A single reef can be home for up to 3,000 different animal species. Those animals who are not able to move independently from main water motion and so drift along can be summarized as zooplankton. The Greek word "planktos" means wanderer or drifter. Although at the mercy of fast horizontal currents, these organisms are able to swim, avoid predators or to capture prey. Zooplankton include crustaceans like copepods or shrimps, as well as worms, medusae or even smaller animals only being seen under the microscope like single-celled protozoans. Crustaceans are the main group, making up about 70% of zooplankton species.
These countless tiny animals play a truly important role within the coral reef food web as they feed on algae (herbivorous) in the surface waters or prey on other smaller animals (carnivorous). Some of them even switch their diets. Their predators are reef fishes, larger crustaceans or filter feeders that are associated with the coral habitat like mussels or sessile worms and of course corals. Up to 17% of the total coral metabolic requirements may be covered by predation on zooplankton compared to about 71% from primary production by the symbiotic algae within the coral and 12% from absorption of dissolved organic matter present in the seawater.
More than 50% of reef zooplankton undergo marked vertical migration. At night they ascend to the surface water over the corals and descend in the morning to hide within the reef. For animals less than a millimeter in size, moving through seawater is very energy consuming. You could compare it to humans trying to move in molasses. So why do they migrate? This nocturnal movement allows them to utilize planktonic food sources and also to mate or migrate to new settling or feeding sites. Corals seem to adjust their feeding behavior as the majority expand their polyps at night to principally feed on zooplankton.
Vertical migration might also be a way to avoid predation. Demersal (in the water column) zooplankton show highly visible swimming movements that makes them easy prey for visual daytime predators such as planktivorous fish(link to Sergeant Major). It is also a way to escape from tactile benthic predators in the bottom of the reef, who are active at night.
The number of emerging zooplankton is related to the structural heterogeneity and three-dimensionality of the reef. Branching corals generate more demersal plankton than plating corals as animals live within the interstices of the reef by day. Some corals even show preferences for certain types of zooplankton. The diet of the coral Millepora complanata mainly consists of copepods, while Alcyonium siderium shows a strong positive selectivity for ascidian larvae. (Ascidians are meroplanktonic animals which means they only spend part of their life time, the larval stage, as plankton (The image is a larval copepod). As adults they settle down on the coral reef and they are even able to partially overgrow corals.) Predation on their larvae by corals may even alleviate competition by decreasing the competitor's recruitment. Other meroplankton species would be larvae of crabs, lobsters or mussels.
To support coral reefs with their enormously diverse and dense assemblages of organisms, input of nutrition from waters that overflow the reef is necessary. The open ocean is therefore a source of zooplankton that gets washed over the reef in currents and is stripped from the water by planktivorous fish and filter feeding organisms.
How do corals capture their prey? Cnidarians possess special cells called cnidocytes that are especially abundant in the tentacles. The interior of a cnidocyte cell is filled with a capsule containing a coiled pleated tube. When water rushes into this capsule the pressure within rises, a lid opens and the tube gets everted, turning the whole cell inside out. The end of the tube is pointed and bores its way into the tissues of the prey like a miniature harpoon. A toxin is sometimes injected that paralyzes the prey. The tentacles then pulls the captured prey towards the coral's mouth. The entire discharge process takes less than 3 milliseconds! Cnidocyte cells are only used once and get replaced within 48 hours after discharge. Other cnidocytes have a closed tube that may be unarmed and coiled or have a long spiny shaft. These types are used to wrap around small animals and entangle them. The discharge mechanism is caused by combined chemical as well as mechanical stimuli. The toxins used by most cnidarians are not perceptible for humans but some produce a painful burning sensation, irritation or even death.