Evolving in front of our eyes? Ecological and genetic evidence of divergent hawkfish

Written by Matthew Tietbohl

Coral reefs are full of a wide variety of incredibly beautiful fishes that come in a wide range of colors. There are brilliant purples, electric blues, vibrant yellows, and even creamy browns. The spectrum of colors you can find in coral reef fish is incredible and it not only varies between species, but also within species. Some species, like parrotfishes and other wrasses, undergo dramatic changes as they age throughout their life. Other species have different naturally occurring color morphs that you can find on the same reef! This is especially true for one group of coral reef fishes, known as hawkfish (Family Cirrhitidae). Hawkfishes are a common group of small to medium sized predatory fishes. They are most known for perching on the tops of corals where they keep a sharp eye out for prey, small fishes or invertebrates, to move within striking distance.  Three of the six species of hawkfish in the genus Paracirrhites have multiple color morphs, with the freckled hawkfish (Paracirrhites forsteri), actually having 4 separate color morphs (Figure 1). This level of variation in adult coloration is quite unusual for most fishes and points to potential differences in their ecology. 

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Figure 1. Representation of the different color morphs of the freckled hawkfish (Paracirrhites forsteri). These different morphs are also known as “banana split” (morph 1), “strawberry shortcake” (morph 2), “cherry red” (morph 3), and “crème brûlée” (morph 4). Figure modified from Coker et al. 2017. 

Different individuals of the same species with different colors are not likely to experience their environment  in the same way. Certain color combinations may make fishes more or less likely to be eaten by predators, but it can also make them better (or worse) at catching prey. Within these hawkfish for example, color morph 4 is darker and might actually be better able to hide within the branches of dead corals than other brighter morphs (Coker et al. 2017). These differences could impact how well these fish are able to grow and reproduce, and could affect their abundances on a reef. If different color morphs are found only in certain habitats and mate more within their own color morphs, this could eventually lead to speciation and one species evolving into four!

This is what a recent publication wanted to try and understand: do the different color morphs of freckled hawkfish matter? Dr. Michelle Gaither and colleagues ( Gaither et al. 2020) used a combination of field observations and surveys alongside more analytically complex molecular analysis to investigate if there are actually any differences between these morphs in the Red Sea. They found slight differences in the depth of habitat used between the hawkfish, with morph 2 (also called the “strawberry shortcake” morph) more often in deeper waters and while morphs 3 (“cherry red”) and 4 (“crème brûlée”) slightly preferred water around 8 m deep. They also looked at their ear bones, called otoliths, to compare differences in growth between species (Figure 2.). Overall, the differences in growth and development were greater than ecological differences, and there were no obvious observations of different hunting success or predation on hawkfish that could impact the fitness of different color morphs. However, when they looked at the genes, they found some curious results.

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Figure 2. Top panel shows differences in the length-age relationship between different color morphs. Middle panel displays differences in abundance of different color morphs at different depths on a reef. Bottom panel displays the results of a cluster and discriminate analysis of the genetic structure. The most likely population number was 4 (K = 4) for the different color morphs. Modified from Gaither et al. 2020 figures 1 and 4.

When using a common genetic barcode with mitochondrial DNA, there was no genetic difference between the different color morphs, which is what you would expect because you are looking at the same species. But when the authors used a more intensive DNA analysis they actually found small, but significant population structure in their genes. More specifically, the amount of a certain set of alleles (alleles are different variants of a gene) were unique between different color morphs (Figure 2). Because the authors did not find evidence of differing habitat use or any association between sex and color, it is likely these color morphs are kept stable from non-random mating or a combination of this with other selective processes. More research will be needed to tease apart the exact drivers of these genetic differences and look into which genes differ between morphs. These colorful freckled hawkfish are a great model system to study how species are able to maintain different color morphs in a population and certainly have a lot more to teach us!

References:

Coker DJ, Chaidez, V, Bermen ML (2017). Habitat use and spatial variability of hawkfishes with a focus on colour polymorphism in Paracirrhites forsteri. PloS One 12(1): e0169079. https://doi.org/10.1371/journal.pone.0169079 

Gaither MR, Coker DJ, Greaves S, Sarigol F, Payet SD, Chaidez V, Sinclair-Taylor TH, DiBattista JD, Berumen ML (2020). Does color matter? Molecular and ecological divergence in four sympatric color morphs of a coral reef fish. Ecology and Evolution. https://doi.org/10.1002/ece3.6566

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