Life History Patterns of Discorsopagurus schmitti, a Hermit Crab Inhabiting Polychaete Tubes

10 Pages·1995·4.3 MB·English

by  F Gherardi

Reference: Biol. Bull 188: 68-77. (February/March, 1995)
Life History Patterns of Discorsopagurus schmitti,
a Hermit Crab Inhabiting Polychaete Tubes
FRANCESCA GHERARDI1 AND PAUL M. CASSIDY2
[Dipartimento ill Biologia Animate e Genetica "Leo Pardi, " Universita di Firenie, Via Romana 17,
50125 Firenic, Italy: and2Shannon Point Marine Center, Western Washington University.
1900 Shannon Point Rd.. Anacortcs, Washington 98221
Abstract. Discorsopagurus .schmitti is a hermit crab that coasts from Japan to Puget Sound (McLaughlin, 1974).
inhabitsempty polychaetetubesin the North Pacific. Here In both itsgeographical distribution and itsecological role,
wedescribe someaspectsofitslite history (relativegrowth, this species is strictly dependent on the polychaete Sa-
population structure, reproductive biology, and incidence hellaria ceinentariitin Moore, 1906. The hermit uses at-
ofparasitism) and discuss the relationships among them. tached worm tubesas housingand occupiesa nichewithin
Unlike most hermits, the two sexes ofthis species have the community associated with sabellarian bioherms
similar size distributions. In both sexes, larger body size (Gherardi and Cassidy, 1994a). A bioherm is a rock
is accompanied by a higher reproductive output (larger formed by accretions from sedentary organisms and sur-
D
clutch size in females and more intrasex competitive po- rounded by other kinds ofrocks. Within the habitat,
tential in males). The energy the females expend in egg schmitti has a contagious distribution, the crabs occurring
production might beequaled in thisspeciesbytheenergy with a density averaging 6 specimens per dm2 (Gherardi
the males expend in supporting parasites. In fact, the ex- and Cassidy. 1994b).
tent of infestation by two rhizocephalans [Peltogaster Despite itspeculiarhabitsand widespreaddistribution,
boschmaeand Thilacoplethus(=Thompsonia) reinhardi] the main life history traitsofthe speciesare still unknown.
ismorepronounced in males, especially those in the larger Previous paperswereconcerned only with itsadaptations
sizeclasses. However, rhizocephalans have little effect on to the sessile worm tubes (Caine, 1980) and its ecology
their hosts; growth and secondary sexual characters are (Gherardi and Cassidy, 1994b). Our study investigates the
not influenced. The only morphological modification is relative growth, population structure, reproductive biol-
the more frequent loss of the second pleopod. Infected ogy, and incidence ofparasitism by rhizocephalans in D.
hermits also showed a mock parental behavior, fanning schmitti.
the externae with the pleopods as ovigerous females fan
their eggs. Larvae are released in sequential bursts, and Materials and Methods
hatchingoccursexclusivelyat night, possiblyto minimize
predation by diurnal fishes. Hatching is also synchronized D. schmitti was collected from a wide sabellarian bio-
with neap tides, which might keep the larvae from being herm in BurrowsChannel. Fidalgo Island (northern Puget
flushed out into open waters. In a species whose habitat Sound, Washington). A total of440 specimens were col-
(sabellarian bioherms) is rare and quite unpredictable, it lected: 329 from JunetoAugust 1992, and 1 1 1 from Jan-
isbeneficial to retain larvae nearthe parental population. uary to April 1993. See Gherardi and Cassidy (1994a. b)
for details on habitat and sampling procedure.
Introduction Sixty-four animals were individually weighed to the
nearest 0.01 g. Chelipeds were excluded from the weight
Discorsopagurus schmitti (Stevens, 1925) is an ano- becausetheyarevariableand sometimesabsent. Foreach
muran crab that occurs widely along the North Pacific specimen, we recorded sex, size (shield length, SL, to the
nearest 0.1 mm), missing chelipeds (i.e.. the number of
Received 10 December 1993; accepted IX November 1994. "injured" specimens), and the number and maximum
68
LIFE HISTORY OF A TUBE-DWELLING HERMIT CRAB 69
diameterofanyeggpresent. When possible,themaximum
axis ofthe occupied polychaete tube was measured with
a caliper. The number and position of externae of the
parasitic rhizocephalans Peltogaster boschmae Reinhard
and Thilacopk'thus(=Thompsonia)reinhardiLutzenwere
alsonoted. Becausewedid notassessthepresenceofroot-
lets penetrating major organs (stage of interna), which
precedes the parasite's sexual development, we may have
underestimated the extent of infestation within the
sample.
To describe the format of relative growth (i.e.. the
change in shape with growth; Hartnoll, 1982), we mea- 1.2 2 24 28 32
suredthe length ofthedactyl(DL)and palm (PA)ofboth SIZECLASSES (SL, mm)
chelae, and theirdepth (DE) in 130 hermits. To represent n = 104 i = 100
the patterns ofthe relative growth ofthese measures (y)
with respect to the SLas an independent variable (.\). the Figure 2. Size class distributions compared between sexes from
natural logarithmic transformation (In y = In a + h In x) summercollections.
ofthe exponential function y = a xh was used. This re-
lationshipfitsnearlyalltheinstancesofallometricgrowth Data on egg incubation and hatching were obtained
in crustaceans (Hartnoll, 1982). The values of h define from 19 ovigerous females, collected on January 18 (5),
the type of allometry (b = 1: isometry; b < 1: negative February 8 (5), and April 27 (9) 1993. In the laboratory,
allometry; b > 1: positive allometry). This and the other the females were placed in individual glass bowls 20 cm
parameters of In y on In A, calculated using the Least- in diameter, in filtered seawater with a salinity of 28-
SniqfuiacraensceMeatnhdotdo,caolmlpowaerdeusslotpoesusaensdtainndtaerrcdeptetsstsbeftorwesiegn- a3t1%1o.0TCheunbdoewrlsawelirgehtk:edpatrkinraegciomnestaonft1t4e:m10p.erUanttuirletuhneiyt
groups. released larvae, ovigerous females were checked twice a
Within winter samples, pleopods were examined and day for hatching, placed in bowls ofclean seawater, and
theirconfiguration related to the occurrence ofparasites. fedAnemia. The number oflarvae released daily was re-
The configuration ofpleopods in D. schmittiwas first de- corded for ten ofthe females.
scribed by McLaughlin (1974);thespecies showspleopods For statistical analysis, we followed the methods and
2-5, with the exception ofsome males, in which the sec- recommendations ofSiegel (1956) and Zar (1984). The
ond pleopod is absent. level ofsignificance under which the null hypothesis was
The behavior ofanimals occupyingpieces oftranspar- rejected is a = 0.05.
ent glass tubing was recorded with a Panasonic color
camera and played back on a Mitsubishi recorder. Results
we
Population struct
Figure 1 shows the relationship between SL and body
weight (excluding chelipeds) compared between sexes. No
between-sexdifferencewasfound ineithertheslope(32.12
vs. 43.96, / = 1.549. df= 81. ns) or the intercept (-49.90
vs. -78.98. t = 1.781, df= 82, ns) ofthe regression line.
The sizes ofcrabs (SL) during summer were analyzed
(Fig. 2). Nosignificant difference in sizedistributionswas
found between the two sexes(G = 4.442, df= 7, ns). The
smallest specimens measured 1.1 (prepubertal, showing
mm
no gonopores), 1.4 (male), and 1.9 SL (female), and
-i r the maximum size attained 3.9 mm SL in the two sexes.
1.5 25 3 3.5
SHIELDLENGTH (mm) The sex ratio was 50.98% (104 males to 100 females),
which did not differ from 1:1 (X2 = 0.044, df = 1, ns).
Similarly, the sexes remained balanced when three size
Figure 1. Relationshipbetweensize(shieldlength)andbody(without classesweredistinguished (<2.6 mm SL: X: = 0.5,df= 1,
c(frhoeu=lnid0p.e8dis0n)9,bwodetifhg=htm,4a0lc,eos/m>p(<ar0r.=e0d1)0b..e69t2w.eednfse=xes4.1,A/>po<sit0i.v0e1c)o,rraenldatifoenmawlaess nXs2;=2.06,-3d.f4=m1m, nsS)L.: X: = 0. df = 1, ns; >3.4 mm SL:
70 F. GHERARDI AND P. M. CASSIDY
Table I
Isometricorallomeiricgrowth withsueof Discorsopagurusschmitti (shieldlength) ofthreemeasuresofboth themajor(right) andtheminor(left)
chela, comparedbetween sexes
In shield length vs.:
Si 99
b*
1
DC
In (majorchela)
LIFE HISTORY OF A TUBE-DWELLING HERMIT CRAB 71
value of the correlation coefficient did not significantly
n = 30 differ from 3 (/ = 0.495. df = 28, ns): that is, clutch size
is proportional to the cube of the SL (roughly equaling
the body mass).
N The mean eggdiameterwas 722 ^m (SE = 19, n = 30),
co
ranging from 455 to 990 ^m. A positive correlation was
also found between the SL ofthe female and the average
diameter of her eggs (r = 0.586, df == 28, P <0.01.
b = 0.14, a = 0.40) (Fig. 3B), showingthat biggerfemales
produce larger (and more numerous) eggs.
Eggs are attached to the second through the fourth
i 1 1 1
OS 1 12 pleopods, about 100 per pleopod, in bunches of7 to 15.
^SHIELDLENGTH(mm) Theyare slightly ovate and attached by a funiculus, mea-
suring around 1.2 mm. Ovigerous females kept inside
B transparent tubing were seen fanning the eggs with a re-
versingcurrent created by thesecond and third pleopods.
1000-1 n = 30
Hatching
Hatchingoccurred between 1 and 75 days(n = 17)after
collection. Because all the analyzed females bore eggs
when collected, this is only a minimum estimate ofthe
actual length ofegg incubation.
The number of larvae per individual ranged from 80
to 541 (average = 226, SE = 46) in the 10 females ana-
115 SHIELDLENGTH25(mm) 35 elygzgesd,pearndbadticdhn(o/t=dif1f.e3r1.sidgfni=fic3a8n.tlnys)f.robmeitnhgeonnumabveerragoef
98.74%- ofthe eggs spawned. Larvae were released in 3-
ldeinaFgmiteghtu)erraen3(d.mbaoxRteihlmatuthimeonansxuhisim,pbsBe)breot(fawftetheeernsatphIeanw-snilzneedtorfeagongvssif.goerrmoautsiofne.maAl)esa(nsdhitehlde 6ofd2a0y9sl(aarvveareagien t=he5f.1oudratyh.daSyE. N=o0.c3o)r.rewliatthioanmwaasxifomuunmd
between the length ofthe hatching period and the female
size (r = 0.074, df= 8, ns), but the former was positively
chelipedswere missing more often than left ones(65.96% related to the overall number oflarvae (Spearman rank
vs. 34.04%, X2 = 4.17, df = 1, P<0.05). correlation test: i\ = 0.742, / = 3.134, df = 8, P < 0.02).
Larvaewere not released at aconstant rate; the percentage
released (Fig. 4) differed significantly throughout the
Eggs
hatchingperiod (Kruskal-Wallisone-wayanalysisofvari-
Egg-bearing females occurred in winter samples only.
They were first found in January and were still present at
the end ofApril, though their percentage was low (20%, 40 -i
9 out of45). Their numbers did not differ from those of
nonovigerous females(January 18: 19 vs. 10, X2 = 2.207,
df= 1, ns; February 1:15 vs. 21, X2 = 0.694, df = 1. ns),
and specimens in the two reproductive states shared the
same size distribution (G = 2.906, df = 5, ns) and fre-
mm
quencypersize class(<2.6 SL 45.10%, vs. a uniform
distribution: G = 0.486, df = 1, ns; >2.6 mm SL 70%,
G = 1.567, df = 1, ns). The smallest and largest females
found bearing eggs measured, respectively, 1.1 and
3.2mm SL.
Eggnumberperclutch rangedfrom 14to496,averaging
287. Female size (SL) was positively correlated with the TIME (days)
number ofeggs (after a In-ln transformation: r = 0.478, Figure 4. Percentage (average SE) ofthe larvae released by K)
df = 28, P < 0.01, b = 2.56, a = 2.48) (Fig. 3A). The femalesplotted against the length ofhatching(in days).
72 F. GHERARDI AND P. M. CASSIDY
ance: H = 29.975, df= 5. P< 0.001 ). peakingin thethird 180-
day and then falling offabruptly after the fifth day. 160 -
Hatching occurred exclusively at night, and mostly 140 -
U
during the neap phase ofthe tide (Mann-Whitney test: S. 120 -
= 0, H = 7 and 7. P < 0.01). when the mean tidal current O> 100 -
is consistently slower (Fig. 5). For our comparison, we LU
defined neap(orspring) phaseastheday ofthe minimum 8(H
(or maximum) tidal excursion fora lunartidal cycle plus oL:L 60 -
the 3 days preceding and following that date. 40 -
20 -
Parasite distribution
Peltogaster boschmae was the most common rhizoce- NUMBEROF EXTERNAE PERHERMIT
phalan parasite in our samples, affecting 18.5% of the
sfepcetceidme6n.s8;%;Tlainidlactohpelettlwnoisr(h=izTohcoeipphsaolnaincsi)cor-eoicnchuarrrdeidiinn- Figure 6. Number ofthe externae of rhizocephalan parasites per
hermit host, compared between sexes.
2.5%. These figures are similar to the percentages reported
by Lutzen (1992) for a previous study in the same area.
Asexual differencein thedegreeofinfestation wasseen
in both the number of externae per individual (males: threeexternae(P. M. Cassidy. pers. obs.),but it isplausible
average = 2.9, SE = 0.6; females: average = 2.3, SE = 0.9; that infestation occurred after spawning. The number of
X2 = 9.312, df= 2, P < 0.01) (Fig. 6) and the prevalence externae was significantly correlated with the host size if
ofparasitism (i.e.. the percentage ofthe parasitized her- the host was male (Spearman rank correlation test: rs
mits; Margolis el al.. 1982) (males v.v. females: 28.17% vs. = 0.413, / = 2.759, df = 37, P < 0.01). but not ifit was
14.39%, X2 = 7.144, df = 1, P < 0.01). Similar results female (r, = 0.299, / = 1.331, df = 18. ns).
wereobtained from thewintersamples, whereparasitized The frequencydistribution persizeclassofthe infested
males and females scored 28.26% and 9.37%. respectively specimens compared with respect to the healthy onesdid
(X2 = 5.424, df= l,P< 0.02). The numberofparasitized not show any difference in the males (G = 4.816, df= 3,
specimensdid not differbetween sampling periods (sum- ns)(Fig. 7A); adifference(though slight)wasfound inthe
mer: 60 out of 281. winter: 19 out of 1 10; X2 = 0.01, females, where parasites occurred more often within
df= 1, ns). smaller size classes (G = 6.913. df = 3. P cu. 0.05) (Fig.
samTphleesmwaaxsim15uminnmuamlebseranodfe1x8teinrnfaeemaflreosm. tWhee scouumnmteerd t7Bw)e.enW-sheexnditfhfreereensciezewcalsassfeosuwnedreindissmtailnlgusipsheecdi,mennosb(eG-
23 externae in one female collected in winter. None of = 0.004, df= 1, ns). but the difference was significant in
tO2hn0el%yoivoningneeornfooeumvsailgfeeemrhaoalusessboenienenso:cuoXrl2lse=catm5ep.dl3be3e4ah,radidfngp=abro\ats.hiPtee<gsg0s(.00a5nv)sd.. lbsiiazgreggeesrotfc:lapGsasrea=ss)i2ti.in7zt4ee4dr.mesddpfieac=tiem:1e.GnPs=cwa1a.0s.06.40135.,8).difTnh=efe1m,ma.ilPne<si0.ma0un1md;
mm
1.4 SL in males.
Peltogaster hoshnuie externae never exceeded two per
thiadtecchyicnlgedsa=ys4=93 individual. They were more frequently found on the left
side ofthe hermit abdomen (left, center, and right vs. a
^ uniform distribution: X2 = 76.513, df = 2, P < 0.001),
Q and at the proximal end, close to the carapace (proximal,
middle, distal v.v. a uniform distribution: X2 = 34.241, df
LuO_U = 1. P < 0.001), without any difference between sexes
(side: G = 1.1 18, df = 2, ns; extremity: X2 = 0.029, df
O<Dr = 1, ns). Their first point oferuption corresponded tothe
z
< position ofthe second pleopod. In contrast, the externae
01 ofThihicopletluis(=Tlioinpsoniu) rcinluinli. ranging from
SPRING NE1AP SPRING 1 to 23, were more clumped on the host, equally distrib-
TIDECYCLE(days) uted on the right and left halves ofthe hermit body, and
more diffused, involving the dorsal side ofthe abdomen,
Figure?. Occurrenceofhatchingwithinalunartidalcycle,compared
ith the mean tidal currentspeed perday (average SE). thecephalothorax, and even thepereiopodsand chelipcds.
LIFE HISTORY OF A TUBE-DWELLING HERMIT CRAB 73
70 - either in males (after a In-ln transformation: b, 0.63 vs.
0.93,? = 1.772, df = 56, ns; a, 0.21 vs. -0.15,? = 1.847,
df = 57. ns) or in females (/>, 0.52 vs. 0.57, / = 0.237, df
= 63, ns: a, 0.40 vs. 0.33. / = 0.01 1, df = 64, ns). Anal-
ogously, parasitesseemed nottoaffect hermitrelativebody
weight in either sex (after In-ln transformation. SL vs.
=
weight without chelae, males: b, 3.45 vs. 5.08, / 1.539,
df = 39, ns; a, -1.13 vs. -3.31, / = 0.043, df = 40, ns;
females: b, 1.70 vs. 3.59, / = 0.778, df = 38, ns; a, 1.32
vs. -1.04, / = 1.216, df= 39, ns). The same wasalso true
when cheliped weight was examined (males: b, 8.80 vs.
24 3.2 7.26, / = 0.537. df = 24, ns; a, -15.41 vs. -11.97,
SIZE CLASSES (SL, mm) t = 0.858, df= 25, ns: females: b, -2.19 vs. 6.41, / = 1.664,
UNINFECTEDn = 102 INFECTEDn =40 df= 22, ns, a, 13.89 v.v. -9.34, / = 1.043, df = 23, ns).
Several parasitized hermits were seen moltingand pre-
served theirexternae aftertheecdysis, even when externae
belonged to the latest stages, already containing oocytes
B and embryos (Liitzen. 1992).
One possible behavioral effect ofparasites is lethargy,
which might reduce the ability of naked hermits to find
empty tubes. However, in pilot experiments in the labo-
ratory, parasitized and unparasitized D. schmitti individ-
uals were about equally matched when competing for a
single empty tube. In the field, no difference was seen in
the relative opening diameter ofthe occupied tubes be-
tween hermits belonging to the two conditions (males: b,
t = 0.025, df = 64, ns, a, t = 0.169, df = 65, ns; females:
b. t = 0.86, df = 68, ns, a,t= 1.913, df = 69, ns).
24 32 Parasitized specimens of both sexes placed inside
SIZE CLASSES (SL, mm) transparent tubingwere seen fanning theexternae ofPel-
UNINFECTEDn=119 INFECTEDn = 20 togaster boschiuae. The behavior was the same as that
I I
already described forovigerous females fanningtheireggs.
Figure 7. Size frequency distributions compared between hermits
that were either uninfected or infected by rhizocephalan parasites in
Discussion
males(A)and females(B).
Thestructure ojthe D. schmitti population
Parasites'effects on the host external morphology and IV)I,nsetxherastpieociinesreolfathioenrmtiotsiczreambosstsltyudfioeldlotwosdtahtee"a(Tnaobml-e
behavior
alous" pattern described by Wenner (1972). This implies
To evaluate the effect of parasites on their hosts, we thatsexesin small sizeclassesareapproximately balanced,
examined various aspects of the hermit external mor- a large excess of females is found in intermediate size
phology. The pleopod number did not show any signifi- classes, andanexcessofmalesisfoundinthelargestones.
cant difference between infested and noninfested speci- Exceptionsarereported byWenner( 1972)in Clibanarius
mens in either sex (distinguishing animals with 4, 3, and zebra and Calcinus latens, by Abrams (1988) in Pagurus
lessthan 3 pleopods, males: G = 1.645,df= 2, ns; females: ochoiensis and P. aleitticus. and by Gherardi and Mc-
G = 0.31, df= 2, ns). However, the second pleopod was Laughlin (1994) in Calcinus laevimanus from the Mas-
absent more often in parasitized specimens (7 out of 16 carenes. A further exception is D. schmitti. in which an
vs. 5 outof54: G= 8.333,df= l,P< 0.01). Thedifference equal number of males and females are represented in
was more pronounced in the males (6 out of 12 vs. 5 out each size class.
of 31: G = 4.576, df = 1, P < 0.05) than in females (1 One obvious bias in the size distribution analysis is the
out of5 v.v. out of23: G = 1.839, df = 1, ns). large- or small-scale habitat segregation between sexes.
The relative growth ofthe DE ofthe major chela, one Species have been reported to show between-sex differ-
"maleness" character, was analyzed. No difference was ences in habitat utilization (e.g., males of Pagurus hir-
found between parasitized and unparasitized specimens. siitiuscnlits occupy high tidepools, but females are dom-
74 F. GHERARDI AND P. M. CASSIDY
Table IV observed in D. schmitti. but the importance for males of
Speciesothermitcrabsreportedfrom theliteraturefollowingthe having larger chelipeds might be associated with the in-
"anomalous"pattern(Wetiner. 1972) inthese.\-ratio-lo-si:erelation trasexual competition to mate. Chelipedsare widely used
in aggressive interactions, both in displays (cheliped ex-
Genus Species Reference tension, waving, and wig-wag display; F. Gherardi, in
CCaolecnionbuixla cloaienvpirmeasnsuuss WWeennnneerr,. 11997722 aprnedp.s)t,roanngderitnheficghhtesli(pheidtss aarned, tghreasmpos)r,ewlhiekerley tthheehbeirgmgietr
lalens Gherardi and McLaughlin, 1994 is to win.
Clibananus digueti Harvey, 1988 In hermit crabs, factorsthatcould reduce thetendency
erythropus Gherardi, 1991 to grow are the interspecific competition for shells and
Diogenes blInrateemvivilirtom.ssalnruiss GGWhhaeelrrteaarrrsddiianaeldndaGt.r,Mifcf1Li9at9hu4sg,hl1i9n8,7 1994 tabhielistcyatrociotcycuopfylaermgpethyopuosliyncghsaewtiethtiunbetshaeshaanbietwath.ouBsyinigt,s
Elassochims tenuimanita Abrams, 1988 D. schmitti has freed itselffrom the harsh war for shells
Pagurisles turgidus Abrams, 1988 that occurswithin the subtidal hermit crab assemblage in
Pagurux grano.simii/uis Abrams, 1988 northern Puget Sound (Abrams et ai, 1986). Its small
hirsutiusculus Abrams, 1988 relativesize must have preadapted thisspeciestothisnar-
samuelis Abrams, 1988
kenncrly Abrams. 1988 row microhabitat, but its body mass is certainly con-
beringanus Abrams, 1988 strained by the size distribution of the available empty
dalli Abrams, 1988 tubes. In his ecological notes on the endemic Bermuda
hermit Ca/cinus verrilli. Markham (1977) observed that
the mean size ofthe crabs occupying attached vermetid
inant in microhabitatswithout standingwateratlowtide; shells was far smaller than that ofcrabs in mobile Ceri-
Abrams, 1988). The size ofclustering species is also seg- thium shells. Members ofthe D. schmitti population an-
regated within clumps (in Clibanarius laevimanus, Ghe- alyzed here occupy the largest tubes at their disposal in
rardi etai, 1994). However, both sexes of/), schmittiare the bioherm, and size in both sexes was positively cor-
restrictedwithin sabellarian bioherms(Gherardi andCas- related with tube opening, suggesting that crabs must
sidy, 1994b), starting from the late megalopa stage, and change their housingwith growth (Gherardi and Cassidy,
although the species has a contagious distribution (Ghe- 1994b).
rardi and Cassidy, 1994b), clumps do not significantly
differ in either sex ratio or size. Thegrowth hypothesis
The growth hypothesis (Abrams, 1988) refers to the
The sexualselection hypothesis
between-sex difference in theavailableenergy forgrowth;
Under the rationale ofthe sexual selection hypothesis the male-biased sex ratio in larger size classes in most
(Bertness, 1981a), thebetween-sex balance in thesizedis- hermit species is attributed to the additional energy that
tribution ofD. schmitti implies that the two sexes get the males can allocate to growth because they do not have to
same benefits (or handicaps) from larger dimension. produce eggs (Bertness, 1981b). Data are still missing for
Reproductive potential might be enhanced with size. the extent ofgrowth through molts in D. schmittiand its
From the perspective ofD. schmitti females, clutch size energy-time budget is unknown, but a number ofclues
significantly increases with the body mass, and larger fe- suggest that thedistribution ofthe rhizocephalan parasites
males also bear more voluminous eggs. might affect growth in this species.
Larger size might also provide a higher reproductive Inthe population weexamined, theextentofinfestation
potential to males. A sexual dimorphism was evident in and parasite prevalence varied significantly between sexes,
the major chela dimensions (dactyl length, and palm reaching in the males an average of2.9 externae per in-
length and width): the chela (especially the biggest) was dividual and a percentage of28 infested specimens. Prev-
more massive in the male than in the female. The func- alence is unaffected by the male host size, but the fre-
tional significance ofthissexual difference hasbeen widely quency ofinfested females decreases in the intermediate
discussed for Brachyura (Hartnoll, 1974), where it was and largersizeclasses, wherethe infestation issignificantly
related to the use ofchelipeds in territorial defense, com- less diffused than in similar sized males.
bat,display, andcourtship. In several hermitcrabs, males Within the framework of the growth hypothesis, one
showed complex precopulatory behaviors, involving the likely conclusion drawn from these data is that if(1) the
chelae, forexample, eitherrotatingand shakingthe female males are more frequently infected than the females, and
(Diogenidae) or jerking her toward himself (Paguridae) if(2) parasites cause a reduction in the growth rate ofthe
(Hazlett, 1966, 1968). Sexual behavior has not yet been host, then the two sexes grow to the same extent because
LIFE HISTORY OF A TUBE-DWELLING HERMIT CRAB 75
the energy the females expend in producing eggs equals The only alteration is the frequent absence ofthe second
thatwhich the malesconsume to support parasites. How- pleopod, which cannot result from an attempt bythepar-
ever, the twoassumptions require furtherclarification and asite to provide a safe accommodation for the externae,
open new questions. but seems instead to be a consequence ofthe eruption of
First, we do not know why the parasites are unequally Peltogasterboschmaeexternae within the softtissue lining
distributed between the sexes. The observed pattern could the host abdomen, which corresponds to the attachment
not be explained by either an increased mortality rate of point ofthe second pleopod.
infested femalesortheoccurrenceofsex reversal, because Neither do infected hermits exhibit behavioral altera-
thesexratiowas50% inallthesizeclasses.Theattachment tions, such as lethargy, that could decrease their ability
ofthe parasite larvae may be impeded by the efficiency in direct or exploitative competition: in the laboratory,
ofcleaningandgrooming(Bauer, 1981), but thetwo sexes parasitized and healthy D. schmitti had the same proba-
did notdifferin eithertheextentorthe modesofcleaning bility of getting an empty polychaete tube, and in the
behavior (Gherardi, 1994). As a third explanation, im- field, they occupied equally sized housings. Besides, rel-
munological responses by the hosts might vary between ativeweight, and thus possibly feedingefficiency, was un-
sexes. Parasitized, but not normal, Carcimis mediterra- affected by the presence ofrhizocephalans. The only be-
neushaveasubstanceintheirbloodthatfixescomplement havioral result ofparasite manipulation is the initiation
in the presence ofextractsofSacculina (reviewed in Bang. ofmock parental care, in which infected hermits ofboth
1983). However, in that parasitic relationship, electro- sexesventilatePeltogasterboschmaeexternaeinthesame
phoregramsdid not show any marked difference between way that gravid females ventilate their eggs.
the parasitized males and females (Herberts. 1978). D.
schmitti females differ in their susceptibility to infection Reproductivepatterns
No
according to their reproductive states. parasitized fe-
males have been found in ovigerous condition (otherex- D. schmittifemalesattain maturity atarelatively small
mm
amples in Hoggarth, 1990, and Liitzen and Jespersen, size: thesmallestegg-bearingspecimen measured 1.1
1992; exceptions in Hoeg and Liitzen, 1985); one expla- SL. On the other hand, the allometry of chela growth
nation is that parasitized females lose their eggs after a should indicate that maturity (at least, functional matu-
few days (Liitzen and Jespersen, 1992), but the reasons rity; Hartnoll, 1969) occurs in males at larger size (over
mm
remain unknown. 3.4 SL).
The second assumption, that growth rate ofthe host is A precocious onset ofsexual behavior in females has
affected by the parasite, is supported by the previous lit- been reported in the decapod literature and associated
erature on rhizocephalan infestation (O'Brien and Van with a reduced possibility ofencountering males; in the
Wyk, 1984; Hawkes et ai. 1986; Hoggarth, 1990; Abello parasitic females ofthe Pinnotheridae (Christensen and
and Macpherson, 1992; Bang, 1983; Overstreet, 1983). McDermott, 1958) and in the freshwater crab Potamon
Nevertheless, a direct investigation of molt frequency is Jhtviatile(Miche\i et al.. 1990)copulation can occureven
lacking and figures on the relative increase at ecdysis in prepubertal females, and sperm are kept intheseminal
compared between infected and uninfected individuals receptacles until ovulation.
are provided only by Liitzen and Jespersen ( 1992). Our A second remarkable featureofreproduction isthelow
findingsthat parasitesdo notinhibit moltingin D. schmitti frequency (50%) ofgravid females in all the size classes.
or influence either body or cheliped weight in either sex This is particularly evident when we consider that D.
make the growth hypothesis questionable, at least in this schmitti breeds only once per year (Nyblade, 1974), and
species. that the breeding period (January-April) is short, but the
time necessary for eggs to mature is relatively long (ex-
Other effects ofparasites ceeOdnineg,exopnlaanvaetriagoen,is1 tmhoantthd)u.e to the shortage of food,
A variety ofmorphological and behavioral alterations females may have limited energy for producing clutches,
exhibited by rhizocephalan-infected decapods and the causing them to skipthe reproductive season. Ifthiswere
hormonal involvement in those phenomena are exten- the case here, we should expect a gradient in the clutch
sively described by Hartnoll (1967), Nielsen (1970), and size depending on the available energy. Nonetheless, egg
Phillips and Cannon (1978) among others (see, e.g.. bib- number is a function offemale size, and the latter is not
liography by Overstreet, 1983). D. schmitti males do not relatedto feedingefficiency(Gherardi, 1994). Inaddition,
undergo the process of feminization observed in other the annual egg production (52.8 mg ofeggs per 100 mg
species (Hartnoll, 1982; O'Brien and Van Wyk. 1984). as female weight per year; Nyblade 1974) is high compared
evidenced by the preservation ofsome "maleness" char- with that of the other hermit crabs in northern Puget
acters (e.g., the high relative depth of the major chela). Sound.
76 F. GHERARDI AND P. M. CASSIDV
Another hypothesis refers again to the difficulty that Shannon Point Marine Center, Anacortes, Washington.
this sedentary species encounters in finding a mate. D Partial funding was provided by M.U.R.S.T. to the first
schmitti is gonochoristic and mating in hermit crabs author.
requirescopulation (Hazlett, 1966). hut it isstill unclear
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