Hydrobiologia 298: 67-72, 1995.
D. Belk, H. 1. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture II.
© 1995 Kluwer Academic Publishers.
67
Is sexual selection operating in the androdioecious clam shrimp,
Eulimnadia texana (Crustacea: Conchostraca)?
Lana Knoll & Naida Zucker
Department of Biology, New Mexico State University, Las Cruces, NM 88003, USA
Key words: androdioecious crustacean, Eulimnadia texana, sexual selection, mate choice, male-male competition
Abstract
Experiments were performed to document the existence of intersexual or intrasexual selection in the clam shrimp,
Eulimnadia texana. Individuals within this species are either males or hermaphrodites. Hermaphrodites can self their
own eggs or outcross with a male, but they cannot outcross with other hermaphrodites. Theoretical considerations
suggest that both intrasexual and intersexual selection could be occurring on the part of the hermaphrodites and
the males. When males were given a choice between two non-gravid hermaphrodites of different sizes, they did
not exhibit a mating preference based upon size. When two males of different sizes were isolated with a single
nongravid receptive hermaphrodite, the hermaphrodite showed no preference between the two males. There was
evidence, however, of male-male competition for receptive hermaphrodites and of mate guarding on the part of the
males. During aggressive encounters between two males, the larger of the two had a significant advantage over the
smaller, and larger males were always the victors when hermaphrodite takeovers occurred as a result of male-male
conflict. Hermaphrodites appear to control the mating process both by struggling with males when they are not
receptive to them and by selfing in the presence of males. This suggests that hermaphrodites withhold receptivity
cues from males, or produce non-receptivity cues, when they are going to self. Though hermaphrodites do not
appear to select males based upon size, they may make a selection between selfing and outcrossing by controlling
the use of receptivity signals.
Introduction
Individual clam shrimp Eulimnadia texana (Crustacea:
Conchostraca) are either male or hermaphroditic. This
type of mating system is known as androdioecy, a
rare condition in animals (Sassaman & Weeks, 1993).
Hermaphrodites are capable of selfing their own eggs
or outcrossing with a male, but they cannot exchange
sperm with other hermaphrodites. Based upon what we
know about the reproductive biology of this species,
there is the potential that sexual selection is an important evolutionary force within this mating system. A
case could be made for hermaphrodite-hermaphrodite
competition (intrasexual selection) and male mate
choice (intersexual selection), as well as for male-male
competition (intrasexual selection) and hermaphrodite
mate choice (intersexual selection).
The case for hermaphrodite-hermaphrodite competition: The sex ratio within this species is highly
skewed in favor of the hermaphrodites (Sassaman,
1989; Knoll, 1994). This often results in competition among members of the common sex for members
of the rare sex (Emlen & Oring, 1977). Thus, one
might predict hermaphrodite-hermaphrodite (,femalefemale') competition for males.
The case for male mate choice: Behavioral observations suggest that males practice precopulatory mate
guarding of hermaphrodites within this species (Knoll,
1994). Mate choice by the males often occurs in systems where mate guarding is present (Parker, 1974;
Thompson & Manning, 1981; Elwood, et al., 1987).
Sexual selection theory predicts that the sex putting the
larger investment into reproduction will be the selective sex during mating (Trivers, 1972). This is typically
the female. When male investment in reproduction is
high, such as in systems where the male aides in offspring rearing or expends large amounts of energy in
spermatophore production, males have been known to
D. Belk, H. 1. Dumont & G. Maier (eds), Studies on Large Branchiopod Biology and Aquaculture II.
© 1995 Kluwer Academic Publishers.
67
Is sexual selection operating in the androdioecious clam shrimp,
Eulimnadia texana (Crustacea: Conchostraca)?
Lana Knoll & Naida Zucker
Department of Biology, New Mexico State University, Las Cruces, NM 88003, USA
Key words: androdioecious crustacean, Eulimnadia texana, sexual selection, mate choice, male-male competition
Abstract
Experiments were performed to document the existence of intersexual or intrasexual selection in the clam shrimp,
Eulimnadia texana. Individuals within this species are either males or hermaphrodites. Hermaphrodites can self their
own eggs or outcross with a male, but they cannot outcross with other hermaphrodites. Theoretical considerations
suggest that both intrasexual and intersexual selection could be occurring on the part of the hermaphrodites and
the males. When males were given a choice between two non-gravid hermaphrodites of different sizes, they did
not exhibit a mating preference based upon size. When two males of different sizes were isolated with a single
nongravid receptive hermaphrodite, the hermaphrodite showed no preference between the two males. There was
evidence, however, of male-male competition for receptive hermaphrodites and of mate guarding on the part of the
males. During aggressive encounters between two males, the larger of the two had a significant advantage over the
smaller, and larger males were always the victors when hermaphrodite takeovers occurred as a result of male-male
conflict. Hermaphrodites appear to control the mating process both by struggling with males when they are not
receptive to them and by selfing in the presence of males. This suggests that hermaphrodites withhold receptivity
cues from males, or produce non-receptivity cues, when they are going to self. Though hermaphrodites do not
appear to select males based upon size, they may make a selection between selfing and outcrossing by controlling
the use of receptivity signals.
Introduction
Individual clam shrimp Eulimnadia texana (Crustacea:
Conchostraca) are either male or hermaphroditic. This
type of mating system is known as androdioecy, a
rare condition in animals (Sassaman & Weeks, 1993).
Hermaphrodites are capable of selfing their own eggs
or outcrossing with a male, but they cannot exchange
sperm with other hermaphrodites. Based upon what we
know about the reproductive biology of this species,
there is the potential that sexual selection is an important evolutionary force within this mating system. A
case could be made for hermaphrodite-hermaphrodite
competition (intrasexual selection) and male mate
choice (intersexual selection), as well as for male-male
competition (intrasexual selection) and hermaphrodite
mate choice (intersexual selection).
The case for hermaphrodite-hermaphrodite competition: The sex ratio within this species is highly
skewed in favor of the hermaphrodites (Sassaman,
1989; Knoll, 1994). This often results in competition among members of the common sex for members
of the rare sex (Emlen & Oring, 1977). Thus, one
might predict hermaphrodite-hermaphrodite (,femalefemale') competition for males.
The case for male mate choice: Behavioral observations suggest that males practice precopulatory mate
guarding of hermaphrodites within this species (Knoll,
1994). Mate choice by the males often occurs in systems where mate guarding is present (Parker, 1974;
Thompson & Manning, 1981; Elwood, et al., 1987).
Sexual selection theory predicts that the sex putting the
larger investment into reproduction will be the selective sex during mating (Trivers, 1972). This is typically
the female. When male investment in reproduction is
high, such as in systems where the male aides in offspring rearing or expends large amounts of energy in
spermatophore production, males have been known to