Journal of the Marine Biological Association of India

Volume 2 Issue 2

Rate of growth of Bankia indica Nair, a shipworm from the Madras Coast.

Balakrishnan Nair N

Even though sixty papers have been published on the different aspects of marine boring organisms of India, very little information is available on the rate of growth of Indian shipworms which cause very serious damage to submerged timber all along the sea coasts. The growth of these specialised, wood boring bivalves is directly related to the damage they effect on timber since each shipworm during its lifetime destroys a column of wood of the same dimension as its largest size and it was therefore interesting to make a study of the rate of growth of a local shipworm Bankia indica. In the present study four aspects have been taken into consideration namely growth in length of the body, (i.e. increase in length of the burrow) increase in weight of the body, increase in the number of ridges of the shell and growth in length and breadth of the shell valves. 
By submerging conditioned test timbers of red-cedar (Cedrela toona) off San Thome, Madras, it was possible to obtain a set of shipworms whose age could be estimated with a considerable degree of accuracy. These test timbers having been placed in the sea during the height of the breeding season (June-August) (when the water was swarming with larvae) may safely be assumed to have been attacked within a few days of submergence. Since this teredine Bankia indica has been found to subsist mainly on the wood into which it bores Nair (1955) and since its growth in length is proportional to the length of the burrow, the latter may be related to the period of tenancy and therefore the age of the animal. Several test panels of red-cedar were suspended (in the sea during June in sites where the sea water was swarming with larvae.) Each month one of the panels was lifte.d out and the settled borers were studied. The longest specimens were selected since they represent those which were first to enter the wood after it was placed in water, as was done by Miller (1922). In the normal living state each shipworm fills its burrow the length of which therefore, may be treated as the length of the animal and carefully measured throughout its course. The average of the lengths of these would give the average length attained by the earliest settlers during the month in question. In the second month another of the panels was removed and the longest burrows were measured, the average of these will give the average length of Bankia settled approximately two months ago. In this way the borers from eight panels were studied from June 18, 1954 to January 23, 1955. The weights of the animals were recorded after drying them between folds of filter paper. The linear dimensions of the shell were measured with a pair of vernier callipers, the very small ones were measured with a micrometer. The dental ridges on the anterior lobe of the shell were counted under a lens. The error in the calculation of the ridges that may arise due to erosion of the ridges by friction or by overgrowth of the shelly material over the ridges are unavoidable and hence has been dealt with as ' negligible for practical purposes' as suggested by Miller (1922). Since by the seventh month the test panels became so riddled by the borers as to become fragile and brittle, observations were limited to a period of 219 days from the time of immersion of the panels. 


bankia indica

Date : 30-12-1960