BIOLOGY OF THE OYSTER-GROUNDS OF SOUTH CAROLINA. 
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VI.— MESSMATES AND ENEMIES; SPAWNING SEASON. 
THE OYSTER ORAB AND ITS RELATION TO ITS HOST. 
The oyster crab as a messmate has doubtless been the subject of comment from 
prehistoric times. Pliny quaintly speaks of it as a discreet doorkeeper, who, in return 
for safe quarters, pinches the oyster, warning it to close its shell in time of need; 
while Plutarch, as if unwilling to be outdone by his voluminous rival, ascribes the 
commensalism to motives of partnership, the crab contributing his eyesight, the oyster 
his entrapping shell. Small fish, sadly deluded and captured, are feasted upon. Prof. 
Verrill has noted that it is the female only that takes refuge within the oyster’s shell. 
Prof. Eyder has added to its interest by showing that to its body are attached clus- 
ters of bell-animalcules (. Zoothamnium ), whose progeny are of food value to the oyster, 
and that in turn even the bell-animalcules pay toll to the common host in the broken- 
off vibriones that infest them. He accordingly regards the crab as a food nursery 
whose presence is of benefit to the oyster.* 
In the Carolina waters excellent opportunity is offered for the study of the oyster 
crab. It was often found in as many as 5 per cent of the oysters opened. All indi- 
viduals were apparently of one species, Pinnotheres ostreum , Say ; more than one female 
was never noted infesting a single oyster. In most instances the crab is found well 
thrust in between the palps, usually between the middle ones. Occupation, at this 
position is evidently annoying to the oyster, for the palps sometimes show thickened 
outgrowths, or are malformed and stunted in size. 
That the crab is of value to the oyster as a purveyor or as a nursery for food 
appears (from many notes that have been made) extremely doubtful. It must be 
admitted on the one hand that, in almost every instance, Zoothamnium colonies 
have been found in every variety of position and abundance. As a rule, how- 
ever, the clusters are small and infrequent. In position they are usually moored 
to the crab’s basal leg segments. In the matter of food there can be no doubt that 
the crab secures many of the small crustaceans which are not normally the oyster’s 
prey. But on the other hand, the stomach contents of the crab consist in great part 
of the minute organisms, diatoms, pine pollen, zoospores, and infusorians, sought by 
the host. And as the closest examination of the oyster’s stomach failed in every case 
to show the presence of the bell- animalcule or its swarmers, as a compensatory food 
tribute, the benefits derived by the oyster from a tenant that can not be ejected ap- 
pear somewhat doubtful. The crab will be found to crowd itself snugly between the 
oyster’s palps, where food organisms are constantly collecting, and at this point may 
readily help itself to a selfish share of the incoming food. 
The hairs of leg tips and mouth parts of the messmate are curiously specialized 
for arresting the slime-coated patches of oyster food. At the distal end of the dactyl- 
opodite the stout recurved hairs are most numerous, holding, as may actually be seen, 
as if with the teeth of a rake, the slime-entangled masses of organisms. The slender 
* Ryder, Maryland Fish Commission, 1881, p. 24. 
