Marine Biology (2+1)

Crustacean wood borers (Limnoria sp. and Sphaeroma sp.)

Another distinct group of marine borers are crustaceans. The principal borers in this group are species of Limnoria

( commonly called as gribble ) and Sphaeroma ( commonly known as pill bug) . Their attack differs from that of the shipworms and the Martesia in that the bore hole is quite shallow; the result is that the wood gradually is thinned from the surface inward through erosion by the combined action of the borers and water erosion. Also, the Limnoria and Sphaeroma do not become imprisoned in the wood but may move freely from place to place.

Species of the gribble Limnoria are found throughout the world and are very destructive to submerged wood. Common gribbles of the Indian coasts include Limnoria bituberculata, L. indica and L. bombayensis. The density of infestation sometimes becomes very high, with 45-60 animals occupying a square centimeter of wood. The superficial layer of the wood rapidly becomes reduced to a spongy mass which is easily removed by the mechanical action of waves. Thus, new layers of wood are exposed to fresh attack, and in course of time the timber may be entirely destroyed.

Limnoria are small, 3 to 4 mm (1/8 to 1/6 in.) long, and bore small burrows in the surface of wood. Although they can change their location, they usually continue to bore in one place. When great numbers of Limnoria are present, their burrows are separated by very thin walls of wood that are easily eroded by the motion of the water or damaged by objects floating upon it. This erosion causes the Limnoria to burrow continually deeper; otherwise, the burrows would probably not become greater than 51 mm (2 in.) long or 13 mm (1/2 in.) deep. In heavily infested harbors, untreated piling can be destroyed by Limnoria within a year. Burrowing is accomplished by the mandibles which are dissimilar in shape. Limnoria swallows the wood which it gnaws during burrowing. It is suggested that they elaborate cellulose and digest wood, but it is not definite whether wood fulfils all of their dietary requirements. It seems that they also get nourishment from the minute plants and animals living on the surface of the wood. Nine species of Limnoria are known to exist in the Indian waters but damage by them is not so serious as that done by other borers.

Sphaeroma are somewhat larger than the Limnorids, sometimes reaching a length of about 13 mm (1/2 in.) and a width of 6 mm (1/4 in.). They make tunnels of twice the length of their body. In general appearance and size, they resemble the common sow bug or pill bug that inhabits damp places. Sphaeroma are widely distributed but are not as plentiful as Limnoria and cause much less damage. Nevertheless, piles in some structures have been ruined by them. Occasionally, they have been found working in fresh water. In types of damage, Sphaeroma action resembles that of Limnoria. It has been reported that Sphaeroma attack even salt-treated wood . They penetrate deeper and deeper and destroy the timber within a brief period. They can tolerate considerable dilution of the sea water, and have therefore, succeeded in spreading into brackish and even fresh water. This great adaptability makes them a serious pest all along the Indian coasts. Five species of sphaeromids such as Sphaeroma terebrans, S. annandalei, S. triste, S. walkeri and S. annandalei travencorensis occur along the Indian coasts. S. terebrans and S. annandalei are the most destructive species along the coastal waters, estuaries and harbours of India.

The amphipod borer Chelura is mostly found associated with Limnoria. They normally work closer to the surface than Limnoria and seem to require the latter’s assistance before they can effectively attack the wood.

Besides the wood-boring animals mentioned here, wood infesting bacteria and fungi especially of the Ascomycetes and of the Deuteromycetes actively participate in the deterioration of wood, by ‘conditioning’ the timber, in order to prepare it for the subsequent attack by borers. This is a biological phenomenon, the importance of which has been stressed only recently. The activity of these fungi leads to the superficial softening of the wood. This softening is effected by the strong cellulose released by the fungi, which hydrolyses the unlignified cell elements of the outer tissues of timber. Even though the fungi do not cause spectacular damage, they prepare the wood for attack by crustaceans and the mollusks. The borers in turn help the fungi to spread deeper and deeper into the timber, thereby enabling them to expand the field of operation from superficial layers to its very core. The fungal infestation on the light timber of the catamarans, dugout canoes and other fishing crafts is deep affecting the entire log, rather than superficial. Marine fungi such as Gnomonia longirostris, Halosphaeria quadricornuata, Torpedospora radiate, Corollospora pulchella and Lulworthia sp. have been recorded from the wooden test panels submerged in the Cochin harbour area of the southwest coast of India