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Small polyps do capture zooplankton
Written by Tim Wijgerde   

Marine scientists currently have reached a general consensus about how corals take up nutrients, which ranges from dissolved nutrients to megazooplankton, and have shown the importance of a variety of nutritive sources for coral growth and survival. A group of scleractinian corals however has remained controversial: the small polyped stony corals, commonly referred to as SPS corals. These animals are often wrongly believed to simply rely on strong lighting and dissolved inorganic nutrients such as ammonium, nitrate and phosphate. Here we present three videos of stony corals, commonly known as SPS corals, capturing zooplankton.

Since early explorers such as Charles Darwin started to study coral reefs in the 19th century, much has been learnt about the enigmatic creatures called corals. One of the most intriguing questions has always been how corals are able to survive in a seemingly empty ocean. The discovery of symbiotic algae, called zooxanthellae, in 1881 was a milestone for  our understanding of coral biology. In the 1970's and 1980's, scientists finally determined that reef building corals can rely entirely on their symbiotic algal partners for their daily energy intake. Without these nutritive algae, thriving in the coral's gastroderm, coral reefs as we know them today may never have existed.

From these findings, most aquarists have concluded that corals harbouring symbiotic algae do not require any supplemental plankton feeding. Although these corals may thrive in aquaria, they still highly benefit from additional plankton feeding (see our article How corals feed in the archive). This is because heterotrophy, the consumption of organic molecules through e.g. plankton, provides the required building blocks for animals to grow. Although zooxanthellae provide corals with ample amounts of energy by translocating carbohydrates and glycerol, additional food sources rich in nitrogen and other elements are crucial as well. Feeding stony corals with zooplankton has been shown to increase growth rates, and may also provide additional resilience against disease. Reproduction is stimulated as well, by allowing more energy to be invested in the production of gametes.

Today, most marine aquaria are well equipped with lighting fixtures but lack the technology to supply plankton or other fine particles. Relatively recent studies have shown however that the waters surrounding coral reefs are no underwater deserts at all. In fact, they are teeming with life such as bacteria, protozoa, phytoplankton and to a lesser extent, zooplankton. Although open ocean water has very low dissolved nutrients such as ammonia, nitrate and phosphate, it is sufficient to allow bacteria, protozoa and phytoplankton to grow. This is because many of these organisms are autotrophic, and are able to utilise the sun's energy to produce organic molecules. This forms the basis of the marine food chain: phytoplankton is consumed by zooplankton, which is subsequently devoured by larger organisms.   

All of this indicates that marine aquaria are chronically underfed in terms of plankton, which is reflected by the often poor biodiversity in these closed systems. Many highly interesting marine organisms such as sponges, tunicates, bivalves and corals that lack symbiotic algae usually do not survive for long in most marine aquaria.

Marine scientists currently have reached a general consensus about how corals take up nutrients, which ranges from dissolved nutrients to megazooplankton, and have shown the importance of a variety of nutritive sources for coral growth and survival. In addition, aquarists and hobbyists have provided detailed photographs and video footage of coral feeding behaviour in the aquarium. A group of scleractinian corals however has remained controversial: the small polyped stony corals, commonly referred to as SPS corals. These animals are often wrongly believed to simply rely on strong lighting and dissolved inorganic nutrients such as ammonium, nitrate and phosphate. Scientists have clearly shown that species from this group, such as Pocillopora damicornis and Stylophora pistillata, strongly benefit from plankton consumption. Actual footage of SPS corals consuming plankton has been sparse, however.

The three videos below clearly demonstrate zooplankton capture by two small polyped scleractinians: Seriatopora caliendrum and Stylophora pistillata. The last one is a highly magnified video of S. pistillata ingesting zooplankton. The take home message therefore still is: feed your corals!

Seriatopora caliendrum capturing Artemia salina nauplii. Nematocytes and mucus aid in capturing prey.

Stylophora pistillata capturing Artemia salina nauplii. Nematocytes and mucus aid in capturing prey.

Stylophora pistillata capturing and ingesting Artemia salina nauplii.