Parenting on Coral Reefs

Dr Anjani Ganase looks at how the next generation is nurtured, on land and in the ocean

Parenting is not just something we do as humans; rather it is an evolutionary trait adopted by animals to care for their offspring during the nesting and juvenile stages of their lifecycle in order to improve the chances of survival through a successful transfer of genetic traits. Because parenting is energy intensive, parents can often only invest time and energy in a few offspring rather than hundreds or even thousands. There is a trade off in what a parent invests against the number offspring being born. Whales, for example, only produce one calf every couple of years. The mother will spend years nurturing the calf, teaching it how to hunt and survive. Conversely, marine turtles come ashore to lay hundreds of eggs, but they don’t stick around to guide the hatchlings to the water or show them how to navigate the open ocean. Their offspring rely heavily on their genetic codes to guide their survival.

While parental care comes in many forms. In mammals, the adaptation is the most extreme. Humans have evolved the ability to gestate and produce milk. For birds, the young are cared for in nests as eggs and then once hatched, chicks learn to fly before fending for themselves. Parenting also exists in insects and fish although it is less common. Insects such as wasps build nests for their young and then feed them, while male water bugs carry their eggs on their backs. This is the alternative strategy to long parental investments: to produce a high number of offspring so that hopefully a few will survive.

 

Yellowtail damselfish in Tobago. Photo by Jonathan Gomez

In the marine world, parenting is rare, but if there was one place to find examples of parenting, it would be on coral reefs which have high density and diversity of marine life. Most fish carry out broadcast spawning, releasing eggs and sperm into the water column for fertilization during spawning aggregations. The larvae are then taken by the currents that deposit them on neighbouring marine habitats. There are some species of fish that do care for their young. These include fish species that lay their eggs on the bottom surface and defend their eggs until they hatch and drift away. Other fish species keep their young for even longer, housing their offspring in the cavity of their mouths for protection against predation (aka mouthbrooding). Male seahorses famously house their young in their body cavity for protection.

While parenting is an investment of time and energy to ensure the survival of offspring, there are some organisms that take advantage of this and have others do the job of parenting. This strategy has been observed in birds, most famously the cuckoo that lays its egg in the nest of another species, the Common Redstarts that have an egg of a similar coloration. If the parenting bird does not recognise the egg as an imposter egg immediately, it will eventually raise in cuckoo chick as its own. Considering the cuckoo’s egg tends to hatch before the eggs of the Common Redstart and chicks are much larger in size, the bird may spend a lot of time and resources feeding the cuckoo, which may take away from food needed for their own chicks. The cuckoo chick may also be able to displace the other chick from their nests because of the larger size. This phenomenon is known as brooding parasitism, and is common in birds and freshwater fish species but has not been found in marine habitats such as coral reefs until recently.

A study published earlier this year seems to have discovered the first form of brooding parasitism on the coral reefs of the Philippines, among a group of Damselfish (genus Altrichthys) that are endemic to the area. While most reef fish disperse their eggs and sperm into the water column for broadcast spawning, a few lay fertilised eggs on the substrate, guarding and ventilating them until they hatch and disperse. However, the Altrichthys damselfish continues to care for the new fish juveniles for a few more weeks after hatching until they are big enough to evade predation on their own. In the study, genetic analysis on the offspring of two species of Altrichthys damselfish found that the offspring were always mixed with other species of damselfish that do not care for their young beyond the hatching phase. While it was difficult to speculate how the offspring became part of the host brood, scientists theorised that the foreign offspring may have been adopted after they hatched and joined the Altrichthys damselfish brood. They were being protected by the adult through this association. Scientists also found that these broods had a high number of foreign offspring of the same species but from different parents, typically up to about 20 % of the brood. Scientists suggest that the brood might have been infiltrated by spawning of a second mating pair in the same nest rather that the juveniles moving from another brood, since the threat of predation is very high on these reefs.

 

Three spot damselfish on Angel Reef, Tobago. Photo by Jonathan Gomez

The increase in the brood size, on one hand may disadvantage the adult Altrichthys damselfish as they expend more energy to protect the larger brood; but the additional fish may be a benefit as it dilutes the number of its own offspring potentially getting eaten. The act of brood parasitism by other species of fish also appears to be beneficial in an evolutionary sense, since the survival of both sets of offspring appears to increase by joining the protective brood. Coral reefs are underwater cities because of the restricted resources, space, light and it is not a surprise that these intense interactions, both good and nefarious, take place.

Every generation is responsible for the survival of the species. Every species has developed, and continuously evolves survival strategies. Strategies include prolific and broadcast regeneration, investment in nurturing, and surrogate parenting. All strategies have found their way into human lifestyles. Humans today, however, must secure their species’ survival by accepting responsibility not just for our own species, but for the survival of every other species, on land and in the ocean.

References

Tariel, Juliette, et al. "Alloparental care in the sea: Brood parasitism and adoption within and between two species of coral reef Altrichthys damselfish?." Molecular ecology 28.20 (2019): 4680-4691.

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