Dominance Cues Leading to Sex Change in Blue-Banded Gobies (Lythrypnus dalli)

by Daniel Hohler on Apr.15, 2010, under Writing

INTRODUCTION:
Blue-banded gobies (Lythrypnus dalli) are small hermaphroditic fish found in temperate Pacific waters. They have a stratified social hierarchy. Males are at the top of the hierarchy and dominate a harem of females. Beneath the males, dominance is obtained primarily by body mass (St. Mary 1997). Dominance has implications on a much larger evolutionary scale. Drawin defined sexual selection as “the advantage which certain individuals have over the same sex and species, in exclusive relation to reproduction” (Darwin 1871). Since Darwin’s time, the theory of sexual selection has been refined. For example, in fruit flies males compete for mates, but females can be more selective. This is because females can only have as many offspring as they can have pregnancies in their life time, where males reproductive success is more variable and only limited into to the number of females that they can get to mate with them (Bateman 1948). More recently sexual selection theory has grown to include interspecific competition (male-male or female-female), and intraspecific competition (male-female) in which females discriminate among males based on genes or resources (Parish 2006). However, the theory of sexual selection is complicated when dealing with hermaphrodites. Hermaphrodites go through many of the same selection pressures that other organisms do such as: bizarre and expensive courtship and copulatory behavior, multiple mating and sperm competition, rapid evolution, special structures associated with courtship, and sexual polymorphism (Leonard 2005).
Hermaphroditism in fishes takes on many forms. The most common form is when individuals change sex once in their lifetime. This is called sequential hermaphroditism. Body size is the primary function of sex change in species that undergo sequential hermaphroditism (Warner 1984, Ross 1990). On the opposite side of the spectrum are species that maintain both testes and ovary tissue throughout their lifetime. This type of hermaphroditism is called simultaneous hermaphroditism (Fischer & Petersen 1987). Other species display intermediate patterns of hermaphroditism that resemble both sequential and simultaneous hermaphroditism. Gobies in the genus Lythrypnus display this intermediate type of hermaphroditism (St. Mary 1996).