Fish Sex

Everything you wanted to know, but were afraid to ask

Sexual systems

• Gonorchism- most common, each sex remains same throughout its life.

• Simultaneous hermaphroditism– Each fish is both male and female at the same

time. Each has both ovaries and testes.

• Sequential hermaphroditism– Fish begin life as one sex and may end as the

other.

Simultaneous hermaphroditism

• Fish trade eggs.

• Two come together and one acts as a female and courts the other.

• They clasp and “female” releases eggs while the second fish releases sperm.

• Now get role reversal.

• Continue 6-18 times (don’t shed all eggs at once). Prevents “cheaters”.

Sequential hermaphroditism

• Protandry – Begin life as male and later become female.– Saw clown fish in movie – Largest is female. Keeps others male by

aggression.

• Size advantage model for protandry – Female success determined by size. Bigger

female makes more eggs. Small male still produces sperm. Stay male until big enough to fight others and have lots of eggs.

Sequential hermaphroditism

• Protogyny. Begin life as female and later become male.

• Bluehead wrasse– Secondary male (bluehead) defends mating territory.

• Size advantage model– The larger the female the greater the reproductive

success. So begin life as female. – When large enough to defend territory, increase

success by switch.

Monogamy and Promiscuity

• Monogamy - Butterfly fishes. Mate for life (but beware those oversexed females)

• Promiscuity. Mass spawning of males and females.

Polygamy (harem)• One male to many females. Male defends

his harem. Example: rock beauty

Polygamy (Harem)

• Each day Rock beauty spawns with all his females for up to 6 months.

• This fish has one male to 7-8 females.

Polygamy (lek)

• Male mates with many females, but provides them only with sperm.

• Male has spawning site and female chooses male based on his site. Any male at the best site works.

• Female gets only sperm. No defense, no territory.

Polygamy (lek). Strategy

• Each male will spawn 0-80 times.

• Males partition sperm. Better male is, fewer sperm he releases each time, but number released is proportional to female size.

• Best strategy for smaller female is to go to lesser male.

Blue Headed Wrasse (Thalassoma bifasciatum)

• Pelagic spawners at down-current promontories along reef.

• Hypothetsize that these sites are chosen because spawn will be washed out to sea and egg predators are fewer.

• Evidence that site, not occupier, is selected by females.

Phases

• Initial phase is yellow female.• Terminal phase is bluehead. Most

blueheads started as females.• Some yellow fishes are primary males.

Strategies

• Terminal males always defend spawning territory.

• Can swim for 4 hours to reach

• Spawn with a succession of females from 1-5 PM. Estimate they spawn from 25-40 times a day.

• Yellow males are sneakers.• Covertly spawn with a female or travel in gangs, hoping

to catch a female outside the sight of a secondary male.• Often gather immediately outside the spawning sites of

secondary males, hoping to sneak access to females.• Look like females so race in and spawn while females

are going up to mate with blueheads. – Good for female because 1-2% of bluehead mass is gonad and

10% of yellow male is gonad.– Nevertheless system favors bluehead.

• When bluehead removed, largest fish, usually female, becomes bluehead and replaces him = socially controlled sex change.

Breeding vs. Habitat

• System differs depending on size of reef.• So many wrasses on large reefs that yellow

males very successful.• Primary males distracted and can’t constantly

defend females from “sneakers”.• Production of yellow males increases and can

reach 50%, while blueheads can be as low as 19%.

• On small-medium reefs, there are fewer wrasses. Yellow males have very little success and fewer are produced.