68
become selective for females (Ridley, 1978; Dewsbury, 1982). The energy investment of mate guarding, as seen in E. texana, and the risks likely to be
incurred by this behavior may lead to selectivity on the
part of the male (Parker, 1974; Hatziolos & Caldwell,
1983). Hermaphroditic clam shrimp show a significant
increase in fecundity as they increase in size. Males
that choose larger hermaphrodites could significantly
increase their reproductive success. Thus one might
predict male mate choice of large hermaphrodites.
Should hermaphrodite-hermaphrodite competition and
male mate choice both be occurring, clam shrimp
would exhibit sex role reversal for both intra- and intersexual selection. This is extremely rare in crustaceans
(Hatziolos & Caldwell, 1983).
The case for male-male competition: There is
a complication in this system, however, because
hermaphrodites will self quite readily when males are
not present and even when males are present (Sassaman
& Weeks, 1993; Knoll, 1994). The hermaphrodites
remain gravid for much of their life cycle and can self
within an hour after the previous clutch of eggs have
been laid (Knoll, 1994). The fact that hermaphrodites
self and only have short periods when they are
apparently receptive to mating may actually change
the operational sex ratio, or the ratio of fertilizable
hermaphrodites to sexually active males (Emlen &
Dring, 1977), in favor of males. This could lead to competition among males for the 'rare' non-gravid, receptive hermaphrodites. Fertilization appears to occur
shortly after molting (Knoll, 1994). Male-male competition is particularly common in crustaceans largely
because females are only receptive for short periods of
time when molting occurs (Ward, 1983; Elwood, et al.,
1987; Anstensrud, 1992). In many of these systems,
mate guarding has emerged as a male competition strategy rather than part of a male selective strategy (Parker,
1974).
The case for hermaphrodite mate choice: There is
also the possibility that hermaphrodites exhibit mate
choice for large males. Since clam shrimp continue to grow with age, the larger males are older and
have thus demonstrated the ability to survive. Furthermore, clasping males provide locomotion for the
hermaphrodite (Knoll, 1994); larger males may be better equipped to provide this energy saving service to
the hermaphrodites.
This study was designed to explore each of these
possibilities in E. texana.
Methods
Rearing procedure
Numerous samples of the first 1-2 cm of soil were
collected from the playa portion of the NSF LongTerm Ecological Research site on the New Mexico
State University College Ranch located at the northern edge of the Dona Ana Mountains 40 km NNE of
Las Cruces, Dona Ana County, New Mexico. For a
description of the site see MacKay, et at., 1990. Sample populations were established in the laboratory by
placing 250 ml soil samples in plastic mouse cages
(30 cm x 12 cm x 15 cm) and hydrating them with 4
liters of aged tap water. The tanks were kept on a 24
hour light cycle using incandescent bulbs in reflectors to maintain summer-like temperatures of approximately 30°C and were supplemented with tadpole
food pellets until algal growth was initiated. The clam
shrimp reached sexual maturity within approximately three to five days after hatching at about 3 mm in
carapace length. In sexually mature hermaphrodites,
the eggs are visible in paired ovaries located along
the digestive tract and fertilized clutches are carried
by the hermaphrodite in a dorsal brood chamber for
approximately 24 hours prior to being released onto
the substrate (pers. obs.). There are no visible external
cues to sexual maturity in the males, but males with
carapace lengths as small as 3 mm were seen clasping
hermaphrodites (pers. obs.). Clam shrimp smaller than
3 mm were not used in observations and individual
clam shrimp were only used once.
A test for hermaphrodite intersexual selection and
male intrasexual selection
Two sexually mature males with carapace diameters of
at least a 0.5 mm difference were placed with a sexually mature non-gravid hermaphrodite of a random
size. The trio was measured and then isolated in a plastic 'dixie' cup containing 125 ml of aged tap water.
Size and sexual dimorphism were pronounced enough
that no marking was necessary for individual identification. The behavior of the trio was observed and
detailed records were kept of all interactions between
the clam shrimp using the focal animal techniques outlined by Altmann (1974). Observations generally lasted for two hours or until fertilization was observed,
whichever came first. At the end of 2 hours, however, if a hermaphrodite and male were engaged in an
interaction, the observation was extended until fertil-
become selective for females (Ridley, 1978; Dewsbury, 1982). The energy investment of mate guarding, as seen in E. texana, and the risks likely to be
incurred by this behavior may lead to selectivity on the
part of the male (Parker, 1974; Hatziolos & Caldwell,
1983). Hermaphroditic clam shrimp show a significant
increase in fecundity as they increase in size. Males
that choose larger hermaphrodites could significantly
increase their reproductive success. Thus one might
predict male mate choice of large hermaphrodites.
Should hermaphrodite-hermaphrodite competition and
male mate choice both be occurring, clam shrimp
would exhibit sex role reversal for both intra- and intersexual selection. This is extremely rare in crustaceans
(Hatziolos & Caldwell, 1983).
The case for male-male competition: There is
a complication in this system, however, because
hermaphrodites will self quite readily when males are
not present and even when males are present (Sassaman
& Weeks, 1993; Knoll, 1994). The hermaphrodites
remain gravid for much of their life cycle and can self
within an hour after the previous clutch of eggs have
been laid (Knoll, 1994). The fact that hermaphrodites
self and only have short periods when they are
apparently receptive to mating may actually change
the operational sex ratio, or the ratio of fertilizable
hermaphrodites to sexually active males (Emlen &
Dring, 1977), in favor of males. This could lead to competition among males for the 'rare' non-gravid, receptive hermaphrodites. Fertilization appears to occur
shortly after molting (Knoll, 1994). Male-male competition is particularly common in crustaceans largely
because females are only receptive for short periods of
time when molting occurs (Ward, 1983; Elwood, et al.,
1987; Anstensrud, 1992). In many of these systems,
mate guarding has emerged as a male competition strategy rather than part of a male selective strategy (Parker,
1974).
The case for hermaphrodite mate choice: There is
also the possibility that hermaphrodites exhibit mate
choice for large males. Since clam shrimp continue to grow with age, the larger males are older and
have thus demonstrated the ability to survive. Furthermore, clasping males provide locomotion for the
hermaphrodite (Knoll, 1994); larger males may be better equipped to provide this energy saving service to
the hermaphrodites.
This study was designed to explore each of these
possibilities in E. texana.
Methods
Rearing procedure
Numerous samples of the first 1-2 cm of soil were
collected from the playa portion of the NSF LongTerm Ecological Research site on the New Mexico
State University College Ranch located at the northern edge of the Dona Ana Mountains 40 km NNE of
Las Cruces, Dona Ana County, New Mexico. For a
description of the site see MacKay, et at., 1990. Sample populations were established in the laboratory by
placing 250 ml soil samples in plastic mouse cages
(30 cm x 12 cm x 15 cm) and hydrating them with 4
liters of aged tap water. The tanks were kept on a 24
hour light cycle using incandescent bulbs in reflectors to maintain summer-like temperatures of approximately 30°C and were supplemented with tadpole
food pellets until algal growth was initiated. The clam
shrimp reached sexual maturity within approximately three to five days after hatching at about 3 mm in
carapace length. In sexually mature hermaphrodites,
the eggs are visible in paired ovaries located along
the digestive tract and fertilized clutches are carried
by the hermaphrodite in a dorsal brood chamber for
approximately 24 hours prior to being released onto
the substrate (pers. obs.). There are no visible external
cues to sexual maturity in the males, but males with
carapace lengths as small as 3 mm were seen clasping
hermaphrodites (pers. obs.). Clam shrimp smaller than
3 mm were not used in observations and individual
clam shrimp were only used once.
A test for hermaphrodite intersexual selection and
male intrasexual selection
Two sexually mature males with carapace diameters of
at least a 0.5 mm difference were placed with a sexually mature non-gravid hermaphrodite of a random
size. The trio was measured and then isolated in a plastic 'dixie' cup containing 125 ml of aged tap water.
Size and sexual dimorphism were pronounced enough
that no marking was necessary for individual identification. The behavior of the trio was observed and
detailed records were kept of all interactions between
the clam shrimp using the focal animal techniques outlined by Altmann (1974). Observations generally lasted for two hours or until fertilization was observed,
whichever came first. At the end of 2 hours, however, if a hermaphrodite and male were engaged in an
interaction, the observation was extended until fertil-