Control of Hatching in an Estuarine Terrestrial Crab I. Hatching of Embryos Detached From the Female and Emergence of Mature Larvae

8 Pages·1992·3.3 MB·English

by  M Saigusa

Reference: Biol Bull 183:401-408. (December, 1992)
Control of Hatching in an Estuarine Terrestrial Crab
I. Hatching of Embryos Detached From the Female
and Emergence of Mature Larvae
MASAYUKJ SAIGUSA
College ofLiberalArts & Sciences, Okayama University, Tsushima 2-1-1. Okayama 700. Japan
Abstract. Embryos ofthe terrestrial crab Sesarma hae- mechanisms and ecological significance ofsuch systems
matocheirhatch simultaneouslyjust priorto theirrelease in marine organisms is severely limited. The reasons are
into water. Larval hatching occurs synchronously when evident: relatively inaccessible habitats, technical diffi-
the embryos are attached to a female, and the role ofthe culties in monitoring the activity, and the noise that is
female in this synchrony has been investigated. Clusters often observed in locomotor activity patterns. Notwith-
ofembryos (200-2000 berries in each cluster) were de- standing these restrictions, clearly-demarcated biological
tached from ovigerous females, and their hatching was rhythms associated with reproductive activity have been
compared with that ofembryos attached to the females. demonstrated in a variety ofmarine animals; e.g.. a poly-
Ofthe detached embryos in a cluster, either all hatched, chaete (Franke, 1985), intertidal midges (Koskinen, 1968:
ornonehatched. Aremarkablefeaturewasthatthesuccess Hashimoto, 1976), and estuarinecrabs(DeCoursey, 1979.
of hatching of these detached eggs depended upon the 1983; De Vries and Forward, 1989). This paperalso con-
time ofhatching ofthe eggs still attached to the female. cernsa rhythmical reproductiveactivity, focussingon the
Clusters ofembryos that were detached from the female control oflarval hatching in a marine crustacean.
within 48-49.5 h of the projected time oflarval release Fertilized eggsofmost marinecrustaceansareattached
all hatched successfully, and swimming zoeas appeared. bya funiculustotheabdominal appendages ofthe female,
But embryos that had been detached from the female for and are ventilated by the female during embryonic de-
longer periods did not hatch at all. though they were ob- velopment. When thedevelopment iscomplete, theouter
viouslyalive. These results suggest a hatchingprocessdif- egg membrane breaks, and the larvae hatch. These larvae
ferent from the embryonic development process. The fe- are released into waterwith a special fanning behaviorof
male may trigger this process. In addition, detached eggs the female's abdomen. Larval release isgenerally a short-
hatched later than eggs attached to the female, and their lived event, and the timing is often correlated with such
hatching was less synchronized. These observations sug- environmental periodicities as day-night, tidal, or lunar
gest that the female not only initiates hatching, but also cycles. In lobsters (Ennis, 1973; Branford, 1978: Moller
enhances the synchrony ofhatching. and Branford, 1979), forexample, larvaehatchatthesame
time each night. The fiddler crab Uca also releases larvae
Introduction for several minutes, in synchrony with nocturnal high
water(DeCoursey. 1979, 1983). Sucha short andprecisely
Studies over the past 50 years have demonstrated that timed event implies that the timing oflarval emergence
many biological rhythms are driven by an endogenous from the egg capsule must be synchronized within each
pacemakerwhich, when coupled toan environmental cy- batch ofembryos. Accordingly, we must ask whetherthe
cle, adjuststhe phase ofthe rhythm to local time. In con- timing ofhatching is controlled by the embryo itself, or
trast to the abundance of information about the timing by the female.
systems in terrestrial animals, our knowledge of the Previous studies of this problem have produced con-
flictingresults. ForthelobsterHomarusgammarm, some
Received 2 May 1991;accepted 26August 1992. investigators indicated that an endogenous factor in the
401
402 M. SAIGUSA
embryoitselfscisv gtime(Pandian, 1970;Ennis, bryos still attached to the female. (The terms 'egg' and
1973). Forward . 1983) showed that em- 'embryo' are used synonymously in this paper. The em-
bryoscontrc inning, and suggested that the bryo isconsidered assuch until it hatches; itisthen called
role hronize the development of a zoea larva.)
theembrvos. ei hand, Branford (1978)showed Detached eggs were prepared as follows. A cluster of
that the female jbster controls the timing of hatching. embryos(200-2000 berries) was removed from the female
This study is also aimed at this general problem, and the togetherwith aportionoftheovigeroushairs. Thiscluster
focus is on the distinction between the control of the wasthen suspended bycotton threadtiedtoahorizontal
hatching process and the control ofembryonic develop- nylon thread in the center ofa small plastic container
ment. (usually with a diameter of8 cm and a depth of6 cm).
Females ofthe crab Sesarma haematocheir were used This container was then placed into a 1 1 glass beaker
in these experiments. Each female incubates 20,000- containing about 600 ml ofdiluted, clean seawater. (The
60,000 eggs on her abdomen. While incubating, the fe- seawaterwassterilizedbyboiling,anddilutedwithdistilled
males remain hidden under litter on the slopes ofsteep, watertoasalinityof 10%o). Thewaterwasstronglyaerated
wooded hills overlooking small estuaries. After the em- with an air stone placed in the bottom ofthe container
bryos have developed for a month, the female emerges, (for a figure, see Saigusa, 1992).
descendstotheriverside, and releasesherzoealarvaeinto The embryo cluster was detached from the female at
the water. Larval release occurs at night. Release is syn- varioustimes ofday forexperimental purposes (e.g.. Fig.
chronized with the time of high water (Saigusa, 1982, 2). In the 1988 experiments, detachment from, and bind-
1985), and thetiming isendogenously controlled (Saigusa, ingtothe nylon thread werecarriedoutwith asmall hand-
1986). Field observations (Saigusa, 1992) demonstrated held flashlight during the dark period. The intensity of
that larval hatching occurs on the bank, just before the the light was very much reduced. The procedure was dif-
larval release, and not in the water. ficult with a hand-held light, soa head-attached lightcov-
Thus, release behavior is caused by the stimulus of ered with a few sheet of red cellophane was used in the
hatching; i.e., what is actually controlled is the timing of experiments of 1989 and 1990. To reduce the influence
larval hatching from each egg, not the release behavior of light as much as possible, the manipulation of each
itself. Hence, the main questions to be answered are: ( 1 ) sample was completed rapidly, i.e., within 5 min. Thus,
how do the larvae hatch simultaneously; (2) how is the each female experienced the light at different times, and
timingsynchronized tothetime ofhigh tide at night; and onlybriefly(Fig. 2c). Almostall ofthecrabsreleasedtheir
(3)doesthe female, ordotheembryos, control thetiming zoeas within 4-5 days after having been placed in the ex-
ofhatching? To answer all ofthese questions, many ex- perimental rooms. Therefore, such aweak and brieflight
perimentswillbe necessary. Butasafirststep, thehatching isnot likelytohaveaffected the timingofhatching, either
time and hatching synchrony ofdetached embryos were ofthe isolated embryos, oroftheembryosattached tothe
compared with those ofembryos attached to the female. females.
Detachedeggsthatdid not hatchwereplacedinabeaker
Materials and Methods for 7-10 days, and if vigorous aeration and turbulence
were provided, these isolated embryos remained alive.
Female Sesarma haematocheir bearing eggs that ap- Detached eggs hatched on the same night as those still
pearedlikelytohatchwithin 1-10days(seeSaigusa, 1988 attached tothefemale, and all ofthe larvaeemerged from
for signs) were collected from the field at Kasaoka, Oka- theireggcaseson a single night, orby noon ofthe follow-
yama Prefecture (Saigusa, 1982), and brought into the ing day, at the latest. After the larvae escaped into the
labors The dates ofcollection were: 23 and 27 Sep- water, the empty egg cases still remained attached to the
tembei and 29 July 1989, 6, 14, and 28 August ovigerous hairs. Therefore. I easily determined whether
1989: 2 and 5 September 1990. The females hatching had occurred and what proportion ofthe swim-
were -\perimental rooms where light and ming larvae had emerged. This examination was mostly
tempi.-' <oiled. A 15 h light:9 h dark pho- carried out during the light period, and a stereo-micro-
toperiod, s he field, wasemployed (light- scope was used as required.
on at 5:00: 1 . 0). Temperature was main- Insomeoftheexperiments, the hatch timeofdetached
tained constant ( iroups of 1-5 females eggs was monitored. At intervals of 30 min or 1 h, the
were selected and > fourexperimental rooms un- plastic container in which an eggclusterwas being main-
derthesamecondi.-. nightandtemperature. Embryos tained was transferred to another beaker with a similar
were then detached from these females. Hatching ofthe quantity ofdiluted (10%o) seawater. The original beaker
larvaefrom theseeggs,theswimmingabilityofthelarvae, was then removed from the experimental room, and the
and the timing ofthese events were compared with em- numberofswimming zoeaswas counted with the help of
LARVAL HATCHING OF A TERRESTRIAL CRAB 403
a pipette. This exchange ofbeakers was made during the Time of day
darkphaseundertheillumination ofahand-held flashlight 18 24
covered with red cellophane.
Each female from which some embryos had been de- 20,000-1
16,200
tached was also monitored, and the time oflarval release
recorded. The recordingsystem consisted ofasensorunit oN
(infrared source-receiver) and a controller unit with a
photoelectric switch. The sensor unit was placed in the
10,000-
experimental room, andthecontrollerunitwassetoutside 6,870
the room. The larval release could thus be monitored
without a change in ambient light. The female was con-
fined in a perforated plastic cage suspended from the rim
ofa 1 or2 1 glassbeakercontainingdiluted, clean seawater
(500 or 750 ml; salinity at about 10%o). Only the bottom 0:40
ofthebeakerwasimmersed inthewater. When thelarvae D 60-
O)
were released, they fell through the perforations and into
the beaker, where they triggered the photoelectric switch.
The output of this system was monitored by an event
recorder (Saigusa. 1986). Females release their larvae as
soonashatchingiscompleted, sothetimeofreleaseclearly
marks the completion ofhatching.
The time ofdaythat each female released herattached
larvae was compared with the time ofhatching from the
eggs that had been detached from her. However, the day
during which a female released her larvae could not be
precisely estimated. The signsofhatching only appearon
the day oflarval release (i.e., a few zoeas begin to swim
in theglassbeakeronly a few hoursbefore larval release).
Many ofthe data were like those set out in the uppertwo
sectionsofTable I. Hence, manycomparisonsofdetached
and attached embryos were necessary; i.e.. 250 samples
from 1 10 females were examined.
Results
404 M. SAIGUSA
Time of day differ from that of an attached cluster and may be less
24 suitable for development. In that case, the induction of
hatching and the appearance ofswimming larvae should
be dependent on the interval between detachment and
natural (female-attached) hatching. If this possibility is
accepted, then the hatchingrateofthe isolatedeggsshould
gradually decrease as the time ofseparation from the fe-
male increases.
As a test ofthis hypothesis, embryos were detached at
varioustimesofdaybeforethelarval releasebythefemale,
and the hatching success rate was recorded. Forthis pur-
pose, manyovigerousfemalesthatwereexpectedtorelease
larvae within a fewdayswere collected from the field and
brought to the laboratory. They were set individually in
the apparatus used to record larval release. Eggs were de-
tached from these females several times per day, and the
success of their hatching was monitored. Table I sum-
marizes the results from three specimens that released
their larvae on 16, 20. and 18 August, respectively. Eggs
separated from female B-5 all hatched on the same night
as the larval release by the female, but the embryos de-
tached from another female. A-15, all failed to hatch.
Finally, ofthe embryos from female C-14. most failed,
but the embryos contained in the last cluster ofeggs to
be detached did hatch. The time ofeach trial, from de-
tachment ofthe eggs to the larval release by the mother
female, is listed in Table I.
Aug.28 Aug.29.1989
Experiments similarto those presented in Table I were
Figure 2. Comparison ofthe hatchingfrom theeggsdetached from carried out with 250 samples from a total of 1 10 females.
asingle femaleat different timeson theday oflarval release. Timesof Theseexperimentsdemonstratethat inductionofhatching
removal(downwardarrows):(a) 15:12:(b) 18:06;(c)21:25on28August
1989.Infigure2caredlightwasusedfor5mintoremovetheeg cluster.
Larval release ofthe female: 22:20on 28 August 1989(upwardarrow).
N: total numberoflarvaeemerged. Time of day
n ' ' r 12 24 12
30-
dayoflarval releaseandonedayearlierall hatch regardless
20-
oflight conditions.
In contrast to these results, most embryos detached 10-
more than 2daysbeforelarval releasedid nothatch. When
suchdetachedembryoswereplaced inabeakerforaweek
or longer. 1 f.-hing occurred sporadically. Those larvae
that did ,. had poorly developed telsons, dorsal 30-
spines, inal appendages. Thus, they were pre-
mature hu /oeas), and they almost never de- 20-
veloped mi e prezoeas sank to the bottom of
10-
the beaker.
Embryosend membraneremainedalive,
ifthey were aerate- -at wasclearly recogniz- Aug 6 8, 1989
able under the mio movements of the em- Figure 3. Comparison ofthe hatching from eggs detached from a
bryos were often obser\ through the transparent egg singlefemaleonthedayoflarvalrelease(a)withthehatchingfromeggs
capsule. These movements are a common feature ofem- detachedonedaybeforethelarvalrelease(b). Timesofremoval(down-
bryos attached to the female. Fungi and protozoans were ward arrows): (a) 16:20 on 7 August 1989 (hatchingwas monitored in
darkconditionsafterdetachment);(b)around 16:00on6August 1989.
neverobserved toovergrow the surfaceofeggs. Neverthe- N: total number oflarvae emerged. Larval release ofthis female: 0:55
less, the local environment ofa detached eggcluster may on 8 August 1989 (upwardarrow).
ARVAL HATCHING OF A TERRESTRIAL CRAB 405
1
Table I bytheembryo itself, the female, orboth. Previousstudies
Hutchingprofilesojdetucheilemhryitxandlarvalreleasehyihcfemale have led to different conclusions. Pandian (1970) sug-
gested that a clock that sets the hatching time is within
Female theegg. Ennis(1973)alsoproposed an endogenous factor
identification Timeofday Larval release controlling the timing ofhatching, but was inconclusive
number ol'detaelinient by the female about whether it is in the embryo or in the mother.
B-5 14Aug. 21:07(O),23:06(O) 16 Aug. 00:20 More direct evidence derives from the experiments in
[27:13] [25:14] which aportion ofembryoswereseparated from a female,
15 Aug. 2I:27(O) and the time ofhatching ofthose detached embryos was
[02:53] compared with that ofthe embryos still attached to the
A-15 15 Aug. 22:17 (x) 20 Aug. 01:40 female. In some crustaceans, the embryos can complete
[99:23] their development and hatch as viable larvae even when
16Aug. 02:01 (x), 23:00(x) they are separated from the mother. Branford (1978) re-
17 Aug.[0915::3398](X),[0734::0409](X) ported that Homarm eggs removed more than 10 days
[72:02] [70:31] prior to larval release still hatched. Similarly, in the es-
tuarine crab Rhithropanopeus, hatching occurred when
C-14 14Aug. 20:17 (X). 22:56(X) 18 Aug. 01:00
theembryoswereseparated 1-5 daysbeforelarval release
[76:43] [74:04]
15 Aug. 21:10 (X) by the female (Forward and Lohmann, 1983). Detached
[51:50] embryos ofHomarus hatched rhythmically in a 24 h LD
16Aug. 00:17 (X),02:35 (O) cycle, but arrhythmically under constant light (LL) or
[48:43] [46:25] constant dark (DD) conditions (Branford. 1978). This
O: Aclusterofeggs from which all larvaehatched. might suggest an exogenously cued hatching rhythm. On
X: A clusterofembryos which did not hatch. Hatching time ofthe the other hand, when the ovigerous female was kept in
DD
embryosattachedtothefemaleisexpressedasthetimeoflarval release conditions, larval hatching showed a marked 24 h
bythat female. Thefemaleidentification numberisthatoftheappartus rhythmicity, so Branford (1978) concluded that any en-
in which larval release was monitored.
dogenous component ofthe rhythm is located in the fe-
male. The results ofForward and Lohmann (1983) were
depends upon the time of larval release by the female, somewhat different from those of Branford (1978): em-
not on the time of egg detachment (Fig. 4 top panel). bryos removed from the female within two days oflarval
Those embryos that had been separated from the female release hatched at a similartime to the larvae released by
for more than about 48 h prior to the larval release ex- the female; hatching synchrony deteriorated with longer
hibited neither hatching nor swimming zoeas. Closer ex- removaltimes. From theseresults. Forwardand Lohmann
amination ofthe data presented in Figure 4 (top panel) ( 1983)concludedthatthetimingofhatchingiscontrolled
showsthat thedrastic change in hatchingsuccessoccurred by the embryo, and that the role ofthe female is to syn-
within acritical interval: 48-49.5 h (theshadowed interval chronize embryo development.
in Fig. 4 bottom panel). In Sesurtihi haematocheir, when some eggs were de-
A question that still remains is whether clusters ofem- tached from the female, andtheirhatchingwascompared
bryos detached successively from a single female would with the eggs left attached to the female, the hatching
hatch or not depending on the time oflarval release. To synchrony ofthe detached eggs decreased (Fig. 1 ). These
answer this question, I examined the hatchability ofem- featureshavealsobeen observed inothercrabsinhabiting
bryos obtained from at least two egg clusters that were estuaries: Neopanope sayi, Uca pugilator, and Sesarma
taken from a single female within several hours before or cincrciini (De Vries and Forward, 1991). Desynchroni-
after the 48 h interval leading up to larval release. The zation and delay ofhatch time increased when the eggs
resulting data (similar to those for female C-14 in Table were aerated at 15C (unpub. data). So temperature is
I) were selected from among the values in Figure 3b. All clearly one ofthe factors affecting the timing ofhatching
14 instances showed the same tendency (Table II). A in detached eggs. But the data ofFigure 1 cannot be ex-
comparison ofthe timeatwhich each embryoclusterwas plained in terms oftemperature alone; the desynchroni-
detached from a female, with the time oflarval release by zation and delay ofhatch timecould beduetotheabsence
that female, led to the hypothesis that there is a critical ofsome cue from the female.
interval at 48-49.5 h before the time oflarval release. Furthermore, iftheembryosofSesarma haematocheir
were detached from the female sooner than 48-49.5 h
Discussion
before larval release, then only sporadic hatching (delayed
A major question raised by these observations is by a week) occurred, and the larvae produced did not
whetherthetimingofhatchingin crustaceansiscontrolled swim (Fig. 4 and Table II). To explain the failure of
406 M. SAIGUSA
100
0> I
-1 -2
Time in daysrelative to thereleaseof larvae
00
1 -i
50-
Time from detachment to larval release
Figure-4. Relation between the successofhatching in detached eggclustersand thetime ofseparation
ofthose eggs from the females. Top panel: Hatching profile ofeggs separated from the female 0-7 days
beforethelarval release. Thedatawereobtained from I 10femalescollectedduringJulyand August, 1989.
Blackdotsrepresentthepercentageoflarvaehatched from an isolatedeggcluster. Duetothelargenumber
oftrials(250 samples), especially in the region of24 h, 48 h, and 36 h, some data obtained around those
times are indicated by one dot when the results were identical. Bottom panel: A closer examination of
hatching from eggs detached 10 h before, and 10 h after, a critical interval i.e.. 48-49.5 h before larval
releasebytheirfemales. Thecritical interval(shading)isindicatedby hatching;othersymbolsarethesame
asin thetoppanel.
hatching and swimming in prematurely detached em- Larval release by 5. haematocheir coincides with the
bryos, 1 postulated a process of hatching that, once ini- time ofhigh water at night. The phase ofthis rhythm is
tiated, req;n - 48-49.5 h until the motor activity ofthe endogenously controlled, and the internal period isabout
embryo i>.. ;shigh enough forswimming. Indeed, this 24.5 h (Saigusa, 1986, 1988). Since larval release occurs
process and uial timing ofhatching are controlled asaresult ofsynchronous hatching, thetimingofhatching
by the embi under detached conditions (Fig. 4. would actually be under 24.5 h circa-tidal control. As
top). Ho\u i Uion of the hatching process is mentioned above, the hatching process might start be-
not likely to b' \ within each embryo, as well. tween 48 and 49 h before the larval release. In the field,
Ifthis were i, '.-moved more than two days this would occur roughly two nights before the time of
beforetheaclu. i.gwouldall hatch. Hatch- high tide, although the physiological mechanisms under-
ing ofdetached eggs flowed an "all-or-nothing" lying the commencement ofhatchingare not yet known.
pattern, with a drasti iiiottt 48 h before larval The embryos of most crustaceans are surrounded by
release (Fig. 4, bottom). c results suggest that the two principle membranes: an outer egg-capsule, and a
timing that triggers the start of the hatching process is verydelicate inner membrane investingthe embryo. The
determined bv the female. common properties ofhatching in the Crustacea are that
LARVAL HATCHING OF A TERRESTRIAL CRAB 407
Table II
HatcliniKprofiles oftheembryosdetachedfrom individualfemales
Timeofday theeggsweredetached from a female [Timetolarval release]
408 M. SAIGUSA
Davis ' it hatching in the eggs ofsome fresh-water Franke,H.D.1985. Onaclocklikemechanismtiminglunar-rhythmic
- ' reproductionin Typosyllisprolilera(Polychaeta).J. Comp. Phvsiol.
Davi* she hatching process in aquatic inver- A 156: 553-561.
closionintheAmericanlobster,Homarus Hashimoto, H. 1976. Non-biting midgesofmarinehabitats(Diptera:
ards(Astacura.Homandae).Am.Midi.Nat. Chironormdae). Pp. 377-414inMarineInsects. L.Cheng,ed.North-
Holland PublishingCompany. Amsterdam.
A study ofthe hatching process in aquatic inver- koskinen, R. 1968. Seasonal emergence ofClunio marinus Haliday
trbiaies. XX. The blue crab, Callinectes sapidus. Rathbun. Ches- (Dipt.,Chironomidae)inwesternNorway.AnnalesZoologiciFennici
apeakeScience6: 201-208. 5: 71-75.
DeCoursey,P.J.1979. Egg-hatchingrhythmsinthreespeciesoftiddler Marshall, S. M., and A. P. Orr. 1954. Hatching in Calanusfinmar-
crabs. Pp. 399-406in CyclicPhenomena inMarinePlantsandAn- clticusandsomeothercopepods.J.Alar. Biol.Assoc. U.K.33:393-
imals. E. Naylor,andR.G. Hartnoll.eds. PergamonPress.Oxford. 401.
DeCoursey,P.J. 1983. Biologicaltiming.Pp. 107-162in TheBiology Moller,T.H.,andJ.R.Branford. 1979. Acircadianhatchingrhythm
ofCrustacea I'II: BehaviorandEcology. F.J. Vernberg,andW. B. inNephmpsnorvegicux(Crustacea:Decapoda).Pp.391-397inCyclic
Vernberg, eds. Academic Press. Washington. Phenomena in Marine Plants and Animals. E. Naylor and R. G.
DeVries,M.C,andR.B.Forward,Jr. 1989. Rhythmsinlarvalrelease Hartnoll,eds. Pergamon Press. Oxford.
ofthe sublittoral crab Neopanope sayi and the suprahttoral crab Pandian, T. J. 1970. Ecophysiological studies on thedevelopingeggs
Sesarma cinereum (Decapoda: Brachyura). Mar. Biol. 100: 241- and embryos ofthe European lobster Homarus gammarus. Mar.
248. Biol 5: 154-167.
DeV2tri9ie-ms3e,9.iMn.cCra,bsanfdroR.mBd.ifFfoerrewnatrdt,idJalr.h1e9ig9h1t.s. /ConCtrruosltaocfeeagng-Bhiant/chi1n1g: SSaaiiggutusisdaaa,l,McMy..cl1e19s9882i5n..theLTatiredrvaralelsttrrieilmaeliancsgreaorbfhSyletarshvamarlcmoraielnecBaiisodlei.nagBcutwliilv.tith1y6so2iln:ar3no7dna1-y-t3ia8dn6ad.l
environment. Jpn. J Ecol 35: 243-251.
Ennish,atGc.hinPg.i1n97t3h.e loEbsntdeorgHeonmouasrurshygtahmmmicairtuys.asJs.ocMiaatre.dBwiiotlhAlsasrovca.l Saigucsuab,atMin.g1c9r8a6b.SesTahremac.irJca-Ctiodmalp.rhPyhytshimo/o.fAla1rv5a9l:r2e1l-e3a1s.e in the in-
U.K. 53: 531-538. Saigusa, M. 1988. Entrainment oftidal and semilunar rhythms by
Fish,J.D.,andA.Mills. 1979. ThereproductivebiologyofCorophiitm artificial moonlightcycles. Biol. Bull. 174: 126-138.
volittalorand C. arenanum(Crustacea: Amphipoda). / Mar. Biol. Saigusa,M. 1992. Observationsonegghatchingintheterrestrialcrab
Assoc. U.K. 59: 355-368. Sesarma haematochen: Pac. Sci. 46:484-494.
Forward, R. B.Jr.,and K.J. Lohmann. 1983. Controlofegghatching Vonge, C. M. 1937. The nature and significance ofthe membranes
inthecrabRhilhropanopeusharrisii(Gould). Biol. Bull. 165: 154- surroundingthedevelopingeggsofHomarusvulgarisandotherDe-
166. capoda. Pmc. Zoo/. Soc. Loud. A 107:499-517.