Enemies of the Mollusca
The juicy flesh and defenceless condition of many of the Mollusca make them the favourite food and often the easy prey of a host of enemies besides man. Gulls are especially partial to bivalves, and may be noticed, in our large sandy bays at the recess of the tide, busily devouring Tellina, Mactra, Mya, Syndosmya, and Solen. On the Irish coast near Drogheda a herring gull has been observed[128] to take a large mussel, fly up with it in the air over some shingly ground and let it fall. On alighting and finding that the shell was unbroken it again took it up and repeated the process a number of times, flying higher and higher with it until the shell was broken. Hooded crows, after many unavailing attempts to break open mussels with their beak, have been seen to behave in a similar way.[129] Crows, vultures, and aquatic birds carry thousands of mussels, etc., up to the top of the mountains above Cape Town, where their empty shells lie in enormous heaps about the cliffs.[130]
The common limpet is the favourite food of the oyster-catcher, whose strong bill, with its flattened end, is admirably calculated to dislodge the limpet from its seat on the rock. When the limpet is young, the bird swallows shell and all, and it has been calculated that a single flock of oyster-catchers, frequenting a small Scotch loch, must consume hundreds of thousands of limpets in the course of a single year. Rats are exceedingly fond of limpets, whose shells are frequently found in heaps at the mouth of rat holes, especially where a cliff shelves gradually towards a rocky shore. A rat jerks the limpet off with a sudden movement of his powerful jaw, and, judging from the size of the empty shells about the holes, has no difficulty in dislodging the largest specimens. ‘I once landed,’ relates a shepherd to Mr. W. Anderson Smith,[131] ‘on the I. of Dunstaffnage to cut grass, and it was so full of rats that I was afraid to go on; and the grass was so full of limpets that I could scarcely use the scythe, and had to keep sharpening it all the time.’ Sometimes, however, the limpet gets the better both of bird and beast. The same writer mentions the case of a rat being caught by the lip by a limpet shell, which it was trying to dislodge. A workman once observed[132] a bird on Plymouth breakwater fluttering in rather an extraordinary manner, and, on going to the spot, found that a ring dotterel had somehow got its toe under a limpet, which, in closing instantly to the rock, held it fast. Similar cases of the capture of ducks by powerful bivalves are not uncommon, and it is said that on some parts of the American coasts, where clams abound, it is impossible to keep ducks at all,[133] for they are sure to be caught by the molluscs and drowned by the rising tide.
The Weekly Bulletin of San Francisco, 17th May 1893, contains an account of the trapping of a coyote, or prairie wolf, at Punta Banda, San Diego Co., by a Haliotis Cracherodii. The coyote had evidently been hunting for a fish breakfast, and finding the Haliotis partially clinging to the rock, had inserted his muzzle underneath to detach it, when the Haliotis instantly closed down upon him and kept him fast prisoner.
Rats devour the ponderous Uniones of North America. When Unio moves, the foot projects half an inch or more beyond the valves. If, when in this condition, the valves are tightly pinched, the foot is caught, and if the pinching is continued the animal becomes paralysed and unable to make use of the adductor muscles, and consequently flies open even if the pressure is relaxed. The musk-rat (Fiber zibethicus) seizes the Unio in his jaws, and by the time he reaches his hole, the Unio is ready to gape.[134] Rats also eat Vivipara, and even Limnaea, in every part of the world.
Every kind of slug and snail is eaten greedily by blackbirds, thrushes, chaffinches, and in fact by many species of birds. A thrush will very often have a special sacrificial stone, on which he dashes the shells of Helix aspersa and nemoralis, holding them by the lip with his beak, until the upper whorls are broken; heaps of empty shells will be found lying about the place of slaughter. The bearded Titmouse (Parus biarmicus) consumes quantities of Succinea putris and small Pupa, which are swallowed whole and become triturated in the bird’s stomach by the aid of numerous angular fragments of quartz.[135]
Frogs and toads are very partial to land Mollusca. A garden attached to the Laboratory of Agricultural Chemistry at Rouen had been abandoned for three years to weeds and slugs. The director introduced 100 toads and 90 frogs, and in less than a month all the slugs were destroyed, and all kinds of vegetables and flowers, whose cultivation had until then been impossible, were enabled to flourish.[136]
Certain Coleoptera are known to prey upon Helices and other land Mollusca. Récluz noticed, near Agde, a beetle (Staphylinus olens) attack Helix ericetorum when crawling among herbage, sticking its sharp mandibles into its head. Every time the snail retreated into its shell the beetle waited patiently for its reappearance, until at last the snail succumbed to the repeated assaults. M. Lucas noticed, at Oran, the larva of a Drilus attacking a Cyclostoma. The Drilus stood sentinel at the mouth of a shell, which was closed by the operculum, until the animal began to issue forth. The Drilus then with its mandibles cut the muscle which attaches the operculum to the foot, disabling it sufficiently to prevent its being securely closed, upon which it entered and took possession of the body of its defenceless host, completing its metamorphosis inside the shell, after a period of six weeks.[137] The female glow-worm (Lampyris noctiluca) attacks and kills Helix nemoralis.
Among the Clavicornia, some species of Silpha carry on a determined warfare against small Helices. They seize the shell in their mandibles, and then, throwing their head backwards, break the shell by striking it against their prothorax.
The common water beetle, Dytiscus marginalis, from its strength and savage disposition, is a dangerous enemy to fresh-water Mollusca. One Dytiscus, kept in an aquarium, has been noticed to kill and devour seven Limnaea stagnalis in the course of one afternoon. The beetles also eat L. peregra, but apparently prefer stagnalis, for when equal quantities of both species were placed within their reach, they fixed on the latter species first.[138]