleta have similar shapes, but Ilyanassa obsoleta is somewhat wider. Hence, small Nassarius trivittutus are more similar in shape to large Z~y~nassaobsoletu than are small shells of the latter species. RESULTS

Table I displays the data from the shell treatment tests using regressions. In the first pair of regressions, the logarithm of the product of preferred shell width and length is regressed on the logarithm of crab weight. These regressions compare preferred shell size, independent of shell weight, to crab size for each shell treatment, and the results for weathered and mint shells are virtually identical. In the second pair of regressions, preferred shell weight is regressed on crab weight, and the results for weathered and mint shells show a slight difference. TABLEI Preferred shell size of 21 male Pugurus longicarpus tested in two shell treatments, weathered and mint Ilyanussu obsoletu: each individual was tested first with weathered shells then mint shells; for each treatment, the logarithm of the product of shell width and shell length is regressed on the logarithm of crab weight, and shell weight is regressed on crab weight. Slope intercept R* Weathered shells

Mint shells

Log (width x length) = f (log crab weight)

0.479

6.027

0.94

0.482

6.029

0.93

Shell weight = f (crab weight)

2.65

0.08

0.86

2.84

0.10

0.85

Because these data are from paired comparisons, analysis of covariance tests for homogeneity of slopes were inappropriate; rather, t-tests for paired comparisons were used. Differences between the widths, lengths, and weights of preferred weathered and mint shells were calculated, and in each case t-tests were used to test the null hypothesis (mean of the differences = 0). Results of these tests are shown in Table II; no significant differences in the widths or lengths were found, but weights are significantly different (t = 3.49, P = 0.002). This indicates that preferred mint shells are the same width and length as preferred weathered shells, but that preferred mint shells are heavier, probably because of the additional weight of the retained periostracum. Perhaps because of the slightly greater weight of preferred mint shells, the 10 males tested for preference between weathered and mint shells showed a preference for weathered shells. Based on 15 observations per individual, total occupancy was 21: 129 (Table III). It may also be that the retained periostracum disguises the calcareous nature of the mint shell and this biases shell preference (cf. Mesce, 1982). Thus with respect

HISTORY AND SHELL PREFERENCE

229

IN HERMIT CRABS

TABLE II

r-Tests for paired comparisons done on the widths, lengths, and weights of preferred mint and weathered shells: shown are the means, standard deviations, standard errors, t statistics, and PvaIues calculated from the differences between treatments (value for mint shell minus value for weathered shell),

Shell width (mm) Shell length (mm) Shell weight (g)

Mean

SD

SE

i

P

- 0.033 0.029 0.048

0.227 0.582 0.062

0.049 0.127 0.014

- 0.67 0.22 3.49

0.5 1 0.82 0.002

to preferred shell size, tests done with weathered shells provide equivalent results to tests done with mint shells, but the former shell type should be used when comparing preference between shell species which differ in periostracum retention. Hence, weathered shells were used in the crab history preference experiments. As a further justification for using weathered shells, it is notable that tests with these shells do not produce detectably more variable results than tests with mint shells (compare R2 values in Table I). If shell history was an important factor, greater variability in the weathered shell tests would have been expected because these shells lack a standardized shell history. TABLE III

Summary table of shell preference tested between shell treatments and shell species: for each test are shown the number of individuals used(N), mean size (anterior shield length, ASL), total observed occupancy, and P value from Fisher’s exact probab~ity test.

Test Weathered : mint

I/. cinerea : L. liltorea U. cinerea : L . littorea N. ~~.vi~iat~s: L. littorea I. obsoleta : L. littorea

N 10 12

.7 10 10

Mean ASL (mm)

Totai occupancy (15 obs./ind.)

3.1 3.9 2.3

129: 21 14: 166 14: 91 150: 0

3.1

7: 143

5.2

P<

_ 0.000 1 0.0001 0.0~1 0.0001 0.0001

The results of the crab history preference tests are shown in Table III. The 12 males which inhabited Uro~a~inx cinerea shells for 2-3 months nevertheless preferred Littorina Iittorea shells to Urosalpinx cinerea shells. (Using these 12 crabs as an example, Table IV shows the probabilistic nature of shell preference in this hermit crab species. Note that the occupancy of each individual and the total occupancy is significantly biased toward Littorina littorea shells, though at any given time crabs may be found in ~rosa~inx cinerea shells.) Similarly, the 7 males which ~habited high-speed shells for over 2 yr (a year of which was spent in Urosalpinx cinerea shells} still preferred Littorina littorea shells to Urosalpinx cinerea shells.

NEIL W. BLACKSTONE

230

TABLEIV Shell preference data from 12 male Pagurus longikurpus (ASL, anterior shield length): each individual was offered a choice between its preferred size shells of Littorina littorea and Urosalpinx cinereu and occupancy was observed twice daily; shown are occupancy of U. cinerea (U) and L. Iittoreu (L) shells (-, no observation), total occupancy, and P values from Fisher’s exact probability tests. Crab size

Total

Day

(A=)

1

2

3

4

5

6

I

8

3.4 3.5 3.6 3.6 3.8 4.0 4.0 4.0 4.0 4.4 4.4 4.4

LL -L -L -L -L LL -L -L -L -L uu -L

LL LL LL LL UL LU LL LL LL LL LL LU

LL LL LL LL LL LU LL LL LU UL LL LL

LL LL LL LL LL LL LL LL LL LL LL LL

LL LL LL LL LL LL LL LL UL LL LL LL

UL LL LL LL uu LL LL LL LL UL LL LL

LL LL LL LL LL LL LL LL LL LU LL LL

LLL LL LL LL LLL LL LL LL LLL

(U : L) 1 0 0 II 3 2 0 0 2 3 2 1

14 15 15 15 12 13 15 15 13 12 13 14

14 : 166

Totals

P

0.0010 0.000 1 0.000 1 0.000 1 0.0352 0.0074 0.000 1 0.000 1 0.0074 0.0352 0.0074 0.0010 < 0.000 1

The 10 crabs used in the ontogenetic tests of shell preference showed reversal in shell preference. At small sizes these crabs preferred Nassarius trivittatusto Littorina littorea shells, 150 : 0 (Table III). Despite rearing in Zlyanassaobsoleta shells, preference reversed when these crabs were tested at a larger size (7 : 143, I. obsoleta: L. littorea). Again note that small Nassarius trivittatusshells are very similar in shape to large Zlyanassa obsoleta shells.

DISCUSSION

Reese (1963) and Elwood et al. (1979) address the question of history and shell preference and obtained different results. Reese (1963) held 46 Calcinus laevimanus in the shell known to be the least preferred, Olivella biplicata. After 3 months of this enforced utilization, 80 y0 of the crabs still selected Acathina spirata or Tegula funebralis shells in preference to Olivella biplicata shells. On the other hand, Elwood et al. (1979) field collected Pagurus berhardus in Nucella lapillus and Littorina littorea shells. When the shell preference of these crabs was tested, they found a slight preference of crabs for the occupied shells species. (Nucella lapillus crabs tended to prefer N. lapillus over Littorina littorea shells and L. littorea crabs tended to prefer L. littorea over Nucella lapillus shells). These results suggest a modification of shell preference by shell utilization, and differ from the results of Reese (1963). In Pagurus longicarpus field-collected in Long Island Sound, shell utilization and shell

HISTORY

AND SHELL PREFERENCE

IN HERMIT CRABS

231

preference show a correlation (Blackstone & Joslyn, 1984). Small FU~I.GUY lon@mpu utilize and prefer high-spired shells ~N~sa~~ ~vittat~, IZyan~sa obsoleta, and Uro~a~in~ cinerea) while large male Pagwus lon~~a~~ (females never attain large sizes) utilize and prefer low-spired Littorina Zittorea shells. This correlation could be caused by shell utilization entraining the crabs to prefer whatever shells they use (utilization and preference might then reflect shell availability). The results presented here, however, show that males which inhabit high-spired shells still prefer Litforina ~ittoreawhen they attain large sizes; thus shell preference is not modified by utilized shells. These results support the conclusions of Reese (1963) and may provide insight into why Elwood et al. (1979) obtained different results. As presented, the data of Elwood et al. (1979) can be explained by behavioral modification or by an endogenous ontogenetic reversal of shell preference in Pagwus ~~~rd~s. Their field data on shell utilization clearly show that P. ~~~rd~ occupying Littorina Zittorea shells are larger than those occupying ~~cei~a lap$lm shells. Because they do not present quantitative data on the sizes of the crabs tested for shell preference, it may be that the Littorina littorea crabs tested were slightly larger than the Nucelia lapilrtdscrabs tested. If there is an ontogenetic reversal of shell preference in Pagzms be~hurd~s, the observed preference pattern could be explained by a slight size difference in tested crabs. This can be seen clearly by comparing Pagwws be~hardus to Pagums longicalpus. Data from Blackstone & Joslyn (1984) show that in the latter species: (1) the mean size of crabs inhabiting high-spired shells is less than the mean size of crabs inhabiting Littorina littorea shells, (2) small crabs prefer high-spired shells, and (3) large crabs prefer Littorina Iittorea shells. Tests of randomly collected crabs would thus show that (large) crabs in ~itto~~a fittorea shells would tend to prefer Littorina Zittoreashells to high-spired shells. Similarly, (small) crabs in high-spired shells would tend to prefer these shells to Littorina littorea. Yet the data presented here show that this is due not to behavioral modification but to the endogenous ontogenetic reversal of shell preference in each individual coupled with the size differences between the tested indi~du~s. This expl~ation may also apply to backs be~hard#s. Nevertheless, at this time the lack of consensus in the literature should be acknowledged and experiments on other hermit crab species should be done. It may be that hermit crab species differ in the amount that shell preference can be modified by shell utilization. It should be emphasized that this question is fairly fundamental to an underst~ding of the evoIution~ ecology of these organisms. If shell preference is unaffected by utilization, it is likely endogenous and under genetic control. Shell utilization would therefore be under genetic control to the extent that preference influences utilization. If this is the case, variation of shell preference and utilization patterns among sympatric hermit crab species could be interpreted in terms of selection and adaptation or other evolutionary processes (for examples of such an approach, see Bertness, 1981b, 1982). Similarly, v~iation in the shell preference and utilization patterns ofgeographic populations of single hermit crab species could also be interpreted in evohitionary terms.

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NEIL W. BLACKSTONE

In this context, a brief discussion of some of the other work done on the shell preference of Pagurus longicarpus can provide insight into the sorts of results that can be expected from geographic comparisons. First, it may be that such comparisons of shell preference indicate no ditTerences. For instance, Mitchell (1975) studied Pagurus Zongicarpusat Beaufort, North Carolina. Preference tests indicated that crabs of weight 0.05-o. 15 g preferred Ilyanassa obsoleta shells while crabs of weights > 0.23 g preferred Littorina irrorata shells. While it is not clear that Mitchell’s methodology controls for differences in size between the shell species used in the preference tests, there is nevertheless a striking correspondence between his results and those presented here and in Blackstone & Joslyn (1984). Preference reversal from Zlyanassa obsoleta shells to Littorina littorea shells occurs at 3.2-3.5 mm ASL in Guilford, Connecticut, Pagurus longicurpus. For these crabs, log(crab weight) = 3.38 (logASL)-5.52 (N = 83, R2 = 0.97), so weight of 0.23 g equals 3.3 mm ASL; hence, it is at the same size that Pagurus longicarpus from the two geographic areas exhibit the onset of shell preference for Littorina shells over Ilyanassa obsoleta shells. Note that this similarity in shell preference is despite dramatic differences in the shell utilization of the crabs in the two geographic areas. In Long Island Sound, Littorina littorea shells are commonly inhabited by large male Pagurus longicarpus, but in North Carolina Littorina irrorata shells are rarely inhabited in the field, possibly because Littorina irrorata snails are found in habitats other than those of Pagurus 1ongicaqm.shermit crabs (Mitchell, 1975, p. 212). A second result of geographic comparisons of shell preference may be apparent differences which are likely spurious and the result of morphological variation in the species being compared. For instance, preference tests done with Pagurus longicarpus and Littorina littorea shells from Guilford, Connecticut, (test crabs from this study and from Blackstone & Joslyn, 1984) yield the relationship (preferred shell weight) = 2.85 (crab weight) + 0.05 (N = 59, R2 = 0.96). Two sets of preference tests done by Scully (1979) with Rhode Island Pagurus longicarpus and Littorina littorea shells yielded greater slopes and intercepts for regressions of the same variables. While this may indicate a difference in preferred shell size between the two geographic areas, it is important to note that Littorina littorea shells from Connecticut are considerably lighter in weight than those from Rhode Island (Kemp & Bertness, 1984). Hence, a Connecticut shell of the same width and length as a Rhode Island shell would weigh significantly less. The apparent geographic differences in shell preference could be merely an artifact of the geographic differences in Littorina Zittoreashells. Thus morphological variation must be considered when evaluating variation of shell preference over hermit crab species’ geographic ranges. A third result of geographic comparisons of shell preference may be actual differences. For instance, Scully (1979) studied Pagurus longicarpus at two sites in Rhode Island. Shell preference tests done on crabs from the two sites showed a difference in the sizes of Littorina Zittoreashells preferred. These preference differences cannot be explained by morphological variation in Littorina Iittorea, for differences are apparent when crabs from the two localities are tested in similar shells. However, because tested

HISTORYAND

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crabs were field-collected and not conditioned in the laboratory, it is not clear whether the difference in preference is due to exogenous causes (e.g., shell utilization or other habitat differences at the two sites) or endogenous causes. Thus once an actual geographic difference in shell preference is detected, further testing is necessary to determine if it is due to exogenous or endogenous factors. If the preference differences are endogenous, genetic control is likely and evolutionary hypotheses can be formulated to explain the geographic differences in shell preference. In conclusion, it should be noted that the data presented here are of particular interest with regard to Pagurus longicarpus. Littorina littorea is a European species which only in historical times has become common on the Atlantic coast of North America (Bequaert, 1943; Vermeij, 1978, 1982). It has been barely a century since Littorina littorea arrived in Long Island Sound (Perkins, 1869). Yet large male Pagurus longicarpus prefer Littorina littorea shells over commonly utilized native shells (Blackstone & Joslyn, 1984) and this preference seems to be endogenous. If preference is genetically based, it may be that Pagurus longicarpus has adapted to the presence of Littorina littorea, though this need not be the case. Reese (1962) demonstrates that hermit crabs can exhibit preferences for shell species with which they have had no prior experience. Perhaps the preference of Pagurus longicarpus for Littorina Iittorea existed before this shell was introduced. The above comparisons with Mitchell’s (1975) data suggest that preference of Pagurus Iongicarpus for the shells of the large native littorine, Littorina irrorata, may be equivalent to their preference for the shells of Littorina littorea. Littorina irrorata is presently extinct in Long Island Sound, but Bequaert (1943) and Vermeij (1978) indicate that Littorina irrorata occurred in Connecticut until the early twentieth century (though Perkins, 1869, describes it as not at all common in New Haven Harbor). In earlier centuries when Littorina irrorata was common, large male Pagurus Zongicarpusmay have been selected to prefer Littorina irrorata shells. With this snail’s extinction in Long Island Sound, large shells may have become scarce; hence, the introduction of Littorina littorea and the availability of its shells may represent fortuitous circumstances for Pagurus longicarpus.

ACKNOWLEDGEMENTS

Use of the Guilford field station and Bingham sea-water system of the Yale Peabody Museum greatly facilitated this study, and comments of two reviewers were useful in preparing the manuscript. REFERENCES ABRAMS, P., 1980. Resource partitioning and interspecific Oecologia (Berlin), Vol. 46, pp. 365-319. BEQUAERT, J., 1943. The genus Littorina in the western

competition Atlantic.

in a tropical

hermit crab community.

Johnsonia, Vol.1,No. 7, pp. 1-28.

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BERTNESS,M.D., 1980. Shell preference and utilization patterns in littoral hermit crabs in the Bay of Panama. J. Exp. Mar. Biol. Ecol., Vol. 48, pp. l-16. BERTNESS,M.D., 198la. Competitive dynamics of a tropical hermit crab assemblage. Ecology, Vol. 62, pp. 751-761. BERTNESS,M.D., 198lb. Conflicting advantages in resource utilization: the hermit crab housing dilemma. Am. Nat., Vol. 118, pp. 432-437. BERTNESS,M.D., 1982. Shell utilization, predation pressure, and thermal stress in Panamanian hermit crabs: an interoceanic comparison. J. Exp. Mar. Biol. Ecol., Vol. 64, pp. 159-187. BLACKSTONE, N. W. & A. R. JOSLYN,1984. Utilization and preference for the introduced gastropod Littorina littorea (L.) by the hermit crab Pagurus longikarpus (Say) at Guilford, Connecticut. J. Exp. Mar. Biol. Ecol., Vol. 80, pp. l-9. CHILDRESS,J.R., 1972. Behavioral ecology and fitness theory in a tropical hermit crab. Ecology, Vol. 53, pp. 960-964. ELWOOD,R. W., A. MCCLEAN& L. WEBB, 1979. The development of shell preference by the hermit crab Pagurus bernhardus. Anim. Behav., Vol. 27, pp. 940-946. HAZLETT,B.A., 1970. Interspecific shell fighting in three sympatric species of hermit crabs in Hawaii. Pac. Sci., Vol. 24, pp. 472-482. KELLOGG,C. W., 1976. Gastropod shells: a potentially limiting resource for hermit crabs. J. Exp. Mar. Biol. Ecol., Vol. 22, pp. 101-I 11. KEMP, P. & M.D. BERTNESS.1984. Snail shape and growth rates: Evidence for plastic allometry in Littorina littorea. Proc. Natl. Acad. Sci. USA, Vol. 81, pp. 81 l-813. MESCE, K.A., 1982. Calcium-bearing objects elicit shell selection behavior in a hermit crab (Pagurus hirsutiusculus hirsutiusculus). Science, Vol. 215, pp. 993-995. MITCHELL, K.A., 1975. An analysis of shell occupation by two sympatric species of hermit crab. I. Ecological factors. Biol. Bull. (Woods Hole, Mass.), Vol. 149, pp. 205-213. PERKINS, G.H., 1869. The Molluscan fauna of New Haven harbor. Proc. Best. Sot. Nat. Hist., Vol. 13, pp. 109-163. PROVENZANO,A. J., JR., 1960. Notes on Bermuda hermit crabs (Crustacea; Anomura). Bull. Mar. Sci. Gurf Caribb., Vol. 10, pp. 117-124. REESE, E. S., 1962. Shell selection behavior of hermit crabs. Anim. Behav., Vol. 10, pp. 347-360. REESE, E.S., 1963. The behavioral mechanisms underlying shell selection by hermit crabs, Behaviour, Vol. 21, pp. 78-126. SCULLY,E. P., 1979. The effects ofgastropod shell availability and habitat characteristics on shell utilization by the intertidal hermit crab Pagurus longicarpus Say. J. Exp. Mar. Biol. Ecol., Vol. 37, pp. 139-152. SPIGHT,T. M., 1977. Availability and use of shells by intertidal hermit crabs. Biol. Bull. (Woods Hole, Mass.), Vol. 152, pp. 120-133. VANCE, R. R., 1972. Competition and mechanisms of coexistence in three sympatric species of intertidal hermit crabs. Ecology, Vol. 53, pp. 1062-1074. VERMEIJ,G. J., 1978. Biogeography and adaptation. Harvard Press, Cambridge, 332 pp. VERMEIJ,G. J., 1982. Environmental change and the evolutionary history ofthe periwinkle (Littorina littorea) in North America. Evolution, Vol. 36, pp. 561-580.