Chalk Bass Cooperate When Mating

By on May 18, 2016
Chalk bass pair getting ready to spawn on a reef near Bocas Research Station, part of the Smithsonian Tropical Research Institute in Panama. Chalk bass spawn in the pelagic column with individuals taking turns releasing eggs for their partner to fertilize. (Credit: Mary Hart / University of Florida)

There’s something to be said about the power of working together. Mankind has arguably used our potential to band together to achieve numerous advances throughout history. But for a certain type of fish, the chalk bass, working together with others of its kind helps to make sure everyone gets to mate.

Part of why this is possible is because chalk bass have both male and female reproductive tissues. This would probably be a problem for other types of fish that tend to compete with one another for mates, but it’s no issue for chalk bass. They take turns fertilizing their partner’s eggs.

Scientists at the University of Florida have assessed the mating patterns of the fish in a work that is published in the journal Behavioral Ecology.

Chalk bass go up into the water column and spawn once with one individual as female and the other individual as male. They’ll come back down to the substrate, do some courtship behaviors and then switch gender roles and do it again, researchers say.

The fish repeat these spawning bouts over and over in a reproductive tactic called egg parceling. In return for letting their partner fertilize a parcel of their eggs, they receive a parcel of eggs from their partner to fertilize.

Egg parceling is a classic example of reciprocity between unrelated individuals. Both partners benefit – by getting their own eggs fertilized, and by fertilizing a partner’s eggs – but the benefits depend on partners trading equal numbers of eggs.

The situation involves the risk of exploitation, meaning one individual fertilizes its partners’ eggs but holds back its own eggs. This is less likely if the same individuals have many interactions together and reciprocate cooperative actions, so long-term relationships may be important for egg parceling.

U. of Florida scientists studied the fish over six months, looking at whether chalk bass mating involved long-term partner fidelity, reciprocation in the number of egg parcels exchanged and matching in the number of eggs produced in a single day by fish within pairs.

Chalk bass. (Credit: Williams, J. T.; Carpenter, K. E.; Van Tassell, J. L.; Hoetjes, P.; Toller, W.; Etnoyer, P.; Smith, M. via Creative Commons 2.5)

Chalk bass. (Credit: Williams, J. T.; Carpenter, K. E.; Van Tassell, J. L.; Hoetjes, P.; Toller, W.; Etnoyer, P.; Smith, M. via Creative Commons 2.5)

For the duration of the study, all chalk bass pairs remained together with their original partners until one or both of them disappeared from the study site. This means that chalk bass pairs stay together for much, if not all, of their adult lives.

Researchers were expecting more pairs to break up over the study period, especially if partners did not take turns releasing eggs. While the researchers did not observe any breakups, they did see that the fish matched reproductive investment, meaning pairs tended to be of similar size and fertility.

Scientists found that both daily egg production and the number of egg parcels exchanged within pairs were coordinated: Pairs tended to alternate gender roles and match the number of egg parcels exchanged between partners.

Even in one case where a pair of chalk bass did not appear to be taking turns over a single spawning period, the pair remained together. The fish appear to stay together for long periods of time, so researchers guess it may not matter as much if one partner doesn’t reciprocate one day. It may have also been that the fish were in a new relationship and hadn’t begun mating yet.

About 20 percent of the fish studied exchanged a few egg parcels with fish that were not their long-term partners. The team also observed some streaking, a behavior in which a fish rushes up to a spawning pair and releases its sperm. Egg parceling, extra-pair mating, and streaking may all be used as reproductive tactics by chalk bass to maximize their spawning success.

It’s thought that egg parceling helps maintain the stability of simultaneous hermaphroditism. It keeps individuals from being exploited.

Researchers note a novel finding from their study that the fish live in stable sites at high density. It’s one of the first examples where there are large social groups in which alternative partners are present, yet chalk bass still have strong fidelity to their partners. It suggests that matching reproductive investment can help long-term cooperation among non-related animals, even when there are opportunities to mate with other fish.

In the future, scientists would like to learn more about the dynamics of chalk bass mating, including how matching is maintained, or if there are coercive behaviors that go on during the partnerships. They would also like to learn more about the costs to chalk bass of losing a partner as well as how they treat fish that don’t reciprocate or have less fertility.

Top image: Chalk bass pair getting ready to spawn on a reef near Bocas Research Station, part of the Smithsonian Tropical Research Institute in Panama. Chalk bass spawn in the pelagic column with individuals taking turns releasing eggs for their partner to fertilize. (Credit: Mary Hart / University of Florida)

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