SOME COMMENSALS OF INDIAN ALCI ONARIANS. 
931 
coasts. In the Gulf of Mannar on rough bottom these sea-fans 
and sea-whips are particularly characteristic. Unlike Spongodes their 
surface texture is hard and coriaceous, and as their branching 
is usually in one plane, they do not afford such excellent hiding places to those 
small animals that elect to live as commensals. But as they are well defended 
from attack by their hardness and spicular texture, a number of animals never- 
theless do seek protection by association with them. These are much better 
known to zoologists than those of Spongodes and Solenocaulon ; it has long 
been noted, for instance by Thurston*, that delicate Ophiuroids commonly 
chng in numbers to the branches of the larger Gorgonids, while the beautiful 
little Cypraeid, Ovula {Sadhis) foimosa, is sometimes found crawling along the 
branches and mimicing their colour exactly, whether red or yellowish-brown. 
Aviculids (A. radiata) have also been noted, settled on several species of sea-fans, 
their narrow elongated shells oriented in such way that, at least when j-oung, 
they blend in shape with the branches and so escape detection ; they usually 
settle on colonies of brownish tint similar to their own. (Fig. 1.) 
All these I have frequently met ■with on Gorgonids on both sides of the Gulf 
of Mannar, together with several other less common or conspicuous commensals. 
Tiny Ophiuroids are by far the most plentiful. On one great specimen of 
Leptogorgia austaliensis having a spread of 6J in. in height by 11 inches in width, 
I counted 952 individuals, and on two smaller ones from the same locality (north 
end of the Periya Par, Ceylon), 159 and 109 were noted I’espectively. I had 
understood from previous \mters that these Ophiuroids followed the common 
commensal rule of assimilating in colour to that of their host. My actual ex- 
perience showed a noteworthy discrepancy. Taking the three specimens above 
noted, all of dark claret colour, two colour varieties of the ophiuroid were 
represented, one claret-coloured, the other orange. The former however predo- 
minated ; of the 952 on the largest Gorgonid, 622 were of this hue, while 330 
only were orange ; upon the second, 85 were claret, 74 orange, while those on 
the third were divided into 69 claret and 40 orange. All the Gorgonid trees 
taken at this particular place were claret coloured. On another occasion, on 
the outer Vangali Par (Cejdon) in depths of to 9 fathoms, the same species 
was found in remarkable abundance, over 250 colonies being obtained in one 
morning ; the great majority were claret coloured as in the previous case but a 
few were orange -coloured. All colonies, irrespective of colour, were infested 
heamly with the same little Ophiotrix which again showed the same two colour 
varieties. Whether on the orange or the claret coloured Gorgonids, the majority 
of the ophiuroids were of the latter colour. Correlated wnth this we must note 
that by far the greater number of this (3orgonid (Leptogorgia australensis) are 
claret colomed ; only a small minority are orange. But the same ophiuroid 
is also found on other species and on none more commonly than on Lophogorgia 
lutkeni. Now this species is characteristically and consistently orange in colour 
and wherever I have found it bearing these ophiuroid commensals, 
the latter have all been of the yellow variety. The occurrence of the yellow 
form only on the orange coloured Lophogorgia lutkeni, whereas both coloiu’ va- 
rieties are found commingled on Leptogorgia australiensis, which is predomi- 
nently claret coloured and only rarely yellow, suggests that the yellow variety is 
the original colour of the Ophiuroid and that the claret coloured ones represent a 
variety in process of colour evolution with a view to adaptation to the tint of 
the claret-hued (and predominant) variety of L. australiensis. The colour has not 
yet become stabilized, seeing that a large proportion, roughly 35 per cent., are 
still yellow among those seen on the Gorgonids of claret tint. As the orange 
* Thurston E. “ Littoral Fauna of the Gulf of Manaar”, Madras Government 
Museum Bulletin, No. 3, Madras, 1895, page 104. 
