(Knoll & Zucker, 1994), the unpredictability of this
environment may outweigh any advantages that a male
could obtain from active mate preference.
The selfing ability of the hermaphrodites complicates this system. The occurrence of selfing during
this experiment demonstrates that hermaphrodites will
self readily despite the presence of a male. Thus nongravid, receptive hermaphrodites may be more rare
than was initially predicted. The reproductive success
of a male is largely controlled by the number of times
he is able to mate (Trivers, 1972). Males may be forced
to mate with any available hermaphrodite in order to
achieve any reproductive success. Field data on the
actual number of receptive hermaphrodites in the natural environment would be very helpful. These studies, however, are difficult due to the unpredictability
of pond filling. The last time the LTER Playa filled
sufficiently to allow Branchiopod activity was in 1984
and this playa typically only fills once every three to
ten years (MacKay, et ai., 1990).
The overall level of selfing in the presence of males
(17 occurrences-18% of the observations) was higher
than expected. In many simultaneously hermaphroditic
animals, outcrossing is the primary reproductive
method while selfing is typically used when animals are
isolated and stored sperm has been used up or become
too old (J arne, et ai., 1991; J arne, et al., 1992; Warner, 1993). Many of these species live in permanent
aquatic environments or in environments with stable
water resources. The unpredictability of E. texana's
water resources may be partially responsible for the
high frequency of selfing in this species. Waiting for
the opportunity to outcross with available males could
be counterproductive when ponds may dry rapidly.
When selfing occurred, males never approached
the hermaphrodite. This suggests that males were not
aware that the hermaphrodite was receptive. Information on the types of cues utilized by hermaphrodites to
signal receptivity to the male are unavailable. The use
of pheromones to signal receptivity has been reported in crustaceans (Hazlett, 1970; Tierney & Dunham, 1982). Receptivity cues in E. texana are probably chemical in nature, as well. A chemical cue
could be either a receptivity or a non-receptivity cue.
In this study, some males clasped other males for
prolonged periods of time, but males never clasped
hermaphrodites for more then a few seconds unless outcrossing followed, suggesting that the hermaphrodites
release a non-receptivity cue. In its absence, clasping
continues. Since males are generally more rare then
hermaphrodites (Knoll, 1994), the chance of 1 male
71
randomly clasping onto another would be low. When a
hermaphrodite is going to self, it may not release this
chemical signal or it may release a chemical signaling
its lack of receptivity. Release or non-release of a nonreceptivity signal may be a choice on the part of the
hermaphrodite between selfing and outcrossing respectively. If such a 'choice' is made, it can be considered
an unusual form of mate choice.
Neither males nor hermaphrodites were ever seen
performing any type of assessment behavior between
two individuals that would suggest the occurrence of
intersexual selection within this species. This study
was designed to test for assessment of morphological
characteristics such as size. If mate assessment occurs
at a distance using chemical cues, it would not have
been evident in this study. Further studies designed to
evaluate the role of chemical cues in mate choice are
underway.
As indicated above, a male would clasp another
male occasionally for extended periods of time and
perform what appeared to be mate guarding behavior.
There seems to be little utility in one male attempting to outcross with another male. Gregarious behavior has also been reported for the copepod Lernaeocera branchialis (Anstensrud, 1992). Anstensrud suggested that chemicals given off by juvenile males in
L. branchialis may be confused with those given off
by females in various stages of development. In Gammarus pulex, males were reported to have continued
precopulatory mate guarding with deceased females
(Ward, 1983). Little adaptive significance can be
attributed to these types of behaviors and they suggest
that mating cues are not 100% accurate. The frequency
of these non-adaptive behaviors is low and it may be
that the cost of more effective signaling systems would
override any benefits obtained.
In summary, male-male competition appears to be
the only form of sexual selection operating in this population of the clam shrimp, Eulimnadia texana. Nevertheless, the observation of one hermaphrodite appearing to interfere with the outcrossing attempt of a male
and smaller hermaphrodite suggests that further studies are needed to fully understand this mating system.
Studies under field conditions or larger laboratory populations which better simulate the natural environment
should be performed to that end.
environment may outweigh any advantages that a male
could obtain from active mate preference.
The selfing ability of the hermaphrodites complicates this system. The occurrence of selfing during
this experiment demonstrates that hermaphrodites will
self readily despite the presence of a male. Thus nongravid, receptive hermaphrodites may be more rare
than was initially predicted. The reproductive success
of a male is largely controlled by the number of times
he is able to mate (Trivers, 1972). Males may be forced
to mate with any available hermaphrodite in order to
achieve any reproductive success. Field data on the
actual number of receptive hermaphrodites in the natural environment would be very helpful. These studies, however, are difficult due to the unpredictability
of pond filling. The last time the LTER Playa filled
sufficiently to allow Branchiopod activity was in 1984
and this playa typically only fills once every three to
ten years (MacKay, et ai., 1990).
The overall level of selfing in the presence of males
(17 occurrences-18% of the observations) was higher
than expected. In many simultaneously hermaphroditic
animals, outcrossing is the primary reproductive
method while selfing is typically used when animals are
isolated and stored sperm has been used up or become
too old (J arne, et ai., 1991; J arne, et al., 1992; Warner, 1993). Many of these species live in permanent
aquatic environments or in environments with stable
water resources. The unpredictability of E. texana's
water resources may be partially responsible for the
high frequency of selfing in this species. Waiting for
the opportunity to outcross with available males could
be counterproductive when ponds may dry rapidly.
When selfing occurred, males never approached
the hermaphrodite. This suggests that males were not
aware that the hermaphrodite was receptive. Information on the types of cues utilized by hermaphrodites to
signal receptivity to the male are unavailable. The use
of pheromones to signal receptivity has been reported in crustaceans (Hazlett, 1970; Tierney & Dunham, 1982). Receptivity cues in E. texana are probably chemical in nature, as well. A chemical cue
could be either a receptivity or a non-receptivity cue.
In this study, some males clasped other males for
prolonged periods of time, but males never clasped
hermaphrodites for more then a few seconds unless outcrossing followed, suggesting that the hermaphrodites
release a non-receptivity cue. In its absence, clasping
continues. Since males are generally more rare then
hermaphrodites (Knoll, 1994), the chance of 1 male
71
randomly clasping onto another would be low. When a
hermaphrodite is going to self, it may not release this
chemical signal or it may release a chemical signaling
its lack of receptivity. Release or non-release of a nonreceptivity signal may be a choice on the part of the
hermaphrodite between selfing and outcrossing respectively. If such a 'choice' is made, it can be considered
an unusual form of mate choice.
Neither males nor hermaphrodites were ever seen
performing any type of assessment behavior between
two individuals that would suggest the occurrence of
intersexual selection within this species. This study
was designed to test for assessment of morphological
characteristics such as size. If mate assessment occurs
at a distance using chemical cues, it would not have
been evident in this study. Further studies designed to
evaluate the role of chemical cues in mate choice are
underway.
As indicated above, a male would clasp another
male occasionally for extended periods of time and
perform what appeared to be mate guarding behavior.
There seems to be little utility in one male attempting to outcross with another male. Gregarious behavior has also been reported for the copepod Lernaeocera branchialis (Anstensrud, 1992). Anstensrud suggested that chemicals given off by juvenile males in
L. branchialis may be confused with those given off
by females in various stages of development. In Gammarus pulex, males were reported to have continued
precopulatory mate guarding with deceased females
(Ward, 1983). Little adaptive significance can be
attributed to these types of behaviors and they suggest
that mating cues are not 100% accurate. The frequency
of these non-adaptive behaviors is low and it may be
that the cost of more effective signaling systems would
override any benefits obtained.
In summary, male-male competition appears to be
the only form of sexual selection operating in this population of the clam shrimp, Eulimnadia texana. Nevertheless, the observation of one hermaphrodite appearing to interfere with the outcrossing attempt of a male
and smaller hermaphrodite suggests that further studies are needed to fully understand this mating system.
Studies under field conditions or larger laboratory populations which better simulate the natural environment
should be performed to that end.