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The effect of photoperiod on the reproductive cycle of Gammarus lawrencianus bousfield
J. exp. mar. Biol. Ecol., 1981, Vol. 53, pp. l-7
Elsevier/North-Holland
Biomedical Press
THE EFFECT OF PHOTOPERIOD GAMMARUS
ON THE REPRODUCTIVE CYCLE OF
LA WRENCIANUS
Bousfield
V. J. STEELE DepartmentofBiology,
Memorial University of Newfoundland, St. John
S. Neyfoundiand, Canada, A IB 3X9
Abstract: Gamnzarus ~u~~,renejanusBousfield is an estuarine amphipod endemic to the northwestern Atlantic that breeds between February and October and has a female resting stage from October to February. Specimens were cultured at six experimental photoperiods (OL, 4L, 8L, 12L, 18L, 24L). each for 6.5 months. Egg development ceased and the resting stage was induced by short days, most effectively between 8L and 12L within 2 months, in August, well before the onset of the resting stage in the field. Females maintained in the resting stage in short days (8L, 12L) began to develop eggs spontaneously after 3 months. Under long photoperiod (18L). constant light (24L). and constant darkness (OL), breeding was continuous with no evidence of a resting stage. Females induced experimentally into resting stage by short days (8L) and transferred to long days (18L), developed eggs and released a brood within 6 wk. This suggests an endogenous control of the resting stage cycle normally lasting ~3 months, and that long days will stimulate egg development, but are not necessary for the maintenance of the normal reproductive cycle.
INTRODUCTIVE
In all of the Gammarus sensu latu studied so far in the northwestern Atlantic, the first brood of young is released in late winter and early spring coincident with the appearance of algal food. This timing is accomplished by the onset and termination of the resting stage and varies markedly in the various species (Steele & Steele, 1975). Previous studies (Steele, 1964) have shown that it is photoperiod rather than temperature changes acting as the proximate factor, and this relationship is studied further here. In Gammarus setosus Dementieva, a northern species that normally produces only a single brood of eggs in the autumn which develop slowly over the winter and hatch the following spring, experimental short days (8L: 16D, 12L: 12D) accelerate the termination of the resting stage in the summer months (Steele et al., 1977). Thus it was concluded that the normal termination of the resting stage would be induced by short days of late August and September. The present contribution is a similar study of the effects of photoperiod on G. lawrenciunus Bousfield, a species with quite different geographical distribution and reproductive cycle. It is endemic to estuaries of the northwestern Atlantic where it is found from Labrador south to Long Island Sound (Bous~eld, 1973; Steele & Steele, 1974). It produces successive broods and generations between February and October (Steele & Steele, 1970) and has a female resting stage from 0022-098~~81~0000-00~~~02.50
0 Elsevieri~[~rth-Holland
Biomedical Press
V. J. STEELE
2
October
to February.
This is similar
in most respects
Gammarus sensu latu, such as G. oceunicus terminates Thus,
later, being correIated
its response
to other northwestern
Segerstrale,
with the time taken for its smaller
to photoperiod
would
Atlantic
except that its resting
be expected
to differ
stage
eggs to develop. from
that
of
G. setosus.
METHODS
The animals were collected at Witless Bay near St. John’s, Newfoundland, on 23 May 1979. At this time all the mature females in the population are breeding. Females were selected to include those with setose oostegites and Stage 5 ovary (Steele & Steele, 1970) to establish a baseline for the experiment. Groups of 25-30 females were placed in containers with 4 1 of sea water at 10 + 1 “C and salinity of 30x,. The temperature and salinity are within the normal habitat conditions, and the animals thrived. Six light regimes: OL : 24D, 4L: 20D, 8L : 16D, 12L : 12D, 18L : 6D, 24L : OD were tested. No males were included. The animals at OL: 24D were exposed to light for 30 min on the day of examination and momentarily once every week during feedings. No controls under daylight were maintained because of lack of suitable facilities. The animals were provided with adequate food consisting of filamentous algae, fish muscle, and Tetramin. The culture water was changed on the day of the examination. The experiment started on 30 May and terminated on 15 December 1979. The experimental animals were counted at monthly intervals (except October, as shown in Figs. 1 and 2) and a subsample of 20 from each light regime was examined with a dissecting microscope to record details of their reproductive condition. The numbers of animals were gradually depleted and, in some of the light conditions, mortality decimated the cultures below subsample size by the end of September. On 30 September
a subsample
of 10 from the 8L: 16D culture
was transferred
to
18L : 6D to confirm observations made to that date. The reproductive state of the experimental females was determined from two characteristics: (1) the status of the oostegites - setose or non-setose, and (2) the stage of ovarian development. The results are expressed as numbers of females with setae on oostegites, including those carrying broods, and numbers without setae which are not breeding. The ovarian development is expressed as two classes: (a) undeveloped ovary with oogonia Stages 1 and 2, and (b) developing and mature ovary Stages 3, 4, and 5 which can readily be recognized in living animals by their purple pigmentation and relative quantity of deposited yolk under a dissection microscope with transmitted light.
PHOTOPERIOD
REPRODUCTION
El
d
El
LA WREN(‘1ANU.S
3
LUJlL --
El
GAMMARUS
El
x
c.II, El
El
El
El
I. L 11 El
El
El
Fig. 1. Numbers of Gammn~u.c lawencimus females with non-setose (NS) and setose (LS) oostegites subsamples of cultures from six light regimes: the total numbers of females in cultures are shown squares.
in in
r
V. J. STEELE
10
t
-
-
Ii
I
ISDee
dtB
10
.EL
1
m
ISNov.
3OSep.
3Q Aug.
30 Jul.
l-2 3-i
830 May
24 q
Fig. 2. Numbers of Gammarus /awrencirmus females with undeveloped ovary ovary (3-5) in subsamples of cultures from six light regimes: the total numbers are the same as shown in squares in Fig. I
(I-2) and of females
developing in cultures
PHOTOPERIOD REPRODUCTION GAMMARUS
5
LA WRE,YCIANC’S
RESULTS The occurrence
of females with oostegite
developing
eggs in the ovary in samples
are shown
in Figs.
carried
1 and 2. During
out (May to December),
setae and the stages of the corresponding
of 20 or less females from each light regime the time of year when
egg development
terminates
the experiment in October
was
and in the
subsequent molt the oostegite setae are lost. Under 18L : 6D, constant light, and constant darkness, reproduction was continuous with no evidence of a resting stage. Under 8L : 16D and 12L: 12D on the other hand, egg development stopped within 2 months and most of the females subsequently lost their oostegite setae, well before the time of the normal resting stage. Under 4L : 20D a resting stage was also induced, but more slowly than under either 8L or 12L. It is concluded, therefore, that egg development and most effectively agreement
is terminated and the resting stage is induced by short days, by those between 8 and 12 h of light per day. This is in
with the field observations
short days of September. Females in resting stage induced
where the resting experimentally
stage begins
by short
days
following
the
(4L: 20D)
that
were transferred to long days (18L: 6D) on 30 September rapidly developed eggs and released a brood within 6 wk (Fig. 3). Thus long days will terminate a resting
15Nov.
10
B
r” 5
HOURS Fig. 3. Transfer of 10 Gummarus the follow-up on 15 November:
of LIGHT per DAY
lawrrncianus females from 4L: 20D to 18L : 6D on 30 September and NS, no setae on oostegites; LS, setae on oostegites; I-2, oogonia stages 1,2; 3- 5, oocyte stages 3,4,5.
stage that has been experimentally induced by short days, but in the field the animals would not normally be exposed to long days until April of May. In fact, their egg development commences in February when the days are still short (1OL : 12D in Newfoundland). An explanation for the field situation comes from the observations of animals maintained in the resting stage by exposure to short days (8L, 12L). Their eggs started to develop spontaneously after a period of x 3 months (Fig. 2). This suggests that there is an innate resting stage cycle normally lasting in this species x 3 months. Long days will stimulate egg development, but are not necessary for the normal reproductive cycle to be maintained.
V. J. STEELE
DISCUSSION G. luwrencianus is typical of the Gammarus species that produce young from spring to fall when environmental conditions are most favourable, and reach a peak population in the fall. It has a resting stage from late fall to winter when conditions are less favourable and the population declines until the following spring. The present study shows that short days will induce the onset of the resting stage, at least under the present experimental conditions, and could be expected to do so in the field also. However, a resident species cannot utilize such photoperiods to time its reproductive activities since short days occur in the spring as well as in the fall. For G. luwrenciunus spring brings optimum conditions for the production of young when they are in fact ready to release their first brood for that year. A resting stage at this time would prevent the production of these offspring. Steele et al. (1977) have suggested that G. setosus might escape the spring photoperiods by responding to changes in photoperiod. Thus a two-phase system, such as exposure to long days followed by short days, was suggested as possibly necessary for induction of the resting stage in that species. Such a system could explain why termination of the resting stage does not take place in the spring. It should be noted that the suggestion was of a response to changes in photoperiod (e.g. from 8L to 16L), not to a progressive seasonal change in photoperiods. McQueen & Steel (1980) have seen lit to misrepresent this suggestion and have claimed that a response to changing photoperiods was proposed, but an examination of the results and a reading of the text should have made clear what was meant. In G. lawrencianus the present observations show that such a two-phase system could not operate in the spring, since the extra short days of winter and short days of spring should both induce a resting stage. Instead, the control of the resting stage appears to be endogenous, unless some unknown environmental clue acts as a stimulus in the late winter. It appears that while individuals can breed continuously until they die under the appropriate conditions of light and temperature, the resting stage lasts for only c 3 months under constant conditions. Thus at least in this species,
and possibly
also in G. setosus, the reproductive
cycle has its own
rhythm and the role of photoperiod is to time the cycle. The occurrence of short days near the time of the fall equinox ensures that this timing is synchronized over a range of latitudes. Since most investigators seem to terminate experiments on photoperiod induction soon after an effect is produced, it is possible that the spontaneous termination of the effect as reported here may be of wider occurrence than is known at the present time. It would be of interest therefore, if more long-term experiments on a variety of species were carried out.
PHOTOPERIOD REPRODUCTION
GAMMARUS
7
LA WRENCIANUS
ACKNOWLEDGEMENTS The author acknowledges
D. H. Steele. The study Research
Council
the valuable advice and assistance received from Dr.
was supported
by the Natural
Sciences
and
Engineering
of Canada.
REFERENCES BOUSFIELD. E. L.. 1973. Shallow-water Gammaridean Amphipoda of New England. Cornell University Press, Ithaca, N.Y.. 312 pp. MCQUEEN, D. J. & C. G. H. STEEL, 1980. The role of photoperiod and temperature in the initiation of reproduction in the terrestrial isopod Oniscus asrl1u.r Linnaeus. Can. J. Zoo/., Vol. 53, pp. 235 240. STEELE. V. J., 1964. Reproduction and metabolism in Gammurus oceunicus Segerstrale and Gamnzcrrux SP~OSUSDementieva. Ph.D. thesis, McGill University, Montreal, 164 pp. STEELE. D. H. & V. J. STEELE, 1970. The biology of Gammarus (Crustacea, Amphipoda) in the northwestern Atlantic. IV. Gammarus lawrencianur Bousfield. Can. J. Zool., Vol. 48, pp. 1261-1267. STEELF, D. H. & V. J. STEELE, 1974. The biology of Gammurus (Crustacea, Amphipoda) in the northwestern Atlantic. VIII. Geographic distribution of the northern species. Can. J. Zorjl.. Vol. 52. pp. 1115-1120. S-TEEI.~. D. H. & V. J. STEELE, 1975. The biology of Gammurus (Crustacea, Amphipoda) in the northwestern Atlantic. XI. Comparison and discussion. Gun. J. Zool.. Vol. 53, pp. 1116~1126. STEELE. V. J., D. H. STEELE & B. R. MACPHERSON, 1977. The effect of photoperiod on the reproductive cycle of Garnmcrrus .wtosus Dementieva, 193 1. Cru.~tac.eunu, Suppl. 4, pp. 5% 63.
Elsevier/North-Holland
Biomedical Press
THE EFFECT OF PHOTOPERIOD GAMMARUS
ON THE REPRODUCTIVE CYCLE OF
LA WRENCIANUS
Bousfield
V. J. STEELE DepartmentofBiology,
Memorial University of Newfoundland, St. John
S. Neyfoundiand, Canada, A IB 3X9
Abstract: Gamnzarus ~u~~,renejanusBousfield is an estuarine amphipod endemic to the northwestern Atlantic that breeds between February and October and has a female resting stage from October to February. Specimens were cultured at six experimental photoperiods (OL, 4L, 8L, 12L, 18L, 24L). each for 6.5 months. Egg development ceased and the resting stage was induced by short days, most effectively between 8L and 12L within 2 months, in August, well before the onset of the resting stage in the field. Females maintained in the resting stage in short days (8L, 12L) began to develop eggs spontaneously after 3 months. Under long photoperiod (18L). constant light (24L). and constant darkness (OL), breeding was continuous with no evidence of a resting stage. Females induced experimentally into resting stage by short days (8L) and transferred to long days (18L), developed eggs and released a brood within 6 wk. This suggests an endogenous control of the resting stage cycle normally lasting ~3 months, and that long days will stimulate egg development, but are not necessary for the maintenance of the normal reproductive cycle.
INTRODUCTIVE
In all of the Gammarus sensu latu studied so far in the northwestern Atlantic, the first brood of young is released in late winter and early spring coincident with the appearance of algal food. This timing is accomplished by the onset and termination of the resting stage and varies markedly in the various species (Steele & Steele, 1975). Previous studies (Steele, 1964) have shown that it is photoperiod rather than temperature changes acting as the proximate factor, and this relationship is studied further here. In Gammarus setosus Dementieva, a northern species that normally produces only a single brood of eggs in the autumn which develop slowly over the winter and hatch the following spring, experimental short days (8L: 16D, 12L: 12D) accelerate the termination of the resting stage in the summer months (Steele et al., 1977). Thus it was concluded that the normal termination of the resting stage would be induced by short days of late August and September. The present contribution is a similar study of the effects of photoperiod on G. lawrenciunus Bousfield, a species with quite different geographical distribution and reproductive cycle. It is endemic to estuaries of the northwestern Atlantic where it is found from Labrador south to Long Island Sound (Bous~eld, 1973; Steele & Steele, 1974). It produces successive broods and generations between February and October (Steele & Steele, 1970) and has a female resting stage from 0022-098~~81~0000-00~~~02.50
0 Elsevieri~[~rth-Holland
Biomedical Press
V. J. STEELE
2
October
to February.
This is similar
in most respects
Gammarus sensu latu, such as G. oceunicus terminates Thus,
later, being correIated
its response
to other northwestern
Segerstrale,
with the time taken for its smaller
to photoperiod
would
Atlantic
except that its resting
be expected
to differ
stage
eggs to develop. from
that
of
G. setosus.
METHODS
The animals were collected at Witless Bay near St. John’s, Newfoundland, on 23 May 1979. At this time all the mature females in the population are breeding. Females were selected to include those with setose oostegites and Stage 5 ovary (Steele & Steele, 1970) to establish a baseline for the experiment. Groups of 25-30 females were placed in containers with 4 1 of sea water at 10 + 1 “C and salinity of 30x,. The temperature and salinity are within the normal habitat conditions, and the animals thrived. Six light regimes: OL : 24D, 4L: 20D, 8L : 16D, 12L : 12D, 18L : 6D, 24L : OD were tested. No males were included. The animals at OL: 24D were exposed to light for 30 min on the day of examination and momentarily once every week during feedings. No controls under daylight were maintained because of lack of suitable facilities. The animals were provided with adequate food consisting of filamentous algae, fish muscle, and Tetramin. The culture water was changed on the day of the examination. The experiment started on 30 May and terminated on 15 December 1979. The experimental animals were counted at monthly intervals (except October, as shown in Figs. 1 and 2) and a subsample of 20 from each light regime was examined with a dissecting microscope to record details of their reproductive condition. The numbers of animals were gradually depleted and, in some of the light conditions, mortality decimated the cultures below subsample size by the end of September. On 30 September
a subsample
of 10 from the 8L: 16D culture
was transferred
to
18L : 6D to confirm observations made to that date. The reproductive state of the experimental females was determined from two characteristics: (1) the status of the oostegites - setose or non-setose, and (2) the stage of ovarian development. The results are expressed as numbers of females with setae on oostegites, including those carrying broods, and numbers without setae which are not breeding. The ovarian development is expressed as two classes: (a) undeveloped ovary with oogonia Stages 1 and 2, and (b) developing and mature ovary Stages 3, 4, and 5 which can readily be recognized in living animals by their purple pigmentation and relative quantity of deposited yolk under a dissection microscope with transmitted light.
PHOTOPERIOD
REPRODUCTION
El
d
El
LA WREN(‘1ANU.S
3
LUJlL --
El
GAMMARUS
El
x
c.II, El
El
El
El
I. L 11 El
El
El
Fig. 1. Numbers of Gammn~u.c lawencimus females with non-setose (NS) and setose (LS) oostegites subsamples of cultures from six light regimes: the total numbers of females in cultures are shown squares.
in in
r
V. J. STEELE
10
t
-
-
Ii
I
ISDee
dtB
10
.EL
1
m
ISNov.
3OSep.
3Q Aug.
30 Jul.
l-2 3-i
830 May
24 q
Fig. 2. Numbers of Gammarus /awrencirmus females with undeveloped ovary ovary (3-5) in subsamples of cultures from six light regimes: the total numbers are the same as shown in squares in Fig. I
(I-2) and of females
developing in cultures
PHOTOPERIOD REPRODUCTION GAMMARUS
5
LA WRE,YCIANC’S
RESULTS The occurrence
of females with oostegite
developing
eggs in the ovary in samples
are shown
in Figs.
carried
1 and 2. During
out (May to December),
setae and the stages of the corresponding
of 20 or less females from each light regime the time of year when
egg development
terminates
the experiment in October
was
and in the
subsequent molt the oostegite setae are lost. Under 18L : 6D, constant light, and constant darkness, reproduction was continuous with no evidence of a resting stage. Under 8L : 16D and 12L: 12D on the other hand, egg development stopped within 2 months and most of the females subsequently lost their oostegite setae, well before the time of the normal resting stage. Under 4L : 20D a resting stage was also induced, but more slowly than under either 8L or 12L. It is concluded, therefore, that egg development and most effectively agreement
is terminated and the resting stage is induced by short days, by those between 8 and 12 h of light per day. This is in
with the field observations
short days of September. Females in resting stage induced
where the resting experimentally
stage begins
by short
days
following
the
(4L: 20D)
that
were transferred to long days (18L: 6D) on 30 September rapidly developed eggs and released a brood within 6 wk (Fig. 3). Thus long days will terminate a resting
15Nov.
10
B
r” 5
HOURS Fig. 3. Transfer of 10 Gummarus the follow-up on 15 November:
of LIGHT per DAY
lawrrncianus females from 4L: 20D to 18L : 6D on 30 September and NS, no setae on oostegites; LS, setae on oostegites; I-2, oogonia stages 1,2; 3- 5, oocyte stages 3,4,5.
stage that has been experimentally induced by short days, but in the field the animals would not normally be exposed to long days until April of May. In fact, their egg development commences in February when the days are still short (1OL : 12D in Newfoundland). An explanation for the field situation comes from the observations of animals maintained in the resting stage by exposure to short days (8L, 12L). Their eggs started to develop spontaneously after a period of x 3 months (Fig. 2). This suggests that there is an innate resting stage cycle normally lasting in this species x 3 months. Long days will stimulate egg development, but are not necessary for the normal reproductive cycle to be maintained.
V. J. STEELE
DISCUSSION G. luwrencianus is typical of the Gammarus species that produce young from spring to fall when environmental conditions are most favourable, and reach a peak population in the fall. It has a resting stage from late fall to winter when conditions are less favourable and the population declines until the following spring. The present study shows that short days will induce the onset of the resting stage, at least under the present experimental conditions, and could be expected to do so in the field also. However, a resident species cannot utilize such photoperiods to time its reproductive activities since short days occur in the spring as well as in the fall. For G. luwrenciunus spring brings optimum conditions for the production of young when they are in fact ready to release their first brood for that year. A resting stage at this time would prevent the production of these offspring. Steele et al. (1977) have suggested that G. setosus might escape the spring photoperiods by responding to changes in photoperiod. Thus a two-phase system, such as exposure to long days followed by short days, was suggested as possibly necessary for induction of the resting stage in that species. Such a system could explain why termination of the resting stage does not take place in the spring. It should be noted that the suggestion was of a response to changes in photoperiod (e.g. from 8L to 16L), not to a progressive seasonal change in photoperiods. McQueen & Steel (1980) have seen lit to misrepresent this suggestion and have claimed that a response to changing photoperiods was proposed, but an examination of the results and a reading of the text should have made clear what was meant. In G. lawrencianus the present observations show that such a two-phase system could not operate in the spring, since the extra short days of winter and short days of spring should both induce a resting stage. Instead, the control of the resting stage appears to be endogenous, unless some unknown environmental clue acts as a stimulus in the late winter. It appears that while individuals can breed continuously until they die under the appropriate conditions of light and temperature, the resting stage lasts for only c 3 months under constant conditions. Thus at least in this species,
and possibly
also in G. setosus, the reproductive
cycle has its own
rhythm and the role of photoperiod is to time the cycle. The occurrence of short days near the time of the fall equinox ensures that this timing is synchronized over a range of latitudes. Since most investigators seem to terminate experiments on photoperiod induction soon after an effect is produced, it is possible that the spontaneous termination of the effect as reported here may be of wider occurrence than is known at the present time. It would be of interest therefore, if more long-term experiments on a variety of species were carried out.
PHOTOPERIOD REPRODUCTION
GAMMARUS
7
LA WRENCIANUS
ACKNOWLEDGEMENTS The author acknowledges
D. H. Steele. The study Research
Council
the valuable advice and assistance received from Dr.
was supported
by the Natural
Sciences
and
Engineering
of Canada.
REFERENCES BOUSFIELD. E. L.. 1973. Shallow-water Gammaridean Amphipoda of New England. Cornell University Press, Ithaca, N.Y.. 312 pp. MCQUEEN, D. J. & C. G. H. STEEL, 1980. The role of photoperiod and temperature in the initiation of reproduction in the terrestrial isopod Oniscus asrl1u.r Linnaeus. Can. J. Zoo/., Vol. 53, pp. 235 240. STEELE. V. J., 1964. Reproduction and metabolism in Gammurus oceunicus Segerstrale and Gamnzcrrux SP~OSUSDementieva. Ph.D. thesis, McGill University, Montreal, 164 pp. STEELE. D. H. & V. J. STEELE, 1970. The biology of Gammarus (Crustacea, Amphipoda) in the northwestern Atlantic. IV. Gammarus lawrencianur Bousfield. Can. J. Zool., Vol. 48, pp. 1261-1267. STEELF, D. H. & V. J. STEELE, 1974. The biology of Gammurus (Crustacea, Amphipoda) in the northwestern Atlantic. VIII. Geographic distribution of the northern species. Can. J. Zorjl.. Vol. 52. pp. 1115-1120. S-TEEI.~. D. H. & V. J. STEELE, 1975. The biology of Gammurus (Crustacea, Amphipoda) in the northwestern Atlantic. XI. Comparison and discussion. Gun. J. Zool.. Vol. 53, pp. 1116~1126. STEELE. V. J., D. H. STEELE & B. R. MACPHERSON, 1977. The effect of photoperiod on the reproductive cycle of Garnmcrrus .wtosus Dementieva, 193 1. Cru.~tac.eunu, Suppl. 4, pp. 5% 63.