Written By Carla Elliff
Coral reproduction is full of surprising aspects. For example, many species of stony corals are hermaphrodites, producing both eggs and sperm, that are combined in tiny gamete bundles. However, as sessile creatures, corals can’t move around to exchange gametes in order to guarantee successful reproduction. No, they have to get creative.
Most of these hermaphrodite species are what is known as broadcast spawners, meaning they release their gamete bundles in massive numbers into the water reaching other colonies far away on the reef. This also helps to guarantee genetic diversity to their offspring – and diversity is a great thing! If this wasn’t an impressive enough strategy, corals are able to synchronize with each other to make sure the whole reef is ready for reproduction. They wait for a particular moment in the lunar cycle and then release their eggs and sperm all together over the course of a few magical nights! These spawning events are among nature’s most amazing spectacles.
The connection to the lunar cycle is still unclear, but researchers hypothesize that it could be related to tide conditions. Another peculiarity is that while most species in the Pacific Ocean and the Caribbean spawn during the full moon, corals from Brazil wait for the new moon to get in sync.
Brazilian corals have gained quite the reputation for being sturdy and rather unique. However, the reproductive aspects of the region’s (mostly endemic) coral fauna are largely unknown. With climate change bringing a concerning scenario to the health of reefs worldwide, it becomes increasingly important to get a grasp on what corals are experiencing under these circumstances.
The Coral Vivo Research Network has been working on understanding precisely the fundamental aspects of coral reproduction in the Southwestern Atlantic since 2008, along with many more intriguing subjects in reef science.
Co-founder of Coral Vivo, Professor Débora Pires, explains that the institution was born in the National Museum of Brazil, which is affiliated to the Federal University of Rio de Janeiro, in 2003. In 2008 the project became a network of collaborators and partners, expanding today to 14 different universities. Coral Vivo offers field and laboratory support to researchers from several fields.
Pires is also a co-author in a recent paper that brings some surprising new evidence on coral reproduction. The study was led by Professor Leandro Godoy from the Federal University of Rio Grande do Sul, who is an associate researcher in the Coral Vivo network and the coordinator of the Reefbank Project.
“The main reef-building species in the South Atlantic Ocean are considered more resilient and tolerant to stress than corals from other parts of the world” explains Godoy. “However, the impacts of bleaching on the reproduction of these broadcast spawners were unknown.”
Were. Past tense.
Bleaching is a serious threat to the survival of corals. When ocean temperatures rise, the relationship between a coral and its symbiotic algae is disrupted, eventually leading to the expulsion of the algae and even death of the coral. Even if the coral does not die from being bleached, life is very stressful during this period.
“Gametogenesis, especially the formation of eggs, requires a lot of energy, so the availability of food has a central role before and after this process. After an intense bleaching event, with low or almost no energy being provided through autotrophy, it would be expected that reproduction did not occur,” Godoy ponders.
However, the study showed that reef‑building corals of the Mussismilia genus were able to spawn while fully bleached. Specimens of M. hispida and M. harttii were collected from the Recife de Fora Marine Park, located in northeastern Brazil, and taken to the Coral Vivo Research Base for analyses.
Another co-author of the study, Professor Miguel Mies from the University of São Paulo and scientific coordinator for Coral Vivo explains about the importance of this particular site. “The location of the research base was chosen on purpose. The Recife de Fora coral reef is easy to access and holds almost all the biodiversity of coral species of Brazil. The idea was to use this site as a model reef for studies.”
Not only were the fully bleached corals able to produce their gamete bundles, but the eggs and sperm within these bundles were in good conditions and formed viable larvae. In fact, when compared to healthy corals collected from the same site, parameters such as spermatozoa concentration and motility, egg size, and fertilization were only slightly lower in quality, statistically speaking.
Studies in other locations across the globe have observed corals successfully spawning after recovery from bleaching, but not spawning while fully affected by the condition.
Mies indicates that an important factor to weigh in is heterotrophy. It is likely that the species assessed compensated their energetic and physiological needs this way in the absence of their symbiotic algae. This is a typical characteristic of massive coral species. “These coral species eat a lot.”
This unprecedented finding sheds light on the consequences of bleaching and thermal stress on coral reproduction. It also further supports the idea that corals from the Southwestern Atlantic show particularly resilient traits.
It is important to highlight though that being more resilient does not mean that these corals require less protection or that they are immune to well-known threats such as climate change and chronic human pressure. Unfortunately, at the rate reefs are declining, it is a matter of time until they are pushed over their limits. These thresholds remain unknown, but undoubtedly nearer.
Massive bleaching events are not likely to stop happening, if goals like the Aichi targets, which included minimization of human pressures on coral reefs, are not met.
However, with increasing data about the ecology of corals in a changing environment, science-based decisions can be made regarding coral reef conservation. Coral reef scientists have never before had the opportunity to be so connected and to share knowledge so freely. Initiatives in the science-policy interface and science communication are also high on the global environmental agenda, bringing fresh perspectives and broader audiences.
These concerns are at the core of Coral Vivo’s mission in marine conservation. “Conservation needs the involvement of society. Effective conservation will only be successful if this is obtained,” emphasizes Pires.
With the UN’s Decade of Ocean Science for Sustainable Development only just beginning, why not make the most of this inviting scenario?
To learn more about the work carried out by Coral Vivo, check out: