Written By Matt Tietbohl
Juvenile coral reef fishes are really important. Without them, there would not be any adult fishes on reefs for us to photograph, eat, and study! Juvenile fishes are interesting because they can live in separate places from their parents, and even look very different. Many species of coral reef fishes have unique juvenile color patterns drastically different from adults. This is especially true of parrotfishes. Parrotfish are a type of wrasse with a specialized beak-like jaw that is designed to remove chunks of the reef to feed on a diverse community of microbes and cyanobacteria living in and on the rocks (Nicholson & Clements, 2020). Juvenile parrotfishes are notoriously difficult to study because their coloration is often so different from their initial/terminal phase colors and because they often look similar to each other. This makes it difficult to use underwater visual surveys, the most common way to survey reef fish populations, to study their distribution and populations.
If we do not know how to tell species apart, it makes it harder to study them and how they use different habitats. To learn more about how parrotfishes use different habitats throughout their lives, Sievers et al. (2020) used a series of fish surveys and collections in multiple habitats in the Philippines. They conducted fish surveys on coral reefs, and potential juvenile habitats in seagrass, macroalgae, and lagoons, and collected juvenile parrotfishes (Figure 1). Because many of these juveniles look highly similar, they identified the juveniles they collected by genetic analysis, essentially using a DNA barcode to match them to their correct species. Then, the scientists could determine which species relied on multiple habitats throughout their lives, which they called multi-habitat use, compared to species only found on coral reefs throughout their whole lives.
Throughout their surveys they counted 19 different species of adult parrotfishes across all habitats. In their collections of juvenile parrotfishes from non-reef habitats, they found 10 species they recorded on their surveys, but 5 species they didn’t count at all as adults. This even included Scarus psittacus, which was one of the most abundant juveniles in their collections. The densities of different species varied between habitats, with some species only found in non-reef habitats, and vice versa as adults. The highest densities of fishes were actually for juveniles in non-reef nursery habitat (Figure 2.), though not all juveniles were found only in non-reef habitats. Some were only found in reef habitats, like S. niger.
These findings provide more clear evidence that parrotfishes are not only reliant on coral reef habitats, but need a variety of non-reef habitats as part of their life cycle. Totaling all species collected and counted, over 90% of all parrotfish species could be classified at multi-habitat use throughout their lives. Non-reef habitat for juveniles is especially important as this was where the greatest densities were found for the vast majority of species. Macroalgae and seagrass habitat may provide more food in the forms of small crustacea and epiphytes (things growing on plants) to supply juvenile parrotfishes with enough energy to grow quickly and reach a size where they can move into other reef or reef-adjacent habitats. The scientists also importantly documented that visual surveys did not count one of the most abundant juvenile species, and this is a good reminder that visual surveys may not always detect every species present. Parrotfish identification may be relatively easy with adults, though juvenile identification can be hard with a limited number of guides (Bellwood & Choat, 1990). The use of DNA barcoding greatly improves the accuracy of species identification and collections paired with surveys like this will be a strong way forward to better understanding how parrotfishes, and possibly other key species, use a variety of habitats throughout their lives.
Bellwood DR, Choat JH (1989). A description of the juvenile phase colour patterns of 24 parrotfish species (family Scaridae) from the Great Barrier Reef, Australia. Records of the Australian Museum, 41(1): 1–41.
Nicholson GM, Clements KD (2020). Resolving resource partitioning in parrotfishes (Scarini) using microhistology of feeding substrata. Coral Reefs, 39: 1313–1327.
Sievers KT, Abesamis RA, Bucol AA, Russ GR (2020). Unravelling seascape patterns of cryptic life stages: non-reef habitat use in juvenile parrotfishes. Diversity, 12(376): doi:10.3390/d12100376.