Shelby Gantt wrote this post for Dr. Stacy Krueger-Hadfield’s Scientific Communication course at the University of Alabama at Birmingham. Long enamored with coral reef communities, Shelby completed a B.S. in Biology with a certificate in Marine Biology at the Georgia Institute of Technology and a M.S. in Marine Biology from the University of North Carolina at Wilmington where she studied sponge microbial communities and nutrient cycling on coral reefs. She is currently working towards her PhD in Dr. Dustin Kemp’s lab where she is exploring the relationships between climate change and coral microbial and algal symbiont associations. Shelby tweets at @GanttShelby.
When thinking about parasites, things like tapeworms or malaria probably come to mind. Brood parasites, such as birds (Davies, 2010), the Mochokid (Cuckoo) Catfish (Blažek et al., 2018; Sato, 1986), or even coral reef fish, probably do not come to mind.
Brood parasitism occurs when offspring are deposited into the care of non-biological parents that ultimately incur a cost from the association. Cuckoo birds are an infamous example of this since lay their eggs in the nests of other bird species.
The Cuckoo chicks are raised at high cost to the host parents in addition to the host’s own offspring, which are either out competed for food or are pushed out of the nest to their death by the Cuckoo chick. The former is very similar to your neighbors unexpectedly leaving their monster children with you for the next 18 years. This abandonment strategy allows the Cuckoo adult to avoid the costs of raising its own offspring, while providing the offspring the benefits of doting parentage. This kind of parasitism has yet to be observed in fish of coral reef ecosystems, probably because many of these fish release their gametes into the water column.
Tariel et al. (2019) investigated the occurrence of brood parasitism in coral reef fish by focusing on two species of damselfish (Althrichthys azurelineatus and Althrichthys curatus) known to not only maintain nests with offspring, but also to brood their offspring for a period of weeks. These damselfish species are part of a monophyletic group that all brood their young (Bernardi, 2011) and offer a prime opportunity to study whether brood parasitism has evolved along with brooding in coral reef systems.
The authors distinguished between conspecific or interspecific brood parasitism and/or alloparental care. Alloparental care occurs when an adult (or mated pair) takes care of offspring that aren’t biologically their own, while brood parasitism is defined as the host parent(s) taking care of unwanted foreign offspring from unrelated conspecific individuals (individuals of the same species) or interspecific individuals (individuals of a different species).
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In their study, Tariel et al. (2019) captured ~ 20 broods from each damselfish species. Then, they used the cytochrome oxidase subunit-I gene to identify the species of each individual captured and microsatellites to identify the relationships of all individuals from the same brood. Using these methods, the authors were able to determine that many of the broods had mixed parentage, with 8 out of 40 broods showing interspecific parentage and 30 out of 40 broods showing conspecific parentage. Of the interspecific foreign offspring, all were from one of two damselfish species (Amblyglyphidoden batunai and Pomacentrus smithi), both of which produce larvae that form in the open ocean.
The only A. batunai recruits that remained on the reef for an extended period of time were the individuals associating with damselfish broods. The authors theorize that these individuals evaded predation by associating with damselfish broods that are protected by parent fish.
When assessing conspecific parentage, there were no significant differences in which host broods had mixed heritage, indicating that both species had an equal chance of foreign conspecific offspring being present. Brood size did correlate with the number of foreign offspring. Additionally, there were a large number of singleton offspring, or offspring in the nest without siblings, and no half-siblings were found, demonstrating no promiscuity of the host parents. Therefore, larger broods with many foreign siblings seem to align with adult conspecifics laying eggs in the host nest, while singleton foreign offspring indicate possible post-hatching recruitment of conspecifics to the host brood.
These results are the first published case of probable brood parasitism among coral fishes and correlate with the findings of bird brood parasitism studies, though more work will need to be done to determine if the interactions are truly parasitic to the host parents.
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