Sex On The Reef: The Way They Do It And How To Find It

Author: Alex Mustard

Marine biologist, emminent underwater photographer and Divequest group leader Alex Mustard takes an alternative view of life on the reef.

Whenever I visit the tropics, sex is something I am always on the lookout for! But do you find that you hear that there is loads of it about, but you never seem to get any? This article is here to help.

We are incredibly privileged to dive on coral reefs, for this is a wonderful environment for observing wild animals. The life on coral reefs is not only incredibly colourful and densely packed, it is also particularly accepting of our presence. Encounters can be unbelievably close - wildlife photographers in the Serengeti rarely get the chance to use macro lenses and fisheyes! As long as we respect the rules, we are generally ignored and can watch the animals of the reef going about their daily lives just a few feet away.

As a biologist, I see successful reproduction as the raison d'etre for all species, and witnessing the creation of the next generation of coral reef life as one of the greatest privileges of diving. It is also a poignant reminder of the vivacious vitality of life on reefs, as it struggles against the mounting pressure mankind exerts on this ecosystem.

It is easy to argue that coral reefs are the most diverse ecosystem on the planet. All the species of the animal kingdom are classified into 34 phyla, the major groupings of the different types of animals alive today. The number of different phyla present in a particular ecosystem is a useful measurement of the range of independently evolved animals that live there. In other words, the diversity of life. In tropical rain forests, probably the most diverse terrestrial ecosystem, all the animals that are found there belong to just 9 different phyla. While on coral reefs there are representatives from 32 phyla. And tallied with this great diversity of life is a wide variety in the ways that coral reef species reproduce.

You may think that corals are not the most exciting animals in the world, being basically living rocks, but even corals reproduce in a surprising number of ways. On their own corals reproduce through budding, fragmentation and some even by releasing asexually produced and brooded larvae. Sexual reproduction occurs in two ways. In some corals the maternal polyps absorb sperm from the surrounding water, that has been released by other members of their species, to fertilise their eggs. The most spectacular from a divers point of view is mass coral spawning. This is route the majority of corals follow, where the corals simultaneously release their sperm and eggs, and fertilization takes place in the water column. Coral colonies may be either male or female, or more usually hermaphrodites (both male and female). The ciliated planulae larvae drift and disperse as part of the plankton, for typically between 3 days and 3 weeks, before settling onto the reef and metamorphosing into a tiny polyp.

Nearly all the animals on the reef have this two-phase life cycle consisting of a dispersive planktonic larval phase and the reef based adult phase, that we are more familiar with. Adult population densities in many areas are controlled by larval supply, and many of the adaptations of coral reef animal reproduction are tailored to increase the chances for the larvae. As you might expect, in this hotbed of diversity almost every variation on this theme has been explored by evolution.

For my money, fish are the most astounding, reproducing in just about every way imaginable. The majority of coral reef fish are broadcast spawners, reproducing by releasing eggs and sperm into midwater, mainly as pairs, but a number of species spawn into the water column in small groups or even as dramatic aggregations of many thousands of individuals. For these fish parental care ends here. Other fish deposit their eggs onto the sea bed, where some defended them vigorously until they hatch, ensuring the young join the plankton with a bit more muscle. One species of damselfish (Acanthochromis polyacanthus) guards its eggs on the reef until they develop into young fish, thus missing out the larval planktonic phase completely. Other species brood eggs and young in their mouths (for example, jawfish and cardinalfish) or in pouches (seahorses).

Like corals, reef fish may be males or females, or both. A number of species are sequential hermaphrodites, changing sex during their development, and while it is more common to follow the pattern of being female first (such as wrasses, parrotfish, groupers and the fearsome triggerfish), some species do it the other way round (clownfish). There are also few species, such as the hamlets from the Caribbean, that are both male and female simultaneously, and can spawn as both a male and a female in the same session!

On almost any dive on a coral reef there is some sex going on, but how do we go about spotting it in this bustling marine metropolis? The first step is to know what is what on the reef. There is no need to be an expert taxonomist, but it is important to be able to distinguish the major groups of invertebrates and the major types of fish. Your ID skills should be able to distinguish a Chromis from a Chromodoris, but being able to recant the scientific binomial of every goby species is a bit excessive. Once you know what you are looking at, any unusual goings on stick out like a sore thumb. A bit of knowledge will help you look at the right time, in the right place, at the right thing!

But we are all busy people. What is much more use is if I pass on some of the short cuts I know for observing reproduction on the reef. One of the best bits of advice I have is to go to resorts that are keen on marine life, or to travel with groups with similar interests, or both. And you'll find lots of examples of these in the Divequest 2002 brochure. I have personal experience of one example, that is the Mandarinfish dive in the Lembeh Strait, which I enjoyed during Malcolm Hey's Sulawesi 2001 Expedition. The resort, KBR, organizes this dive around the wildlife not human priorities - the dive is conducted at dusk to catch the show. Dawn and dusk are great times to catch spawning fish, and the rule of thumb is to look out for fish laying onto the seabed at dawn and fish releasing eggs into the water column at dusk. You can help yourself further by choosing a holiday time that coincides with the spawning activity of fish or corals. Or go on a trip with an expedition leader who has expert knowledge of the area you are exploring, or of coral reef life in general. My other top tip is to talk to and benefit from the experience of dive guides, who will always have the best local knowledge.

As a visitor to coral reefs I am always keen to avoid disturbing the resident animals, leaving the reef as I found it. Being able to observe animals going about their everyday lives and witnessing their natural behaviour is as good a measure as any that our presence in their home is not an intrusion. I hope that on your next visit to a coral reef you get lots of sex, and enjoy the privilege of brief acceptance into the lives of coral reef animals.

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