lentieularis (L.), albeit 'rudimentary', has been presented by Zaffagnini (1969) and genetic evidence of it
in Eulimnadia texana Packard by Sassaman & Weeks
(1993). Although they have elsewhere been referred to
as hermaphrodites (Sassaman & Weeks, 1993; Knoll,
1994, Knoll & Zucker, 1995a, b), I will refer to the eggbearing gender in Limnadiinae as females to emphasize
their evolutionary relatedness to other conchostracan
species. In contrast to many other animal and plant
hermaphrodites, these individuals cannot function as
outcrossing males. Because they lack the modification
of anterior limbs as claspers, they cannot effect pairing
and therefore cannot be sperm donors to other individuals. Their endogenously produced sperm can be
used only in self-fertilization, if at all. Yet these individuals function quite normally as outcrossing females
(Sassaman & Weeks, 1993).
Modes of reproduction
The reproductive mode of individual species (sometimes individual populations) of clam shrimps has usually been inferred by the absence or presence and proportionate abundance of males in population samples.
When males are abundant, the species is considered
to be sexual (gonochoric); when males are absent,
the species is considered to be 'parthenogenetic'; and
when males are present but very rare, a mixed-mating
system of sexual and 'parthenogenetic' reproduction
usually is deduced. Qualitative reviews of sex ratio
variation were made by Sars (1902) and by Mathias
(1937), but there has been no recent compendium of
sex ratio variation in conchostracans based on quantitative estimates of male frequencies.
Sex ratio variation
Summarized in Table 1 are estimated sex ratios
(expressed as % males) for 60 species of conchostracans, representing all families and genera and over
25% of the estimated number of species throughout
the world (ef, Hu, 1988). For many species, the only
available data is from the field-collected type material
on which the original description was based. In other
cases, there have been autecological studies of particular species that included periodic sampling of natural
populations. Finally, for some species I have used data
from laboratory rearings of animals from dried sediment samples of natural habitats.
47
Where sample sizes were indicated in the original
report, I have calculated the percentages of males. If
sample sizes were not given, but a qualitative statement
of approximate equality was, I have indicated ~50 for
male percentage.
I have included the best data known to me for
at least one representative of each genus. For some
monotypic genera such as Maghrebestheria Thiery,
Sewellestheria Tiwari, Paraeyclestheria Shen and Dai,
and Limnadopsium Novojilov, data is limited to original species descriptions. For larger genera I have
included several species for which reasonably quantitative data are available. In choosing these cases, I
have not included the sex ratios for type specimens
when it is clear in the original description that the type
material was selected from a larger initial collection
whose sex ratio was not specified, or when the original description was based on samples of fewer than
10 individuals. Previously unpublished information is
detailed in Table A of Appendix 1.
Sex ratios based on field samples from natural populations should be viewed with some caution because:
(1) the two sexes may differ sufficiently in their behavior, ecological preferences, or microhabitat utilizations
so that they are not sampled in proportion to their relative abundances or (2) the two sexes may differ in
survivorship. Eriksen & Brown (1980), for example,
have reported metabolic differences between male and
female Cyzieus ealifornieus related to differences in
behavioral activity, and Michael & Chandran (1967)
have reported more rapid loss of protein and carbohydrate in females than in males during starvation, presumably due to the energetic and material costs of egg
production. Successional sampling of cohorts in natural populations of Eulimnadia have documented differential mortality of males (Strenth & Sissom, 1975;
Vidrine et aI., 1987) and Belk (1989) has even inferred
the presence of yet undiscovered males of Eulimnadia
astraova on the basis of spermatophores attached to
mature females. Roessler (1995a) has described differential mortality of female Metalimnadia serratura in
natural populations, and laboratory observations indicate that this mortality is a cost associated with interacting with males.
These sources of bias can be largely excluded in
estimates obtained by rearing adults from natural sediment samples, provided that the cultures are censused
early enough to precede any inherent difference in survivorship. In this context, it is interesting to note that
there is generally much less deviation from a 1: 1 sex
ratio in, for example, species of Cyzicidae and Lep-
in Eulimnadia texana Packard by Sassaman & Weeks
(1993). Although they have elsewhere been referred to
as hermaphrodites (Sassaman & Weeks, 1993; Knoll,
1994, Knoll & Zucker, 1995a, b), I will refer to the eggbearing gender in Limnadiinae as females to emphasize
their evolutionary relatedness to other conchostracan
species. In contrast to many other animal and plant
hermaphrodites, these individuals cannot function as
outcrossing males. Because they lack the modification
of anterior limbs as claspers, they cannot effect pairing
and therefore cannot be sperm donors to other individuals. Their endogenously produced sperm can be
used only in self-fertilization, if at all. Yet these individuals function quite normally as outcrossing females
(Sassaman & Weeks, 1993).
Modes of reproduction
The reproductive mode of individual species (sometimes individual populations) of clam shrimps has usually been inferred by the absence or presence and proportionate abundance of males in population samples.
When males are abundant, the species is considered
to be sexual (gonochoric); when males are absent,
the species is considered to be 'parthenogenetic'; and
when males are present but very rare, a mixed-mating
system of sexual and 'parthenogenetic' reproduction
usually is deduced. Qualitative reviews of sex ratio
variation were made by Sars (1902) and by Mathias
(1937), but there has been no recent compendium of
sex ratio variation in conchostracans based on quantitative estimates of male frequencies.
Sex ratio variation
Summarized in Table 1 are estimated sex ratios
(expressed as % males) for 60 species of conchostracans, representing all families and genera and over
25% of the estimated number of species throughout
the world (ef, Hu, 1988). For many species, the only
available data is from the field-collected type material
on which the original description was based. In other
cases, there have been autecological studies of particular species that included periodic sampling of natural
populations. Finally, for some species I have used data
from laboratory rearings of animals from dried sediment samples of natural habitats.
47
Where sample sizes were indicated in the original
report, I have calculated the percentages of males. If
sample sizes were not given, but a qualitative statement
of approximate equality was, I have indicated ~50 for
male percentage.
I have included the best data known to me for
at least one representative of each genus. For some
monotypic genera such as Maghrebestheria Thiery,
Sewellestheria Tiwari, Paraeyclestheria Shen and Dai,
and Limnadopsium Novojilov, data is limited to original species descriptions. For larger genera I have
included several species for which reasonably quantitative data are available. In choosing these cases, I
have not included the sex ratios for type specimens
when it is clear in the original description that the type
material was selected from a larger initial collection
whose sex ratio was not specified, or when the original description was based on samples of fewer than
10 individuals. Previously unpublished information is
detailed in Table A of Appendix 1.
Sex ratios based on field samples from natural populations should be viewed with some caution because:
(1) the two sexes may differ sufficiently in their behavior, ecological preferences, or microhabitat utilizations
so that they are not sampled in proportion to their relative abundances or (2) the two sexes may differ in
survivorship. Eriksen & Brown (1980), for example,
have reported metabolic differences between male and
female Cyzieus ealifornieus related to differences in
behavioral activity, and Michael & Chandran (1967)
have reported more rapid loss of protein and carbohydrate in females than in males during starvation, presumably due to the energetic and material costs of egg
production. Successional sampling of cohorts in natural populations of Eulimnadia have documented differential mortality of males (Strenth & Sissom, 1975;
Vidrine et aI., 1987) and Belk (1989) has even inferred
the presence of yet undiscovered males of Eulimnadia
astraova on the basis of spermatophores attached to
mature females. Roessler (1995a) has described differential mortality of female Metalimnadia serratura in
natural populations, and laboratory observations indicate that this mortality is a cost associated with interacting with males.
These sources of bias can be largely excluded in
estimates obtained by rearing adults from natural sediment samples, provided that the cultures are censused
early enough to precede any inherent difference in survivorship. In this context, it is interesting to note that
there is generally much less deviation from a 1: 1 sex
ratio in, for example, species of Cyzicidae and Lep-