Sexual Cycle
The formation of dormant, latent or diapause ('winter')
eggs and the appearance of sexual forms within the life
cycle of Cyclestheria hislopi remained enigmatical for
a long time. The only morphological description of
males are of a young male, found in the brood chamber
of a parthenogenetic reproducing female, given by Sars
(1887) and one individual reported by Daday (1926).
Sissom (1980) mentioned the appearance of males in
a population found in the United States and suspected
of being introduced from Australia.
Sexual females and dormant eggs were thought to
exist, but were not described until Roessler & Sanchez
(1986). It is interesting that, at the same time Sars
was working with his Australian material, Stuhlmann
(1888) reported a particular 'Limnadia' from Zanzibar,
with females with opaque valves and 'layed eggs'.
He supposed them to represent 'possibly sexual reproduction' forms. Furthermore, he mentioned the direct
parthenogenetic development of embryos within the
maternal brood chamber. No doubt, the reported form
was Cyclestheria and the females with opaque valves
were sexual females.
Extensive sampling over more than 15 years in
the warm tropical lowlands of Colombia revealed that
the appearance of males of Cyclestheria is normally
closely related to particular environmental conditions.
The number of males increases as the aquatic habitat matures in its seasonal cycle. Parthenogenetically
reproducing females produce both males and parthenogenetic females in varying ratios within and between
successive broods. In the next generation, females may
again produce males or may produce only parthenogenetic females. Field studies agree with laboratory
data. The origin of sexual females in contrast is not so
clear.
1. Males: The first morphological description of a
young male in the first larval stage and taken from the
brood chamber of a parthenogenetic female was made
by Sars (1887). I have observed that the males have
already reached sexual maturity in this stage, before
they are released by their mother.
Spermatogenesis takes place in the median and
medial-ventral parts of the enlarged tubular testes,
which, like the ovaries, extend along both sides of
the digestive tract. The spermatozoa are cylindrical
with blunt ends. Spermatogenesis is normal in males
up to the oldest I could cultivate in the laboratory.
(Figs 2e; 4). Copulation or mating behavior was never
115
observed, notwithstanding numerous observations of
males in combination with sexual females.
In most field samples, males do not reach ages
corresponding to more than three carapace lines. It is
only in the dryer season that males survive to advanced
stages. I only occasionally found males with up to 10
carapace lines in the field. The optimal morphological
development occurred when the number of males was
at its highest level in the population, which reached
maximally a total female/male ratio of 5: 1. In laboratory cultures, optimal development of males was
observed and the oldest male reached 13 carapace lines
(Fig. 2e). The optimal development of males in the
field, with respect to numbers as well as size, was
observed in December, the beginning of the dry season, which normally extends from December to March
in most parts of Colombia. Nevertheless, male numbers are reduced as drying continues. Further population development until total drying is characterized by
morphologically normal parthenogenetic females and
females that undergo a special process of hypodermic
transformation that will be described later.
As mentioned by Sars (1887) and Sissom (1975),
Cyclestheria hislopi shows a sexual dimorphism.
Males are smaller and, according to my observations,
more acti ve than females. Females of all different types
have 16 postmaxillary appendages. Males have 15 and
the first pair is converted into claspers (Fig. 2e).
2. Sexual females: Unlike the parthenogenetically
reproducing females, the sexual form of the female produces diapause eggs. These females undergo a strong
procuticular reinforcement of the external plates of the
bivalve carapace forming a special type of ephippium
(Figs 2a; 6). The origin of sexual females is unclear.
The females that develop in the same broods along
with males generally appear to be parthenogenetically reproducing females. Nevertheless, and although I
could not yet clarify this aspect, it seems that the sexual females are generated by parthenogenetic females
in the same way as reported above for males.
Sexual females look morphologically similar to the
parthenogenetic forms up to and including the fourth
larval stage (concerning states: see conventions). As
with parthenogenetic females, sexual females start to
mature sexually after the fourth larval molt (i.e., during
the fifth larval stage). Strong morphological changes
begin at that time and ovules mature to, ova. Oviposition occurs shortly after the fifth larval molt (during
the sixth larval stage). Sexual females produce only
The formation of dormant, latent or diapause ('winter')
eggs and the appearance of sexual forms within the life
cycle of Cyclestheria hislopi remained enigmatical for
a long time. The only morphological description of
males are of a young male, found in the brood chamber
of a parthenogenetic reproducing female, given by Sars
(1887) and one individual reported by Daday (1926).
Sissom (1980) mentioned the appearance of males in
a population found in the United States and suspected
of being introduced from Australia.
Sexual females and dormant eggs were thought to
exist, but were not described until Roessler & Sanchez
(1986). It is interesting that, at the same time Sars
was working with his Australian material, Stuhlmann
(1888) reported a particular 'Limnadia' from Zanzibar,
with females with opaque valves and 'layed eggs'.
He supposed them to represent 'possibly sexual reproduction' forms. Furthermore, he mentioned the direct
parthenogenetic development of embryos within the
maternal brood chamber. No doubt, the reported form
was Cyclestheria and the females with opaque valves
were sexual females.
Extensive sampling over more than 15 years in
the warm tropical lowlands of Colombia revealed that
the appearance of males of Cyclestheria is normally
closely related to particular environmental conditions.
The number of males increases as the aquatic habitat matures in its seasonal cycle. Parthenogenetically
reproducing females produce both males and parthenogenetic females in varying ratios within and between
successive broods. In the next generation, females may
again produce males or may produce only parthenogenetic females. Field studies agree with laboratory
data. The origin of sexual females in contrast is not so
clear.
1. Males: The first morphological description of a
young male in the first larval stage and taken from the
brood chamber of a parthenogenetic female was made
by Sars (1887). I have observed that the males have
already reached sexual maturity in this stage, before
they are released by their mother.
Spermatogenesis takes place in the median and
medial-ventral parts of the enlarged tubular testes,
which, like the ovaries, extend along both sides of
the digestive tract. The spermatozoa are cylindrical
with blunt ends. Spermatogenesis is normal in males
up to the oldest I could cultivate in the laboratory.
(Figs 2e; 4). Copulation or mating behavior was never
115
observed, notwithstanding numerous observations of
males in combination with sexual females.
In most field samples, males do not reach ages
corresponding to more than three carapace lines. It is
only in the dryer season that males survive to advanced
stages. I only occasionally found males with up to 10
carapace lines in the field. The optimal morphological
development occurred when the number of males was
at its highest level in the population, which reached
maximally a total female/male ratio of 5: 1. In laboratory cultures, optimal development of males was
observed and the oldest male reached 13 carapace lines
(Fig. 2e). The optimal development of males in the
field, with respect to numbers as well as size, was
observed in December, the beginning of the dry season, which normally extends from December to March
in most parts of Colombia. Nevertheless, male numbers are reduced as drying continues. Further population development until total drying is characterized by
morphologically normal parthenogenetic females and
females that undergo a special process of hypodermic
transformation that will be described later.
As mentioned by Sars (1887) and Sissom (1975),
Cyclestheria hislopi shows a sexual dimorphism.
Males are smaller and, according to my observations,
more acti ve than females. Females of all different types
have 16 postmaxillary appendages. Males have 15 and
the first pair is converted into claspers (Fig. 2e).
2. Sexual females: Unlike the parthenogenetically
reproducing females, the sexual form of the female produces diapause eggs. These females undergo a strong
procuticular reinforcement of the external plates of the
bivalve carapace forming a special type of ephippium
(Figs 2a; 6). The origin of sexual females is unclear.
The females that develop in the same broods along
with males generally appear to be parthenogenetically reproducing females. Nevertheless, and although I
could not yet clarify this aspect, it seems that the sexual females are generated by parthenogenetic females
in the same way as reported above for males.
Sexual females look morphologically similar to the
parthenogenetic forms up to and including the fourth
larval stage (concerning states: see conventions). As
with parthenogenetic females, sexual females start to
mature sexually after the fourth larval molt (i.e., during
the fifth larval stage). Strong morphological changes
begin at that time and ovules mature to, ova. Oviposition occurs shortly after the fifth larval molt (during
the sixth larval stage). Sexual females produce only