Over the last few years, azooxanthellate corals have become very popular amongst scientists and hobbyists. New insights into their diet show that proper nutrition is key to the successful husbandry of these unique species.

When we think of corals, must of us image a zooxanthellate coral.  That is, a species harbouring symbiotic algae. Between depths of 0 to 15 m (50 ft), most reefs are dominated by these so-called hermatypic (or reef-building) corals. Up to 95% of their daily required energy is produced by their symbiotic algae^1,2,3.

Corals, in essence, are heterotrophic organisms, e.g. they require external organic compounds to survive. Although these corals receive a lot of photosynthates from their partners, they need more. Next to carbon, they require nitrogen, sulphur, phosphorous and many trace elements to grow. This is why corals capture all kinds of food, ranging from small molecules such as amino acids, different types of plankton, up to small shrimp and fish, depending on polyp size. 

Azooxanthellate corals

Evolution has generated numerous coral species which can be placed all over the heterotrophic-to- autotrophic spectrum. A significant proportion of coral species lacks symbiotic algae, and are called azooxanthellate species. They rely solely on plankton (phyto-, zoo and bacterio), detritus (POM: particulate organic matter) and dissolved nutrients (DOM: dissolved organic matter, DIM: dissolved inorganic matter).

A nice example is the genus Dendronephthya (family Nephteidae), a group of attractive soft corals which are becoming more and more popular (fig.1), despite them being incompatible with modern reef aquaria.

Figure 1: A Dendronephthya hemprichi specimen. Notice the sclerites, clearly visible as red spikes (courtesy of Charles Delbeek).

Dendronephthya hemprichi

Dendronephthya hemprichi is a species which frequently occurs in the Red Sea, at significant depths with stroming laminar flows. Recently it was found that D. hemprichi is able to grow quite fast; 26 cm (10 inches) large specimens were found to grow in couple of months⁴. This is an impressive feat for a species lacking symbiotic algae. This implies that these corals ingest large amounts of plankton or detritus captured from the water column.

Fabricius and coworkers found that these corals are highly dependent on both ample nutrition and water flow. They found that this species heavily feeds on phytoplankton. Algae contains lots of chlorophyll, a pigment which is used to capture the sun's energy (photosynthesis). By using fluorescence microscopy, the amount of chlorophyll trapped by the coral polyps could be determined (which is of course a measure for the amount of consumed algae).

Figure 2:  left: Chlorophyll A concentration in D. hemprichi polyps after exposure of 3 days at different flow rates. A flow rate of 17,5 cm/s seems optimal for particle ingestion. right: Growth increase of 704 D. hemprichi colonies over 30 days, expressed in percentages (modified from Fabricius et al, 1995).  

They conducted in situ (in the natural environment) experiments, placing flow pumps in the ocean to manipulate water movement. A flow rate of 17,5 cm/s was found to be optimal for D. hemprichi colonies to consume algae. At 7,5 cm/s, this consumption was reduced to about 50% compared to 17,5 cm/s (fig.2). Furthermore, they found that polyp tissue mass decreased at flow rates of 1-3 cm/s, but increased at rates of 14 cm/s and higher. Next to this, it was found that between 12,5 and 22,5 cm/s increase in polyp number was highest (fig.2).  

Several findings indicate that these corals predominantly feed on phytoplankton. They display so-called pinnula; feathery structures which greatly increase the surface area of a polyp’s tentacles, and act as a fine filter mesh (a nice example is Xenia umbellata). The pinnula of D. hemprichi tentacles are interspaced at 60-80 µm (micrometer, 1000^th of a millimeter). This allows them to filter extremely small particles from the water, including all kinds of algae. Furthermore, soft corals from the genus Alcyonium were found to possess plant-digesting enzymes such as amylase and laminarinase, in contrast to scleractinians (stony corals)⁵. Next to this, soft corals possess only small and ineffective nematocytes, unlike many scleractinians⁴. D. hemprichi was found not to be able to effectively stun and ingest zooplankton; after an average time of 50 seconds prey escaped^4,6. Even after capturing the plankton three times over it escaped.

All these findings imply that at least a subset of soft corals, including the genus Dendronephthya, is not adapted to capturing zooplankton. This insight is crucial if future attempts to aquaculture these species are to be successful.

Corallium rubrum

The Mediterranean is home to several coral species, such as Corallium rubrum (fig.3). This species  builds an aragonite skeleton, despite being an Octocoral.  It is also azooxanthellate, thriving in deep waters. It is well-known for its usage as an ornamental in the tourist industrry. It feeds on detritus (POM) and all kinds of plankton. Similar to D. hemprichi, scientists determined the effects of flow rate on particle ingestion of C. rubrum polyps. 

Figure 3: Corallium rubrum specimens in respiration chambers for scientific purposes (photograph: Anna Paijmans). 

No clear effect of flow rate on particle ingestion could be discerned; at 2, 6 and 11 cm/s, C. rubrum polyps ingested equal amounts⁷. This species was found to predominantly feed on detritus, or POM. This equaled to about 93% (or roughly 3 µg C/polyp/day) of the total amount of C (carbon) ingested⁸, which is also seen with other Mediterranean species such as Paramuricea clavata and Leptogorgia sarmentosa⁹.  A second major type of food intake were bacteria; which equaled to 148 ng (nanogram; 1 billionth of a gram) of C/polyp/day, and 28 ng of N (nitrogen)/polyp/day. Altough bacteria only provide 4,5% of the total carbon budget, scientists found that they are the main source of nitrogen and trace elements. Phyto- and zooplankton only yielded about 2% of the diet.

According  to a 20-year study, growth of C. rubrum is dramatically low; 1,78 mm (0.07 inches)/year on average¹¹. This forms a striking contrast with the growth of D. hemprichi. This may be due to the fact that these specimens grew at sites where plankton was low in concentration. The limited availability of plankton forces these animals to mainly feed on detritus, which has a relatively low nutritional value.  

Concluding remarks 

These experiments show how diverse and specific a coral species' diet can be. Understanding the specific diets of azooxanthellate species is key to successful aquaculture and husbandry. Although few aquarists have had (mostly short-term) success with these animals, insight into azooxanthellate coral husbandry is increasing.

References:

• Falkowski, PG, Dubinsky, Z, Muscatine, L, Porter, JW, Light and bioenergetics of a symbiotic coral. Bioscience, 1984, pp 705–709(34)

• Muscatine, L. Porter, JW, Reef corals: mutualistic symbioses adapted to nutrient-poor environments. Bioscience, 1977,  pp 454– 460(27)

• Edmunds, PJ, Davies, SP, An energy budget for Porites porites (Scleractinia). Mar. Biol, 1986,  pp 339– 347(92)

• Fabricius K, Genin A, Benayahu Y, Flow-dependent herbivory and growth in zoöxanthellae-free soft corals, Limnol. and Oceanogr., 1995, pp 1290-1301(40)

• Elyakova, LA, Shevchenko NM, Avaeva SM, A comparative study of carbohydrase activities in marine invertebrates, Comp. Biochem Physiol., 1981,  pp 905-908(69B)

• Fabricius KE, Benayahu Y, Genin A, Herbivory in Asymbiotic Soft Corals,  Science, 1995, pp 90-92(268)

• Ingestion of pico- and nanoplankton by the Mediterranean red coral Corallium rubrum, Picciano M, Ferrier-Pagès C, Marine Biology, 2007, pp 732-782(150)     

• Tsounis G, Rossi S, Laudien J, Bramanti L, Fernandez N, Gili J-M, Arntz W Diet and seasonal prey capture rates in the Mediterranean red coral (Corallium rubrum L.). Mar. Biol., 2005

• Ribes M, Coma R, Gili JM,  Heterogenous feeding in benthic suspension feeders: the natural diet and grazing rate of the temperate gorgonian Paramuricea clavata (Cnidaria: Octocorallia over a year cycle. Mar Ecol Prog Ser, 1999, pp 125–137(183)

• Garrabou J, Harmelin JG, A 20-year study on life-history traits of a harvested long-lived temperate coral in the NW Mediterranean: insights into conservation and management needs, Journal Anim. Eco., 2002, pp 966-978(71)