Giant barrel sponge

Giant barrel sponge
Conservation status
Not evaluated (IUCN 3.1)[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Porifera
Class:	Demospongiae
Order:	Haplosclerida
Family:	Petrosiidae
Genus:	Xestospongia
Species:	X. muta
Binomial name
Xestospongia muta (Schmidt, 1870)[2]
Synonyms
Petrosia muta (Schmidt, 1870) Schmidtia muta Schmidt, 1870

The giant barrel sponge (Xestospongia muta) is the largest species of sponge found growing on Caribbean coral reefs. It is common at depths greater than 10 metres (33 ft) down to 120 metres (390 ft) and can reach a diameter of 1.8 metres (6 feet). It is typically brownish-red to brownish-gray in color, with a hard or stony texture.^[3]

The giant barrel sponge has been called the "

The giant barrel sponge is variable in form. It is very large and firm,[3] typically being barrel-shaped, with a cone-shaped cavity at the apex known as the osculum. However, some individuals within the same population may be low and squat or relatively tall and thin. Similarly, the surface can range from smooth to rough, rugged, and irregular, sometimes with buttresses.^[3] In shallow water the color is brownish-red to brownish-gray, but at greater depths and in caves and under-hangs, or when the sponge is undergoing cyclic bleaching events, it is pinkish or white.^[7]

Distribution, habitat, and climate needs

The giant barrel sponge is common on reefs throughout the

The giant barrel sponge is a filter feeder. Water is continually pumped into the sides of the sponge, through the sponge body, and out of the osculum at the top of the sponge. Small pores in the sponge body are connected to channels lined by collar cells, each with a flagellum, and the beating of these flagellae draws water through the channels. Incoming particles, particularly microscopic bacteria and prochlorophytes, are phagocytosed by the collar cells. Sponges like X. muta also absorb dissolved organic compounds directly from the seawater as part of their diet.^[10]

The giant barrel sponge is probably

Growth models for X. muta have been formulated from digital photographs of the same sponges over a period of 4.5 years.[5] Sponge growth rates ranged from over 400% per year to only 2% per year. The largest sponges on Conch Reef, about the size of an oil barrel, were estimated to be about 130 years old. The largest individual for which a photograph was available (now dead) was estimated to be 2300 years old.^[5] By using the growth model, the age of an individual X. muta can be estimated from the osculum diameter and the base circumference.^[13]

Ecology

The tissues of the giant barrel sponge contain photosynthetic symbiotic cyanobacteria, Synechococcus spongiarum,^[14] which give the sponge its color. Individuals may undergo periodic bleaching, but this is a cyclic event, and the sponge recovers its normal coloration over time.^[7] This cyclical bleaching is likely to be a response by the cyanobacteria rather than by the host sponge, it has no negative effect on the host sponge.^[15] Unlike the circumstances for coral bleaching, X. muta does not appear to rely on its photosynthetic symbionts for nutrition, and they are considered commensals.^[16] Unrelated to cyclic bleaching is a pathogenic condition of X. muta called "sponge orange band" that can result in the death of the sponge.^[17] The cause and transmission of this pathogenic condition remains a mystery.^[18]

The giant barrel sponge is an important member of the reef community. Sponges filter large amounts of water, and are a predominant link in benthic-pelagic coupling on reefs and they harbor diverse assemblages of bacteria that can take part in nitrification and carbon fixation.