Small but mighty: How little fish are key for coral reef function

Written by Louise Anderson

A paper recently published in Science brings the smaller, often overlooked fish species on reefs to the forefront of our attention, and makes the case for their importance in overall ecosystem functioning.

Why? 

It has long been a paradox of coral reefs that they are these incredible oases of life and diversity in otherwise low-nutrient, low productivity ocean ‘deserts’. The intricacies of how exactly coral reef ecosystems exists in this context is not yet fully understood. One piece of this puzzle is the role of cryptobenthic reef fish. Cryptobenthics are the small (<50mm), short-lived fish that live on or in the reef itself (Fig. 1). We tend not to know much about these secretive, ‘cryptic’ species. They are tiny, often well camouflaged, tricky to identify and hidden within the reef, they may not even be picked up in observational reef surveys. This group includes families such as gobies, blennies and cardinalfish. As scientists we don’t really have a good handle on the roles that these fish perform in the ecosystem and how important they are, especially compared to the larger and more visible reef fish.  Much overlooked, these little fish nonetheless play a crucial role in how the reef works that we’re only just beginning to understand. 

A paper out recently by Simon Brandl and co-authors takes a look at the population dynamics of these fish, and how they compare to the bigger species that we already know more about. It focuses primarily on the early life stages of fish, where eggs and larvae float in the water as ichthyoplankton. Ichthyoplankton is a really important source of food on coral reefs, and this research helps us understand how energy moves around a reef ecosystem, and why coral reefs are so diverse and teeming with life in otherwise depauperate surroundings.

Figure 1. The whip-coral goby Bryaninops yongei is a cryptobenthic reef fish.Photo by Louise Anderson in Chuuk, FSM.

How?

The authors looked at the larvae, adult fish populations and employed theoretical models in a consortium of approaches to address this knowledge gap:

  1. A meta-analysis of the fish larvae present in plankton both near reefs and offshore, which is where data from other published studies is combined and analysed for patterns and trends. 
  2. Adult fish population surveys. Visual surveys are pretty good at getting estimates for large reef fish, but cryptobenthics are often missed. To ensure the cryptobenthics were actually represented in the data, fish were collected from an enclosed section of reef, using clove oil as an anaesthetic, at several sites spread across the world’s coral reef areas. 
  3. Demographic models of arrival, growth and death of fish larvae in the theoretical ichthyoplankton, drawing on the literature and field data, to assess the relative contribution of the cryptobenthics.

What did they find?

The researchers found that these cryptobenthic fish contribute two-thirds of the larvae near reefs (Fig.2). Because they grow quickly, have high mortality rates, and there are lots of them, they are a really important food source for other, larger species and account for a significant proportion of  energy transfer through reef food webs. As they are eaten and gone almost as quickly as they are produced, only an approach that looks at both larvae and the adult fish would have picked this up.

Cryptobenthic populations are also doing something very different to the other reef fish. They’re sticking around! All those larvae and eggs stay close to the reefs where they were produced. They have shorter dispersal distances than larger fish species, which tend to have life history stages where they hang around in the water column for longer, and drift between reefs. 

Figure 2. This figure is from the paper (Brandl et al., 2019). It shows that cryptobenthic fish make up most of the icthyoplankton near reefs, and has three key points. (A) Cryptobenthic larvae dominate the near-reef icthyoplankton, but the larger reef fish dominate offshore, (B) this near-reef domination is consistent worldwide and (C) the biggest three fish families that make up near-shore plankton are cryptobenthic. These are Gobiidae (gobies), Blenniiformes (blennies), and Apogonidae (cardinalfish).

What does this mean?

This work highlights how crucial the cryptobenthic fish are for maintaining a coral reef’s functioning, and quantifies some of this contribution. In doing so, we now better understand the way that energy moves around a reef ecosystem and how cryptobenthic fish populations are maintained. It also draws attention to an area of vulnerability: this feature of sticking close to a home reef makes cryptobenthic fish populations more at risk from localised threats such as a bleaching event or a crown of thorns starfish outbreak. This also makes them a useful indicator of ecosystem functioning, as declines in these species could be noted before the changes manifest more broadly in the reef fish population. Indicators like this are useful as they can provide an earlier opportunity for ecosystem managers and conservationists to act when a coral reef is in decline.

Citation:

To check out the original paper for yourself, see: 

Brandl, S. J., L. Tornabene, C. H. R. Goatley, J. M. Casey, R. A. Morais, I. M. Côté, C. C. Baldwin, V. Parravicini, N. M. D. Schiettekatte and D. R. Bellwood (2019). “Demographic dynamics of the smallest marine vertebrates fuel coral reef ecosystem functioning.” Science 364(6446): 1189.

https://science.sciencemag.org/content/364/6446/1189

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