Marine Biology

Pholads

These are the another group of wood-boring molluscs, which clearly resemble clams and therefore are not included with the shipworms. These pholads are commonly called as paddocks and these are not that much specialised forms as that of tredinids. The family Pholadidae includes three subfamilies namely Pholadinae, Martesiinae and Xylophaginae. The members of the genus Martesia are very serious pests of submerged timber. They have a world-wide distribution in tropical and warm temperate seas. Lignopholas, another genus of the subfamily Martesiinae, is restricted to tropical, brackish or fresh waters of South and Central America and Indonesia. Xylophaga occurs in the lower littoral and sub-littoral waters of the colder high latitudes and some species have been recorded from deeper waters. They are entirely encased in their double shells. The Martesia are the best-known species, but another well-known group is the Xylophaga. Like the shipworms, the Martesia enter the wood when they are very small, leaving a small entrance hole, and grow larger as they burrow into the wood. They generally do not exceed 64 mm (2-1/2 in.) long and 25 mm (1 in.) in diameter but are capable of doing considerable damage.

The most widely distributed and destructive species of Martesia is M. striata. This occurs in temperate as well as tropical waters in salinities ranging from normal sea-water to as low as 6%. Martesia drills holes in the wood which are about the size of its body which may reach a length of about 5 centimetres. The Posterior end of the body is narrow and is at the open end of the burrow, through which the inhalant and exhalent siphons project into the ambient water. These borers have free swimming larvae, which are carried to far off places by currents. Two species of Martesia occurs along the Indian coasts, M. striata and M. fragilis, the former chiefly occurs in the wooden structures in coastal area while the latter is found in soft drifting wood in the open sea. M. fragilis is slender and delicate and attacks catamarans and fishing boats. Even though M. striata cannot penetrate as deep as shipworms, successive generations bore deeper and deeper into the wood causing serious damage by weakening the piles and other underwater structures. Martesia does not apparently utilize wood as a source of nourishment but bores for the sake of protection only, feeding entirely on plankton. This habit is different from that of shipworms which exploit the nourishment contained in the very material they bore into, as well as plankton for their nutrition.

Lignopholas having only two species L. clappi (Western Atlantic) and L. rivicola (Borneo and Thailand), is restricted to brackish and fresh water, where they are abundant.

Xylophaga has a world-wide distribution. The majority of the species are borers in deep water where they replace teredines. A few species occur in shallow waters. Xylophaga makes tunnels up to eight times the length of the shell. Of the thirty-four species of Xylophaga, only five occur commonly in shallow water. Among these two, namely X. dorsalis and X. atlantica and X. washingtona in N. America and X. globosa in Chile. X.atlantica occurs in floating wood; X. globosa in test panels submerged at a depth of 3 metres and also in wood dredged from a depth of 270 metres while X. washingtona is reported deep water from 200 to 1,000 metres. Tests indicate that xylophaga utilizes cellulose, although the means by which it does so is not clear.