SPONGES

Phyllum Porifera

Sponges are sessile animals, simply constructed with a single-body cavity with large exhalant pores and smaller inhalant openings lined with special cells called choanocytes, through which nutrient-rich water is passed. The exhalant tube is usually located at the highest point of the animal to allow for waste water to be carried away more efficiently. Sponges have no internal organs. Yet despite their simplicity, they are very successful organisms and are found worldwide in all different habitats. The hard structure of the sponge contains small calcareous or siliceous spikes or spicules which support the sponge. From an ancient class of animals, sponges are often difficult to identify as they change shape to suit their environment. In very exposed locations, sponges will be flattened, often covering large areas of rocky substrate. In calm sheltered conditions, many sponges grow quite spectacularly with large branching arms and delicate formations.

SPIKY SPONGE

Acanthella acuta

Relatively common, but only growing in small clumps in fairly shallow water, this sponge is often found on wrecks where it enjoys a firm holdfast. Obvious by its spiky shape, growing out over 25cm (10in), this orange sponge is variable in shape with obvious large oscula set back into the body and often has a rather fuzzy appearance.

YELLOW TUBE SPONGE

Verongia aerophoba

Enjoying a rocky substrate in fairly shallow water from 5 to 25m (17–83ft), this bright yellow sponge is tinged with a symbiotic blue algae which quite often makes it look greenish or brown in colour. It forms small clumps with large, finger-like tubes approximately 3cm (1¼in) in diameter and up to 12cm (4¾in) long with an obvious exhalant opening at the end of each tube.

YELLOW BRANCHING SPONGE

Axinella damicornis

Often mistaken for Acanthella acuta, the Yellow Branching Sponge tends to be low forming with irregular outcrops of exhaling tubes. Almost lemon-yellow in colour, with a slightly ‘fuzzy’ appearance, this small sponge is rough in texture, and it prefers shaded areas, rarely growing below 15m (50ft) and only 10cm (4in) in height.

BRANCHING TUBE SPONGE

Axinella polypoides

Quite often found in muddy pockets on the lower rock substrate inside caverns, this columnar sponge, yellow or orange in colour, forms tree-like branches, which are cylindrical in shape. Depending on the surge quality of the water, this sponge may develop many small branches, or only one or two very long branches, growing up to 1m (3ft 3in) in length. Quite fragile, it can be easily damaged. Of the three large and similar species of Axinella, this species is recognized by its lack of tuberculous growths and generally it tends to have longer more cylindrical branches. Axinella canabina has many small, more-jagged, irregular branches rising vertically from a single stalk and Axinella verrucosa is fairly smooth and cylindrical, but also is comprised of many small branches and projections. The branches of Axinella polypoides are often so long that they get too heavy to be self supporting and drape themselves over the rocky substrate.

POTATO SPONGE

Chandrilla nucula

Particularly widespread in most areas of the central Mediterranean, this species is especially resistant to surge and rough seas as well as being tolerant of mild pollution from sewage. Resembling small, brown, new potatoes, this sponge only grows to a maximum size of 2cm (¾in) and forms small patches on hard rock. Enjoying full sunlight, it is located on the tops of rocks and in shallow water to a depth of around 7m (23ft).

CLATHRINA

Clathrina coriacea

Rarely found in depths of less than 10m (33ft), this golden-yellow honeycomb sponge is only 3cm (1¼in) in height, but grows extensively over large areas. It prefers shaded areas and is common in caverns and under overhangs. Constructed of many small tubules, which link and weave together, it forms a raised pad, which quite often hangs down from cave walls. Looking somewhat like a lump of small netting, the exhalant valves or oscula are found at the ends of the interlocking tubes. A similar species, Clathrina clathrus is white in colour and is also found in both the Mediterranean and Atlantic. Clathrina is commonly associated with large colonies of the tunicate Dendrodoa, with which it interleaves, creating a marked colour contrast between the white and yellow of the sponge and the red of the tunicate.

BORING SPONGE

Cliona viridis

In varying shades of yellow and green, this species is probably much more common than originally thought, as its small size is so easily overlooked by marinelife observers. Looking like a series of excavated holes in the top of limestone rocks, the Boring Sponge has many inhalant and exhalant holes lined with calcareous spicules to trap organic matter. This sponge bores deeply into the soft rock and forms large colonies, the large holes of which are quite often used as refuge by small crustaceans or gobies.

OYSTER SPONGE

Crambe crambe

The Oyster Sponge is quite common and found in fairly well-lit waters from a depth of 5 to 30m (17–100ft). Commonly associated with Thorny Oysters and other sedentary bivalve molluscs, this brilliant red encrusting sponge covers the outer shell of the bivalve. Ridged by raised oscula found along the exhaling channels, the colonies grow from 10–20cm (4–8in).

BREADCRUMB SPONGE

Dyscidea fragilis

Forming large mats of over 1.50m (5ft) in low light conditions, principally under overhangs or in caverns, the Breadcrumb Sponge is a uniform grey in colour tinged with light purple. This sponge forms many raised oscula and is quite unmistakable from any other species.

BLACK SPONGE

Ircinia spinosa

Particularly common in central and eastern Mediterranean regions, the Black Sponge grows to approximately 20cm (8in) in diameter and is a low encrusting species found on the tops of well-illuminated rocky surfaces in all depths.

LOBED SPONGE

Oscarella lobularis

Commonly found in caves and caverns, this fleshy demosponge has neither spicules or spongin. Forming large sheets over 10cm (4in) thick, it shies away from natural sunlight. Often forming large interconnected tubes pale cream in colour with brownish tips, this sponge can cover large areas of cliff wall or overhang and will colonize rocks and algae.

PINK CAVE SPONGE

Petrosia ficiformis

This common sponge is found in most cave and cavern situations and can cover large expanses of cavern wall in long, sometimes connecting, lobes. With highly visible oscula, the colours are variable and will be a combination of peach to grey/white and sometimes purple. This is a favourite food species of the Spotted Doris nudibranch Discodoris atromaculata, seen opposite.

ORANGE SPONGE

Spirastrella cunctatrix

One of the more common sponges found in the Mediterranean, the Orange Sponge grows over rocky walls and covers most other organisms. Bright orange in colour with quite obvious channels that run over the body leading to the oscula, it is often mistaken for Crambe crambe, growing over a much wider area, often taking over a rocky wall.

GREEK BATH SPONGE

Spongia officinalis

Lacking calcereous spicules, the Greek Bath Sponge has a complex structured skeleton of spongin fibres making it extremely flexible. A light pink or uniform grey in colour, this sponge is collected commercially for sale as a bath sponge. The oscula are sparse but conspicuous, being slightly raised from the textured body. Common in caves and areas of low light from shallow water to over 40m (130ft).

LARGE ENCRUSTING SPONGE

Suberites domuncula

This is a large sponge developing irregular mounds and spheres with obvious oscula. It can grow to over 1m (3ft 3in) wide and 7cm (2¾in) thick. Quite hard and leathery in texture, it has three different types of spicules in its formation. It is quite often found growing on a Hermit Crab’s shell, where it eventually dissolves the shell, creating a new secondary home for the Hermit Crab. This allows for the crab’s growth pattern to get larger, without the need to change shells.

PURSE SPONGE

Sycon ciliatum

A quite distinct small sponge with a singular ovoid or spherical shaped tube around 7.5cm (3in) in length. It has a single, large oscula at the top surrounded by a stiff, spiky collar. Preferring shallow water and found in beds of mixed algae, this is an annual species, releasing its larvae in the spring.