Effects of Photoperiod and Temperature on Egg-Laying Behavior in a Marine Mollusk, Aplysia californica

7 Pages·1992·2.7 MB·English

by  N L Wayne

Reference: Biol. Bull. 182: 8-14. (February, 1992)
Effects of Photoperiod and Temperature on
Egg-Laying Behavior in a Marine Mollusk,
Aplysia californica
NANCY L. WAYNE AND GENE D. BLOCK
Department ofBiology, UniversityofVirginia, C/iarlottesville, Virginia 22901
Abstract. The primary purpose ofthese studies was to animals. These results suggest that the eyes play a role
determine whether photoperiodic signals could influence in mediating the effects of photoperiod on egg laying
seasonalegg-layingbehaviorinthemarinemollusk,Aply- behavior.
sia californica. Egg-laying behavior was monitored from
groupsofanimalsthatwerecollectedatfourtimesofyear Introduction
and maintained in different temperature and photoperi-
odic conditions in the laboratory. Animals that were ob- Like many animals living in the temperate zone, the
tained in autumn and kept in warm waterlaid eggs more marine mollusk Aplysia californica breeds seasonally.
frequently than those in cold water, regardless ofphoto- Both field and laboratory observations indicate that this
period. Furthermore, animals maintained on short days species is reproductively competent during the summer
and warm water laid eggs more frequently than those on and autumn, and reproductively quiescent during the
longdaysand warm water. Animalsin cold watershowed winter and spring (Strumwasser el al. 1969; Audesirk,
little to no egg laying, and a photoperiodic response was 1979; Berry, 1982). The onset ofthe breeding season is
not evident. Animals that were collected in either winter indicatedbyasignificant increase intheincidenceofcop-
or spring and maintained in warm water showed little or ulationandegglaying(Strumwassereta/., 1969;Audesirk,
nospontaneousegglayingthroughoutthestudy, regardless 1979), as well as increased synthesis ofthe hormone that
ofphotoperiod. Aswith the autumn animals, Aplysia in- controls egg laying (egg laying hormone; Berry, 1982).
dividualsobtainedinsummerand keptonshortdaysand Earlier work has shown that egg laying hormone, a
warm waterlaid eggs more frequently than those kept on peptide synthesized and secreted by the neuroendocrine
longdaysand warm water. These resultspArovide the first bagcells, is responsible fortriggering egg-laying behavior
evidence that the reproductive system of californica is (Strumwasseretal.. 1969;Kupfermann, 1970;Arch, 1972;
responsive to photoperiod. Overall, the data suggest that Dudek et al.. 1980; Stuart et al.. 1980; Chiu and Strum-
warm water is permissive for egg laying, and that short wasser, 1981; Blankenship et al., 1983). Although much
days can further stimulate this behavior. However, there isknown aboutthe molecularbiologyofbag-cell peptides
is a strong inhibition of spontaneous egg laying during (Chmetal.. 1979; Helleretal.. 1980;Scheller?//.. 1982;
thewinterandspring, which neitherwarm waternorshort Mahon and Scheller. 1983) and about the electrophysi-
photoperiod can overcome. The role ofthe eyes in me- ological propertiesofbagcells(Kupfermannand Kandel.
diating the photoperiodic response was also investigated. 1970; Kaczmarek et al.. 1978, 1982; Kaczmarek and
A control group ofintact animals kept on shortdays laid Strumwasser, 1981), the seasonal regulation of bag-cell
eggs more frequently than those on long days, but this activity and egg laying remains obscure. Thegeneral goal
photoperiodic response was not evident in eyeless ofthis and future studies istogain insight into the mech-
anisms underlying seasonal fluctuations in egg laying be-
Received 12 August 1991;accepted 31 October 1991. havior and in reproductive neuroendocrine function.
LIGHT AND TEMPERATURE AFFECT EGG LAYING
The occurrence ofreproductive activity at a particular gland extract, while immature animals will not lay eggs.
time ofyear suggests the involvement ofsome environ- An Aplysia that did not lay eggs spontaneously during
mental timing agent (e.g., ambient temperature, photo- thecourseofthestudieswasagain treatedwithatrialgland
period, foodavailability, specific nutritionalcue). Previous extract at the end ofeach study to assess maturity. Only
studies in Aplysia have shown that warm watercan stim- thoseanimalsthatwere reproductively maturebytheend
ulate egg laying, whereas cold temperatures inhibit this ofthestudieswereincluded in theanalysis. Animalswere
behavior (Berry, 1984; Pinsker and Parsons, 1985). Al- fed acombination ofRomaine lettuce and dried seaweed
thoughtheauthorsinterpretedtheirresultstosuggestthat (Msubi Nori, Japan Food Corp.) daily. Egg masses were
changes in the rate ofegg laying are solely dependent on recorded daily from individual buckets. Because Aplysia
seasonal cyclesoftemperature, thestudiesdid nottest for lays eggs at a maximal rate ofonce per day and does not
effects ofother environmental variables, such as photo- consume its own eggs (unpub. obs.), the presence or ab-
sence ofan eggmassisan excellent indication ofwhether
period.
The annual cycle of photoperiod is the most regular an animal exhibited egglaying behavioron anygiven day.
and predictable environmental factor, and is therefore Theeffectsofphotoperiod on egglayingbehaviorwere
used by a wide variety oftemperate-zone species to time determined as follows. Specimens of Aplysia were col-
reproduction totheappropriateseason(mammals: Turek lected and shipped to our seawater facilities at four dif-
and Campbell, 1979; birds: Rowan, 1926; reptiles: Licht, AfeUreTntUtMimNes of year. Animals that arrived in the early
1967: insects: Lees, 1966;terrestrial slugs: Sokoloveetai, 1988 (Sept. 22)wereall reproductivelymature
at the beginning ofthe study and were divided into four
1984). A. californica are intertidal organisms, spending
much of their time near the water surface (Audesirk, treatmentgroups. Aplysiaindividualswerekepteitheron
1979); thus they would be exposed to annual changes in shortdays(8 h light/day)oron longdays(16 h light/day);
animals on these two photoperiods were further divided
day length. A. californica might use photoperiodic infor- and maintained either in warm (20C) or in cold (15C)
mation, as well as temperature cues, to synchronize re-
tphriosdsutcutdiyonwatosatopadrettiecurlmairnteiwmheeotfheyerare.ggThleayimnagibneghoaavlioorf twmaeatmiepnret.raTaihtnuuersdetiohnenccoeolgdmgbwliaanytieendrgercfaofreeuclltyds,obiffeepivhneovrte,ostlpiaegyraeitdoedde.gagAnsn,diswmoaatwleesr
can be influenced by photoperiodic signals. dropped the cold-watergroup from the remaining studies.
WINTER
Materials and Methods A1p9l8y9si(aJani.nd3i)viadnudaltshetehaartlyarSrPivReIdNiGn t1h9e89ea(rMlayr. 31)were
General reproductively immature at the beginning ofthe studies;
but they had all reached maturity by the end ofthe ex-
Specimens of Aplysia californica (200-300 g)3w4erNe periments. In these two studies, all animals were main-
captured offthe coast ofCalifornia (approximately tained in warm waterand kept eitheron short oron long
latitude) by Alacrity Marine Supply, Redondo Beach, days. Aplysia individuals that arrived in the early SUM-
California. At the collection sites, the annual20raCnge in MER 1989 (June 23) were reproductively mature at the
watertemperatureisfrom approximately 10to (Dan beginningofthestudyandweremaintainedinwarmwater
Stark, Alacrity Marine Supply, pers. comm.), and thean- and kept either on short or on long days.
nual range in photoperiod is from 1 1 to 15.5 h light/day The role ofthe eyes in mediating the effects ofphoto-
(includes 1 h civil twilight). Uponarrival inthe laboratory, period on egg laying was investigated with specimens of
animals were maintained in temperature- and light-con- Aplysia that were brought to the laboratory in the late
trolled seawater tanks (475 liters; light intensity at water SUMMER 1990 (Aug. 7) and maintained in warm water
surface was 700 lux as measured with aphotographic light and 14.25 hlight/day(photoperiodin mid-Augustat34N
meter). Waterwas recirculated through undergravel filters latitude) for three days. All of these animals were im-
within the tanks. Treatment groups (initially, 12 animals mobilized with MgCl: (injected into hemolymph); halfof
per group; 0-3 animals/group died during the course of them were bilaterally enucleated, andtheotherhalfserved
the studies) were maintained in separate tanks, and all as intact controls. Followingsurgery, animalswere further
animalswerekeptin single, perforatedplasticbuckets(20 dividedand kepteitheron short (8 hlight/day) oron long
cm in diameter) so that each individual could be moni- days (16 h light/day), making a total of four treatment
tored throughout the studies. groups.
To document the egg laying capability (i.e.. reproduc-
tive maturity) of each animal, atrial gland extract was Analysis ofdata
injected into the hemolymph ofall animals upon arrival Differencesinegglayingbetweentreatmentgroupswere
inthelaboratory(Nagle etal, 1985). Animalswith a ma- assessed byChi-square analysis. Valuesweresignificantly
P<
ture reproductivesystem will layeggsin responsetoatrial different if 0.05.
10 N. L. WAYNE AND G. D. BLOCK
AUTUMN
100
Ishortdays, warm A. . shortdays, cold B. C.
I long days, warm long days, cold
6-10 11-15 16-21 1-5 6-10 11-15 16-21 warm cold
Days of experiment shortday brgday shortday bngday
AUTUMN
Figure 1. Percentof.-iplysiaindividualslayingeggsduringtheearly 1988. PanelA:Animals
were kept in warm (20C) waterand eithershort (8 h light/day) orlong (16 h light/day)days. Data were
averaged (+sem) into 5-day bins. Panel B: Animals were kept in cold (15C) water and either short (8 h
light/day)orlong(16 h light/day)days. Dataarepresented asin panel A. Panel C: Data fromthe4groups
arepresentedasthepercentofanimalslayingeggseachday,averaged(+sem)overtheentire21-daystudy.
Differentlettersindicate valuesaresignificantlydifferent(atleast P< 0.05).
Results toperiodonegglaying. In theSUMMER(Fig. 4),weonce
again observed the emergence ofa photoperiodic effect:
Photoperiodic effects on egg laying Aplysia maintained on short days and warm water laid
Overall, photoperiod and temperature can both affect eggs more frequently than those kept on long days and
the frequency of egg laying. In the AUTUMN, Aplysia warm water. This photoperiodic response in the summer
individuals kept in warm water laid eggs more frequently wasnotasrobustasthat seenduringthepreviousautumn
than those kept in cold water (Fig. 1). Furthermore, an- (compare Figs. Ic and 4b).
imalsmaintained on short daysandwarm waterlaid eggs
more frequently than those kept on long days and warm Photoperiodic effects in intact vs. eyeless animals
water. However, in the WINTER (Fig. 2) and in the The eyes appear to play a role in transducing photo-
SPRING (Fig. 3), egg laying frequency overall was sup- periodic information to the reproductive axisresponsible
pressed in all groups (even though animals were repro- for regulating egg laying (Fig. 5). Once again, control an-
ductively mature by the end ofthe studies; see Materials imals kept in short days and warm water laid eggs more
and Methods), and there was no apparent effect ofpho- frequently than those kept on longdaysand warm water.
WINTER
100 50
shortdays, warm B.
80" long days, warm 40 >
"S ~-
^Q) *<
60- 30
0)
ra 4fj- 20 | |
~
<Q. 20- 10 I
1-5 6-10 11-15 16-20 21-25 26-30 31-36 shortdays long days
Daysofexperiment
Figure2. PercentofAplysiaindividualslayingeggsduringtheearlyWINTER 1989.Animalswerekept
in warm (20C) water and either short (8 h light/day) or long (16 h light/day) days. Panel A: Data were
averaged (+sem) into 5-day bins. Panel B: Data are presented as the percent ofanimals laying eggs each
day, averaged (+sem) over the entire study. There was no significant difference between the values ofthe
twogroups.
LIGHT AND TEMPERATURE AFFECT EGG LAYING II
SPRING
12 N. L. WAYNE AND G. D. BLOCK
100
shortdays, intact D shortdays, eyeless
long days, intact long days, eyeless
1-5 6-1011-1516-2021-2526-30 1-5 6-1011-1516-2021-2526-30 intact eyeless
Daysof experiment short-day long-day short-day long-day
SUMMER
Figure 5. Percent ofAplysia individualslayingeggsduringthe late 1990. All animalswere
maintained in warm (20C) water. Panel A: Intact, control animals were kept on short (8 h light/day) or
long (16 h light/day) days. Data were averaged (+sem) into 5-day bins. Panel B: Bilaterally enucleated
animals were kept on short (8 h light/day) or long(16 h light/day) days. Data are represented as in panel
A. Panel C: Data from the4groupsare presentedasthepercent ofanimals layingeggseachday, averaged
(+sem) over the entire 30-day study. The letters a and b indicate values that are significantly different (P
< 0.05). 'Indicates that valuesapproached significant difference compared to that ofthe intact, long-day
controlgroup(P<0.10).
A common phenomenon among some seasonally Further, our results suggest that the eyes play a role in
breeding vertebrates is a spontaneous shutdown of the mediatingphotoperiodic information tothereproductive
reproductive system duringthe non-breeding season (liz- axis responsible for regulating egg laying behavior. Spe-
ard: Cueller and Cueller, 1977; birds: Hamner, 1967; cifically, photoperiod had no effect on egg layingin those
Robinson and Follett, 1982; mammal: Robinson and animals that were bilaterally enucleated. The eyes of
Karsch, 1984). During this period of reproductive qui- Aplysia contain both photoreceptors and a circadian
escence,previouslyinductivephotoperiodiccuesnolonger pacemaker (Jacklet, 1969; Eskin, 1971). The circadian
stimulatereproductiveactivity. Thisperiodofinsensitivity system is involved in the neural pathway mediating pho-
to stimulatory photoperiod (commonly labelled 'photo- toperiodic responsesin mostanimalsinvestigated(Follett
refractoriness') isan endogenous processand can be'bro- and Sharp, 1969; Elliott, 1976; Almeida and Lincoln,
ken' by exposing the animal to a bout ofinhibitory pho- 1982). Furthermore, both ocular and extraocular photo-
toperiod, followed by a stimulatory day length (Jackson receptors mediate photoperiodic responses in avariety of
el ai, 1988). In Aplysia. we have shown that previously species (Reiter, 1969; Follett et a/.. 1975; Legan and
stimulatory environmental cues(warm water, shortdays) Karsch, 1983; Fosterand Follett, 1985). InAplysia, pho-
did not stimulate spontaneousegglayingduringthe non- toreceptors are found not only in the eye, but also in
breeding season in winter and spring. Aplysia may there- structures asdiverse astheabdominal ganglion (Andresen
forebehave likemanyotherseasonal breedersandbecome and Brown, 1982),thecerebralganglion(BlockandSmith,
refractory to stimulatory signals. If this is so, then pre- 1973), the rhinophores(Jacklet, 1980), the oral veil (Cook
treatment with long days and cold temperatures should etul.. 1991). andthesiphon(LukowiakandJacklet, 1972).
be able to break refractoriness to stimulatory short days Therefore, the relative roles ofthe ocular photoreceptors
and warm temperatures. and ocular pacemakers in mediating the effects of pho-
But mechanisms other than an endogenous refracto- toperiod on egg laying are not clear. For instance, both
riness to an environmental signal might equally well un- theocularphotoreceptorsand ocularpacemakers maybe
derlie the cessation ofspontaneous egg laying byAplysia playing a role in photoperiodic time measurement. Al-
during winter and spring. For instance, one or more key ternatively, extraocular photoreceptors may be transmit-
components ofthe reproductive neural axis may be de- ting light signalsto the ocularcircadian pacemaker, which
velopmentally immature during the winter and spring then sendsitssignalstothe next stepin thephotoperiodic
(even though the reproductive tract can mature in the response system.
laboratory). Alternatively, some environmental cue (e.g., Nevertheless, we must stressthatourresultsaredifficult
food orother nutritional item necessary forhigh levels of to interpret, because the photoperiodic response in the
spontaneous egglaying) may be missingduringthattime intact controls in the lastexperiment wasweakcompared
ofyear. to that seen in the first experiment (compare Fig. 1 with
LIGHT AND TEMPERATURE AFFECT EGG LAYING 13
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