What are the hidden roles of parasites in reef communities?
Coral reefs are among the most biodiverse ecosystems on Earth, yet surprisingly little is known about the ecological roles of parasites within these environments. As a result, our knowledge about how parasite infections are changing through time – and the implications of such changes – is severely limited. In the Caribbean Sea, we are studying the ecology and emergence of parasites in coral reef ecosystems. By combining information from archival museum samples, infection surveys among reefs and across countries, and detailed assessments of host behavior and ecology, our group aims to evaluate the factors driving large-scale changes in infection as well as its consequences for reef communities.Ìý
The project
One of our main projects is focused on the ecology and emergence of parasites in the genus Scaphanocephalus, which cause Black Spot Syndrome in reef fishes. Beginning in 2011, scientists and members of the public began reporting large numbers of reef fish with dermopathies (skin lesions) in the Southern Caribbean. Surgeonfishes and parrotfishes – which are considered keystone species for their roles as grazers of algae that can threaten coral growth – were often the most afflicted, raising concerns about the effects for reef systems. In 2019, we linked this condition to infection by trematode parasites in the genus Scaphanocephalus (Kohl et al. 2019). While not previously known to infect fish in the Caribbean Sea, we detected infections in ~30 species and often at high infection loads around the islands of Curacao and Bonaire (Malawauw et al. 2024). The parasite has a multi-host life cycle involving three host species: marine mollusks serve as the likely first intermediate host, releasing free-swimming stages that infect the skin, fins, and muscles of fish that function as the second intermediate host. The definitive hosts are osprey, which become infected by consuming infected fish and then disperse the parasite, sometimes over long distances. The black spots may increase the fish’s conspicuousness to visual predators such as osprey, thereby enhancing the probability of predation (and transmission).Ìý
Evidence also suggests that infections by Scaphanocephalus in reef fish have increased by nearly 50-fold over the last century (Johnson et al. 2024). Examination of archival museum samples collected in Curacao and Bonaire (1905 to 1966) indicate that Scaphanocephalus was rare historically and, when present, at low intensity. In contrast, fish collected between 2017 and 2022 were 7x more likely to be infected and supported up to 1000 parasites. We are working to understand whether such emergence patterns are evident in other locations and identify the environmental factors that drive such changes.
One of the valuable features of this system is the opportunity for non-invasive surveillance. Because the number of spots (dermal lesions) per fish correlates with the number of encysted parasites, we can use imagery and video analysis to indirectly quantify infection. This facilitates surveys of infection patterns across far more fish, reefs, and locations than would otherwise be possible – all while minimizing impacts to the fish hosts. For instance, De Wit and Johnson (2024) used video transects to survey 34 reef sites across the leeward coast of Curacao, revealing both that BSS was widespread (>70% of over 5,000 inspected surgeonfish) and that its severity correlated with nutrient runoff, fishing pressure, and wave action. We are currently using community science imagery from the platform iNaturalist to survey species and sites across the Caribbean with the aim of identifying broad scale geographic patterns in infection.
Looking forward, one of the key questions focuses on the consequences of BSS for ecologically and economically important herbivores. High levels of Scaphanocephalus infection are associated with reduced body condition and increased oxidative stress, yet how such effects extend to the population and community scales remains unknown. Addressing this question takes on a heightened importance in light of evidence that Scaphanocephalus is emerging in the region.
Project publications
Johnson, P. T. J., Malawauw, R. J., Piaskowy, J., Calhoun, D. M., Kohl, Z., ter Horst, L. J., and D. A. Zelmer (2024). Emergence of black spot syndrome in Caribbean reefs: a century of fish collections reveal long-term increases in Scaphanocephalus ¾±²Ô´Ú±ð³¦³Ù¾±´Ç²Ô.ÌýProceedings B,Ìý291: 20242065. .Ìý
Locke, S. A.,ÌýCalhoun, D. M., Valencia Cruz, J. M., Ebbs, E. T., Diaz Pernett, S. C., Tkach, V. V., Kinsella, J. M., Freeman, M. A., Blanar, C. A., and P. T. J. Johnson (2024). Expanding on expansus: A new species of Scaphanocephalus from North America and the Caribbean based on molecular and morphological data. Parasitology, 1–51.Ìý.Ìý
De Wit, C. D. G. andÌýJohnson, P. T. J (2024). Black Spot Syndrome in ocean surgeonfish: using video-based surveillance to quantify disease severity and test environmental drivers. Marine Biology,Ìý171: 110.Ìý.Ìý
Malawauw, R. J., Piaskowy, J., Ter Horst, L. J., Calhoun, D. M., and P. T. J.,ÌýJohnson (2024).ÌýParasitism in reef fish communities: evaluating the roles of host traits, habitat use, and phylogeny on infection byÌýScaphanocephalusÌý(Trematoda).ÌýCoral Reefs,Ìý43: 509–522.Ìý.Ìý
Kohl, Z. F., Calhoun, D. M., Elmer, F., Peachey, R. B. J., Leslie, K. L., Tkach, V., Kinsella, J. M., and P. T. J. JohnsonÌý(2019). Black-spot syndrome in Caribbean fishes linked to trematode parasite infection (Scaphanocephalus expansus).ÌýCoral Reefs 38: 917-930. .Ìý
Elmer, F., Kohl, Z. F., Johnson, P. T. J., and R. B. J. Peachey (2019).ÌýBlack spot syndrome in reef fishes: using archival imagery and field surveys to characterize spatial and temporal distribution in the Caribbean. Coral Reefs 38:Ìý1303-1315. .Ìý