Author: Sancia van der Meij and Alan Powderham

When contemplating grazers on a coral reef, adorable little sheep probably do not leap into your imagination. Yet, there are exotic creatures that do exactly that: graze on sunlight. Some species of gill-less sea slugs, Costasiella kuroshimae, even look disarmingly similar to a tiny sheep.

These charming little slugs measure only about 5 millimetres and live on green Avrainvillea and Halimeda algae. Mind you, they are so tiny that they are far from being readily obvious. You need to know their environment and where to look. Their nickname, ‘leaf sheep’, attributes to its dark eyes and two rhinophores that stick out from the top of its head, resembling sheep ears. The leaf-like protrusions on its back, called cerata, aid in breathing. As noted in chapter 7 of Coral Triangle Cameos, these tiny exotica are also fondly known as ‘Shaun the Sheep’, after the popular animated character.

What truly makes these species of marine slugs special is their ability to perform kleptoplasty — a process in which an animal stores chloroplasts from plant cells and reuses them to derive energy from photosynthesis. In the bright, clear waters of coral reefs, these chloroplasts remain active and can maintain this process of producing energy through photosynthesis for weeks—one of nature’s smartest energy hacks.

On dark winter days, it’s lovely to daydream of adventures on colorful coral reefs. The recently published Coral Triangle Cameos: Biodiversity and the Small Majority, presents a photographic smorgasbord of the smallest fish and invertebrates of this amazingly diverse region. These tiny residents not only contribute enormously to the rich variation of the marine life, but their inter-relationships also offer fascinating insights to the creative expressions of evolution.

A chillingly beautiful example is the colonial sea anemone Nemanthus annamensis, known by the suggestive English name “gorgonian wrapper.” This anemone forms colonies around the branches of black corals and gorgonian sea fans. Unfortunately, it is a one-sided love affair: the coral is systematically overgrown and slowly dies under the anemone’s suffocating embrace. However, for the anemone, this parasitic form of symbiosis offers many benefits. Within the branches of the doomed corals, the anemone establishes a prime location on the reef to feed in plankton-rich currents – thus usurping the precious domain of the corals.

But this enforced diversity of species doesn’t end there. The anemone often hosts the symbiotic shrimp Izucaris masudai, but which is hardly noticed because it has adopted the exact same colours and patterns as its host.