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Book Review

Stuck in the middle with you: The trophic ecology of Caribbean reef sharks and large teleost coral reef predators

Bond, M.E., J. Valentin-Albanese, E.A. Babcock, N.E. Hussey, M.R. Heithaus, and D.D. Chapman. 2018. The trophic ecology of Caribbean reef sharks (Carcharhinus perezi) relative to other large teleost predators on an isolated coral atoll. Marine Biology 165:67 https://doi.org/10.1007/s00227-018-3322-2

We often think of sharks as the top of the ocean food web, chowing down on seals and big fish to their heart’s content. That is often not the case! Where does the Caribbean reef shark fall in this hierarchy? Let’s find out.

Background

A healthy coral reef can be home to an abundance of fishes from a diversity of species. With this much food around many large predators, including sharks, groupers, and barracuda can live on the same reef. Often in this food web, sharks occupy the top position as apex predators, with a trophic level greater than four, while the groupers and barracuda are considered mesopredators, feeding in the middle of the food web with a trophic level around three. Models of food webs and trophic interactions on Caribbean reefs have shown that Caribbean reef sharks (Carcharhinus perezi), as the apex predator, have strong regulating control over populations of lower trophic level species. If this is true, the loss of Caribbean reef sharks could have catastrophic effects on the balance and biodiversity of coral reef ecosystems.

Caribbean reef sharks can be found from North Carolina, USA through the Caribbean to Brazil, but are uncommon north of the Florida Keys. They grow much larger than their Indo-Pacific relatives and, therefore, could occupy the role of top predator on a reef, feeding mainly on large predators like barracuda and Nassau grouper. However, they could also feed more like the smaller Indo-Pacific reef sharks, eating a variety of fish, both carnivorous and herbivorous, and functioning as mesopredators. In reality, we know very little about what Caribbean reef sharks eat and how they fit in among large reef visitors, like tiger sharks, and reef mesopredators. The assumption that they are apex predators could be invalid, but there is no way to know that without collecting the data; so researchers set out to better quantify the Caribbean reef food web.

A Carribbean reef shark navigates a reef, searching for food (Wiki Creative Commons).

Research Questions

There were three questions researchers wanted to answer:

  1. Are adult Caribbean reef sharks apex predators on coral reefs?
  2. Are young Caribbean reef sharks mesopredators?
  3. Does a shift in foraging location coincide with the shift from mesopredator to apex predator?

Methods

Dr. Mark Bond and his team used stable isotope analysis to place Caribbean reef sharks within the coral reef food web. Stable isotope analysis is a non-lethal method of studying diet and works by comparing the ratios of nitrogen and carbon in specific body tissues. Nitrogen and carbon are elements that are found in everything that we eat so scientists can trace them through the food web from plants, to primary consumers, mesopredators, and apex predators. In general the nitrogen stable isotope ratio increases stepwise as you move higher up in the food web. This allows researchers to assign trophic levels to species. The carbon ratios generally stay the same regardless of trophic position, which allows researchers to determine where an organism is feeding based on the carbon signature of the primary consumers in the area. Check out this article from PBS for a deeper dive into isotope analysis across scientific disciplines.

To determine what trophic level Caribbean reef sharks occupy, researchers sampled white muscle from 86 individuals at Glover’s Reef Atoll, a barrier reef in Belize. They then compared the nitrogen and carbon ratios from the reef sharks to samples from three mesopredators: Nassau grouper, black grouper, and great barracuda. They also collected muscle tissue from multiple lower trophic level species in specific areas of the reef: ocean facing forereef, lagoon patch reef, and the lagoon’s seagrass meadow. The carbon ratios from these fish were used to determine where the reef sharks were foraging and if that location changes with age. In total, 262 samples were taken. 

Results

Remember, the researcher’s first questions were are adult Caribbean reef sharks apex predators on the reef and are juveniles mesopredators. When they compared the nitrogen ratios for adult Caribbean reef sharks to the mesopredators with ANOVA statistical tests, they found that nitrogen values for reef sharks were significantly lower than those for barracuda and black grouper and not significantly different from Nassau grouper. Similarly, juveniles had significantly lower nitrogen values than barracuda and were not significantly different from the two grouper species. Caribbean reef shark nitrogen values also did not change significantly with size. They found that there was significant dietary overlap among the four species and all were assigned a trophic level of approximately 3.1 to 3.5.

The relative nitrogen and carbon ratios plotted for Caribbean reef sharks (all individuals Zii orange, adults Zii_A red, juveniles Zii_J yellow), great barracuda (Sba green), black grouper (Mbo purple) and Nassau grouper (Est pink). This graph shows significant overlap in nitrogen values indicating that all species feed at a similar trophic level. (Bond et al. 2018). The carbon values on the x-axis are color coded based on carbon source with dark blue representing open ocean sources, light blue as patch reef, and green as seagrass (Bond et al. 2018, Figure 2).

So now we know that Caribbean reef sharks are feeding firmly as mesopredators, below the apex predator trophic level, and this does not change throughout the shark’s lifetime. But does where they are foraging shift as they grow older? When comparing carbon values, they found that there was no significant difference between the four mesopredator species, but juvenile reef sharks had lower values than black grouper and barracuda. In general, carbon values became more enriched in larger sharks. Juveniles appear to spend similar proportions of time feeding in the forereef, lagoon patch reef, and lagoon seagrass habitats, but adults focus the vast majority of their feeding in the seagrass lagoon, which caused the enriched carbon values.

What’s it all mean?

The trophic position of Caribbean reef sharks on coral reefs has previously been overestimated, making the dynamics of the reef more complicated than scientists once thought. Instead of feeding on large mesopredators, like barracuda and grouper, reef sharks are likely competing with these fish for resources throughout their life cycle. The lower trophic position means that Caribbean reef sharks are feeding not just on carnivorous fish species, but also herbivorous fish. They are dietary generalists, foraging in a variety of habitats across the reef ecosystem. It is likely that larger, more mobile shark species like tiger sharks and great hammerhead sharks occupy the apex predator position on Caribbean reefs.

Just because Caribbean reef sharks are feeding at lower trophic level does not make them unimportant to the reef ecosystem. Because they forage in multiple habitats across the reef, they are likely important for nutrient transfer throughout the reef ecosystem. Though they may not exert strong top down predatory pressure on specific species like an apex predator would, their presence on the reef is likely felt across multiple trophic levels, potentially keeping populations of many different species in balance on the reef. Generally as a predator gets larger it eats larger prey which occupy higher trophic levels, but the complexity and diversity of coral reefs allow for the existence of quite large fish that eat mainly algae and coral. The presence of these species is likely what keeps adult Caribbean reef sharks feeding at a lower than expected trophic level. The Caribbean reef shark is relatively understudied, but clearly an integral player in the complex coral reef food web. The authors note that we will need more research into individual foraging behavior and habitats to better understand their role on the reef.

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