THE BIOLOGY O F WOOD-BORING TEREDINID MOLLUSCS
443
system capable of digesting starch, sucrose, glycogen, maltose and
lactose, cellulose in wood, regenerated filter paper, cellobiose and gum
arabic, and also comparatively weak proteolytic and lipolytic enzymes
(Nair, 1956~). This weak protease activity in Bankia is in conformity
with the observation that the nitrogen content of adult shipworms is
low (Greenfield, 1953). Thus, the amount and nature of the enzymes
produced by the digestive diverticula probably depend more or less on
the natural diet and on the amount of food available. There does not
seem to be any appreciable difference in enzyme equipment between
species having different diets. There appears to be no exclusion of certain enzymes in relation to food specialization in the Bivalvia. Most
animals are incapable of digesting cellulose because they lack the
necessary enzyme equipment but shipworms which chiefly live on wood
are an exception possessing a strong carbohydrase system in the crystalline style and the digestive diverticula. The capacity to elaborate
cellulase, cellobiase and enzymes capable of hydrolysing pentosans in
addition to the carbohydrase enzymes is as much an adaptation to their
peculiar diet as the modification of the alimentary canal, the shell or
the foot. This fact is obvious when we find that the capacity of other
bivalves to split not only carbohydrates but also proteins and fats is
retained by B. indica. Closely correlated with this emphasis on the
digestion of carbohydrates is the capacity for the storage of great
quantities of glycogen (Lane, Posner and Greenfield, 1952 ; Greenfield,
1953; Lane, 1955; Saraswathy and Nair, 1969a). The conclusions of
Johnson et al. (1936) and Bartsch (1922) that the shipworm can live on
plankton alone even after all the available wood is exhausted appear to
contradict the findings of Potts (1923), Roch (1932), Lane (1955) and
Becker (1959) that the supply of wood is the principal food constituent.
Since shipworms are equipped with all three sets of digestive enzymes,
they must be capable of living not only on wood but also on other items
of food both living and non-living brought into the body along with the
respiratory current of water. Conclusive tests based on the exclusion
of all water-borne materials are difficult. Even under normal conditions shipworms may not be living on wood alone but also on plankton
(see Fig. 47), as shown by Dore and Miller (1923) for Teredo navalis,
Lasker and Lane (1953) for Teredo and Quayle (1959) for Bankia
setacea. However, it is not improbable that other factors such as the
high specialization in the sorting mechanisms which permit only a small
quantity of very small particles such as the nanoplankton to enter the
gut, the major part being rejected as pseudofaeces might be one cause
for their reported mortality after the wood supply is exhausted. Becker
(1959) was able to rear Lyrodus pedicellatus through four generations in
443
system capable of digesting starch, sucrose, glycogen, maltose and
lactose, cellulose in wood, regenerated filter paper, cellobiose and gum
arabic, and also comparatively weak proteolytic and lipolytic enzymes
(Nair, 1956~). This weak protease activity in Bankia is in conformity
with the observation that the nitrogen content of adult shipworms is
low (Greenfield, 1953). Thus, the amount and nature of the enzymes
produced by the digestive diverticula probably depend more or less on
the natural diet and on the amount of food available. There does not
seem to be any appreciable difference in enzyme equipment between
species having different diets. There appears to be no exclusion of certain enzymes in relation to food specialization in the Bivalvia. Most
animals are incapable of digesting cellulose because they lack the
necessary enzyme equipment but shipworms which chiefly live on wood
are an exception possessing a strong carbohydrase system in the crystalline style and the digestive diverticula. The capacity to elaborate
cellulase, cellobiase and enzymes capable of hydrolysing pentosans in
addition to the carbohydrase enzymes is as much an adaptation to their
peculiar diet as the modification of the alimentary canal, the shell or
the foot. This fact is obvious when we find that the capacity of other
bivalves to split not only carbohydrates but also proteins and fats is
retained by B. indica. Closely correlated with this emphasis on the
digestion of carbohydrates is the capacity for the storage of great
quantities of glycogen (Lane, Posner and Greenfield, 1952 ; Greenfield,
1953; Lane, 1955; Saraswathy and Nair, 1969a). The conclusions of
Johnson et al. (1936) and Bartsch (1922) that the shipworm can live on
plankton alone even after all the available wood is exhausted appear to
contradict the findings of Potts (1923), Roch (1932), Lane (1955) and
Becker (1959) that the supply of wood is the principal food constituent.
Since shipworms are equipped with all three sets of digestive enzymes,
they must be capable of living not only on wood but also on other items
of food both living and non-living brought into the body along with the
respiratory current of water. Conclusive tests based on the exclusion
of all water-borne materials are difficult. Even under normal conditions shipworms may not be living on wood alone but also on plankton
(see Fig. 47), as shown by Dore and Miller (1923) for Teredo navalis,
Lasker and Lane (1953) for Teredo and Quayle (1959) for Bankia
setacea. However, it is not improbable that other factors such as the
high specialization in the sorting mechanisms which permit only a small
quantity of very small particles such as the nanoplankton to enter the
gut, the major part being rejected as pseudofaeces might be one cause
for their reported mortality after the wood supply is exhausted. Becker
(1959) was able to rear Lyrodus pedicellatus through four generations in