296
although it is still restricted. Zhang & Lefcort (1991)
reported that polyploids survive better at extreme temperatures and have a wider thermal preferece range
than diploids.
Artemia females of both strains can reproduce
either ovoviviparously and oviparously. In fact, the
former mode of reproduction may be interpreted as an
adaptation to the hipersaline conditions that Artemia
experiences since this reproduction route has not been
reported for freshwater Anostracans (Belk, personal communication). Nauplius reproduction allows for
rapid population growth, whereas the production of
diapause cysts ensures the survival of a population
through unfavorable conditions (Lenz & Dana, 1987).
The factors influencing cyst formation in Artemia and
in their hatchability have been studied mainly in the
Artemia populations of North America (Drinkwater
& Clegg, 1991). Berthelemy-Okazaki & Hedgecock
(1987) found that a number of factors, both intrinsic and extrinsic, affect the reproductive mode of the
San Francisco Bay Artemia including brood number,
density, photoperiod, salinity and temperature. As in
the case of diapause induction, the cues which can
act as a signal for cyst diapause are habitat specific. This is understandable since these abiotic factors
are useful as regulatory cues, precisely because they
forecast the coming of favorable conditions (both abiotic and biotic) (Drinkwater & Clegg, 1991). Evidence
that the production and hatchability of resting eggs
have a different response to temperature in sexual and
in parthenogenetic populations (Browne et ai., 1984,
1988; Amat, 1982; Vanhaecke & Sorgeloos, 1989)
indicates the potential of dormancy to function as an
adaptation regulating their seasonal occurrence.
Temperature is clearly an important factor controlling reproductive rates and population success,
and therefore might affect the seasonal succession
of Artemia populations in Spanish salterns. In this
study, we examine the life-history responses as well
as the hatchability and cyst production of sexual
and parthenogenetic (diploid and tetraploid) Artemia
species that live on the Iberian peninsula (Amat, 1980):
the sexual diploid Artemia tunisiana which is the
dominant winter-spring species; and the diploid and
tetraploid Artemia parthenogenetica strains, that dominate during spring-summer. The experiments were
performed at three temperatures, 15 0, 24 ° and
29.5°C.
Material and methods
Experimental animals
Artemia nauplii hatched from cysts of the following
populations were used (harvest date in parentheses):
(1) Sexual diploid Artemia tunisiana from Bonmatf
saltworks, Alicante, Spain (1990); (2) Parthenogenetic diploid A. parthenogenetica strain from La Mata
lagoon, Alicante, Spain (1988); (3) Parthenogenetic
tetraploid A.parthenogenetica strain from La Trinidad
saltworks, Tarragona, Spain (1984).
Cultures
Nauplii were obtained from mass hatching of cysts
in one liter of 38 ppt sea water at 28 ° C after 48 h
(Hontoria et al., 1989). Nauplii were adapted to high
salinity in 60 ppt brine during 24 h, with the microalgae
Dunaliella sp. as food.
Initial culture density was set to 100 nauplii / I in
2 I containers filled with 90 ppt brine. Every two or
three days suspensions of brine containing the alga
Dunaliella sp. were added ad libitum. Cultures were
conducted for each population at three temperatures
15 0, 24 ° and 29.5 °C, in thermostatized incubators
(±1 0C). Cultures were maintained at 12D:12L photoperiod pattern with fluorescent lighting under constant aeration. When females presented visible spots
along their ovaries, and males had their claspers full
developed, individuals were removed from the mass
cultures and placed in single jars containing 50 ml of
90 ppt brine and algal culture. One female for parthenogenetic populations or one male-female for the sexual
population was placed in each jar. Males were replaced
at death. At least every other day jars were examined
for deaths or offspring production. Cysts or nauplii
were removed by pipette and counted. Excess algae
and accumulated feces were removed and the volume
replaced with fresh brine containing algal suspension.
Salinity ofthe individual cultures was also maintained
about constant by addition or dilution of brine. Thirty replicates were used for each treatment. The cysts
produced in the cultures at 15 ° and 24°C were kept
in saturated brine at the same temperatures. At the
end of the life-history experiments they were sieved
through 160 p,m mesh, washed and resuspended in
38 ppt sea water at the same temperature at which they
were obtained. Five samples of' hydrated cysts from
the sexual and the La Trinidad populations at 15°C,
and from the La Mata and the La Trinidad popula-
although it is still restricted. Zhang & Lefcort (1991)
reported that polyploids survive better at extreme temperatures and have a wider thermal preferece range
than diploids.
Artemia females of both strains can reproduce
either ovoviviparously and oviparously. In fact, the
former mode of reproduction may be interpreted as an
adaptation to the hipersaline conditions that Artemia
experiences since this reproduction route has not been
reported for freshwater Anostracans (Belk, personal communication). Nauplius reproduction allows for
rapid population growth, whereas the production of
diapause cysts ensures the survival of a population
through unfavorable conditions (Lenz & Dana, 1987).
The factors influencing cyst formation in Artemia and
in their hatchability have been studied mainly in the
Artemia populations of North America (Drinkwater
& Clegg, 1991). Berthelemy-Okazaki & Hedgecock
(1987) found that a number of factors, both intrinsic and extrinsic, affect the reproductive mode of the
San Francisco Bay Artemia including brood number,
density, photoperiod, salinity and temperature. As in
the case of diapause induction, the cues which can
act as a signal for cyst diapause are habitat specific. This is understandable since these abiotic factors
are useful as regulatory cues, precisely because they
forecast the coming of favorable conditions (both abiotic and biotic) (Drinkwater & Clegg, 1991). Evidence
that the production and hatchability of resting eggs
have a different response to temperature in sexual and
in parthenogenetic populations (Browne et ai., 1984,
1988; Amat, 1982; Vanhaecke & Sorgeloos, 1989)
indicates the potential of dormancy to function as an
adaptation regulating their seasonal occurrence.
Temperature is clearly an important factor controlling reproductive rates and population success,
and therefore might affect the seasonal succession
of Artemia populations in Spanish salterns. In this
study, we examine the life-history responses as well
as the hatchability and cyst production of sexual
and parthenogenetic (diploid and tetraploid) Artemia
species that live on the Iberian peninsula (Amat, 1980):
the sexual diploid Artemia tunisiana which is the
dominant winter-spring species; and the diploid and
tetraploid Artemia parthenogenetica strains, that dominate during spring-summer. The experiments were
performed at three temperatures, 15 0, 24 ° and
29.5°C.
Material and methods
Experimental animals
Artemia nauplii hatched from cysts of the following
populations were used (harvest date in parentheses):
(1) Sexual diploid Artemia tunisiana from Bonmatf
saltworks, Alicante, Spain (1990); (2) Parthenogenetic diploid A. parthenogenetica strain from La Mata
lagoon, Alicante, Spain (1988); (3) Parthenogenetic
tetraploid A.parthenogenetica strain from La Trinidad
saltworks, Tarragona, Spain (1984).
Cultures
Nauplii were obtained from mass hatching of cysts
in one liter of 38 ppt sea water at 28 ° C after 48 h
(Hontoria et al., 1989). Nauplii were adapted to high
salinity in 60 ppt brine during 24 h, with the microalgae
Dunaliella sp. as food.
Initial culture density was set to 100 nauplii / I in
2 I containers filled with 90 ppt brine. Every two or
three days suspensions of brine containing the alga
Dunaliella sp. were added ad libitum. Cultures were
conducted for each population at three temperatures
15 0, 24 ° and 29.5 °C, in thermostatized incubators
(±1 0C). Cultures were maintained at 12D:12L photoperiod pattern with fluorescent lighting under constant aeration. When females presented visible spots
along their ovaries, and males had their claspers full
developed, individuals were removed from the mass
cultures and placed in single jars containing 50 ml of
90 ppt brine and algal culture. One female for parthenogenetic populations or one male-female for the sexual
population was placed in each jar. Males were replaced
at death. At least every other day jars were examined
for deaths or offspring production. Cysts or nauplii
were removed by pipette and counted. Excess algae
and accumulated feces were removed and the volume
replaced with fresh brine containing algal suspension.
Salinity ofthe individual cultures was also maintained
about constant by addition or dilution of brine. Thirty replicates were used for each treatment. The cysts
produced in the cultures at 15 ° and 24°C were kept
in saturated brine at the same temperatures. At the
end of the life-history experiments they were sieved
through 160 p,m mesh, washed and resuspended in
38 ppt sea water at the same temperature at which they
were obtained. Five samples of' hydrated cysts from
the sexual and the La Trinidad populations at 15°C,
and from the La Mata and the La Trinidad popula-