Bumble-bees of the reef: fishes help to spread coral symbionts

Written by Matthew Tietbohl

Edited by Cassie Wilson

Corals are keystone species that create the structure of coral reefs. These cousins to anemones create a calcium carbonate (limestone-like) skeleton, as they grow. They live in partnership with microscopic, photosynthesizing, symbiotic algae, known as dinoflagellates, from the family Symbiodiniacea (LaJeunesse et al., 2018). Corals rely on their symbionts to provide them with key nutrients while corals protect these microscopic algae inside their tissues. There are two main ways corals can establish their relationship with Symbiodiniacea: either by receiving them from their parents or from the environment. Environmental pools of these symbionts can be at very low densities in seawater or sediments or growing on algae, but they can be important ways for corals to pick up different kinds of symbionts with different tolerances to environmental conditions. While environmental sources of these algae have been previously studied, a new study has found strong evidence for how fishes can play a role in the distribution of these symbiotic algae.

Working in Moorea in French Polynesia, Grupstra et al. (2021) looked into how different types of fishes may play a role in distributing symbionts over a reef. They looked at several species that actually feed on corals (corallivores) as part of their diet, including species that exclusively feed on corals (obligate – largely butterflyfishes), those that only occasionally feed on corals (facultative – largely parrotfishes) and several that feed on algae (surgeonfishes) as a control. Previous studies have shown that Symbiodiniacea can survive passage through the guts of some fishes (Castro-Sanguino & Sánchez, 2012); so, to find out if this was the case, Grupstra and co. looked through the feces from freshly collected fishes. They wanted to know if these fishes could help distribute Symbiodiniacea. 

What they found was astonishing! Live Symbiodiniacea could be found in all samples of obligate corallivores, over 80% of facultative corallivores, and even in over 30% of non-coral feeder’s feces. Obligate corallivores had 4-7 orders of magnitude more live symbionts in their feces than sediment or water samples, and facultative corallivores also had several orders of magnitude more live Symbiodiniacea (Figure 1). It appears these species have a great capacity to export live symbionts from their diet to other corals. The authors even tried to calculate the approximate transfer of symbiont cells from fish feces. By measuring rates of fecal discharge and combining this with the density of live symbionts standardized per volume of feces, the authors were able to estimate that millions of symbionts are potentially released by obligate corallivores alone, enriching the environment with Symbiodiniacea that could be used by coral species.

Figure 1. Cell densities of the coral symbionts in the family Symbiodiniacea from fecal samples collected from freshly killed fishes. Large points represent means with +/- standard error. The letters above points indicate statistically significant differences between points. Species abbreviations are as follows: obligate corallivores; CHRE Chaetodon reticulatus, CHLU C. lunulatus, CHOR C. ornatissimus, AMSC Amanses scopas, facultative corallivores; CHPE C. pelewensis, CHCI C. citrinellus, CHSP Chlorurus spilurus, grazer/detritivore: CTST Ctenochaetus striatus, CTFL Ctenochaetus flavicauda, SED sediment, WAT water. Figure modified from Figure 2 in Grupstra et al., 2021. 

This research highlights some important questions about how we think about corallivory on coral reefs. Typically, this is assumed to be a bad deal for corals, as feeding scars can make them vulnerable to disease. However, to some degree, coral feeding could be net beneficial to a coral reef as whole via this distribution mechanism of fish feces. By feeding on live corals and then distributing their symbionts across the reef as they “use the toilet”, fishes could expose corals to a wider pool of potential symbionts than they would be exposed to in the absence of fishes. After bleaching or other stressful events that kill corals, these fishes could even help distribute symbionts with adaptations to these stressors by feeding on surviving corals, potentially helping corals to recover faster. Much more work is needed in this field to understand this mechanism, how it operates on a reef scale, and if it is net positive or not for individual corals and the reef as a whole, but for now, we know much more about how important fishes can be for spreading coral symbionts around the reef!

Literature Cited:

Grupstra CGB, Rabbitt KM, Howe-Kerr LI, Correa AMS. 2021. Fish predation on corals promotes the dispersal of coral symbionts. Animal Microbiome, 3(25): 1-12. 

LaJeunesse TC, Parkinson JE, Gabrielson PW, Jeong HJ, Reimer JD, Voolstra CR, Santos SR. 2018 Systematic revision of Symbiodiniaceae highlights the antiquity and diversity of coral Endosymbionts. Current Biology, 28: 2570–80. 

Castro-Sanguino C, Sánchez JA. 2012. Dispersal of symbiodinium by the stoplight parrotfish Sparisoma viride. Biology Letters, 8(2): 282–6.

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