Fluorescence finds coral recruits

Fluorescence is proving to be a very useful tool for research on coral recruitment and survivorship. The challenge is that newly settled corals are about 1 millimeter in diameter. In the past it has been almost impossible to locate juvenile corals in the field until they are at least 5 mm in diameter. By the time corals can be found easily by conventional techniques they are between 6 months and a year old. This misses an important early part of their life history, and makes it difficult to estimate survivorship in natural conditions.

With fluorescence you can find juvenile or even newly settled corals that are 1 mm or smaller in diameter. Detecting small things is all about contrast, and fluorescence provides the contrast you need. If a coral fluoresces it will generally appear as a bright green against a dark background. We have found corals 5 mm in diameter from more than 2 meters away, and corals as small as 1 mm in diameter in routine sweeps of patches of reef.

White-light (left) and fluorescence (right) images of a coral recruit (glowing green) on a natural reef surface, Bonaire. The yellow arrow in the white-light shot points to the location of the specimen. There is NO way to have found this specimen in daylight or with a white light at night. The small scale lines are in millimeters, so this specimen is less than 1 mm diameter.

Two recent publications in Coral Reefs deal specifically with this application of fluorescence.

Piniak, G. A., N. D. Fogarty, C. M. Addison, and J. Kenworthy, 2005. Fluorescence census techniques for coral recruits. Coral Reefs, 24:496-500.

Baird, A. H ., A. Salih, and A. Trevor-Jones, 2005. Fluorescence census techniques for the early detection of coral recruits. Coral Reefs, 25:73-76.

Photograph of a small coral on a reef structure, Boca Chita Pass, Florida. Scale bar is in cm. Green polyp is approximately 2 mm diameter.

Answers to some common questions:

Can you find ALL recruits with fluorescence?

No. Not all coral recruits fluoresce, and some that do fluoresce do not glow brightly enough to be found easily. Keep in mind that not all adult corals fluoresce either, and even within a species there may be both fluorescent and non-fluorescent morphs.

How far away can you spot a 1 mm recruit?

From our own experience, we have certainly seen many, many small bright dots from a meter or more away without looking very hard.

If you do find a small (1 - 2 mm) fluorescing feature, is it definitely a coral?

No again. Just to keep life interesting, there are non-corals that fluoresce, like some anemones, corallimorphs, zoanthids, hydroids, etc. Even small mobile invertebrates such as some polychaetes fluoresce. It can be a challenge to know what is a coral and what is not.

How do you identify what you have found?

Identifying to species is a matter of experience and of the basic difficulty of the problem - recruits can be hard to identify when they are very small. In the field you at least want to be able to distinguish coral from non-coral. The NightSea Modular Underwater Magnifier can help with this.

Do you have to dive at night?

Not any more. NightSea has recently developed a specialized light for finding coral recruits in the daytime.