371 



UNATTACHED SPONGES 



In discussing the possible means by which the distribution of sponges is effected 

 (see p. 358), the assumption is usually made that there is no such thing as a free-living 

 sponge capable of being transported in the adult stage by currents. Certain specimens 

 of Tedania teniiicapitota, Ridley, in the present collections indicate the possibility that 

 even adult sponges, often of large size, may pass their life floating just above the sea- 

 floor, or at least resting unattached on the sea-bottom and capable of being lifted by 

 currents of moderate force. This^ may help to explain a number of anomalies pre- 

 viously observed in other species ; and, in view of the importance of such observations as 

 possibly affecting geographical distribution and the bionomics of sponges as a whole, 

 it will be of interest to deal fully with the subject. 



The sole means of observing sponges in the living state is confined to observation 

 between tide marks, in aquaria, and in exceptionally favourable circumstances, with 

 a diving helmet. In the first two, it is obvious that nothing in the nature of floating 

 sponges can be observed, and in the third, the results of such investigations have 

 never been recorded. It is necessary therefore to rely on indirect observation of 

 preserved material. 



Tedania temiicapitata, Ridley, is represented in the present collections by some 200 

 specimens. Most of these are damaged or at least incomplete at the base, presumably 

 where the trawl has torn them from the substratum. A few are, however, entire, and some 

 of these show no visible sign of having been attached either to the sea-floor or to any such 

 object as a hydroid or polyzoon. For the most part, such specimens are sub-spherical 

 and of small size. There are, in addition, three discoid specimens which seem to offer 

 convincing evidence in support of the suggestion that some sponges may not be 

 permanently fixed to the substratum and, under favourable circumstances, slightly 

 buoyant. 



The first of these specimens (Fig. 56 a) is discoid, 16 cm. across, i2 cm. high and 

 3 cm. at the thickest point. The margin is even, and a cross-section in any direction 

 shows a fairly regular ellipse (Fig. 56 b). At the base, the sponge is attached to the concave 

 inner surface of a bivalve shell, and between the shell and the adjacent sponge tissue 

 is a small quantity of sand. The margins of the sponge which encroach on the outer 

 surface of the shell are, however, entire, without sand inclusions, and show no sign 

 whatsoever of having at any time been attached to the substratum, rock, stone or hy- 

 droid, or of having been in contact with, or buried in sand. Further, there are several 

 hydroids growing on the shell, and in each case these point downwards. The only 

 possible assumption is that the sponge was free-living and, since it was obtained in a 

 trawl, floating just above the floor of the sea. 



1 The idea is not novel, for Bowerbank (i866,p.97) szys of Halknetnia patera : "The habit of the sponge of 

 including a small pebble in its centre, apparently as ballast, is very remarkable, and this is the only case in 

 which there is an indication of a natural tendency to locomotion belonging to the adult species that I have ever 

 observed among the Spongiadae". 



