Diversions of a Naturalist

by Sir E. Ray Lankester

CHAPTER XVII
SAND-HOPPERS

WHEREVER there is a sandy seashore with here and there masses of dead seaweed and corallines thrown up by the waves, you will find sand-hoppers feeding on the debris. They are crustaceans, like crabs, shrimps, and barnacles, but in general aspect resemble enormous fleas. I hope that this comparison will not enable any reader at once to picture the less familiar by the more familiar. A good-sized sand-hopper is about half an inch long, and jumps not by means of a specially large pair of legs as the flea does, but by the stroke of the hind body, the jointed rings of which are carried curled downwards and ready to give a sudden blow. The sand-hopper (Fig. 20, a) has some of the rings or segments of the mid-body distinct, and not fused with those of the head or overhung by a great shield as in the lobster, crab, and shrimp. His walking legs and jaw-legs are also not quite of the same shape, though similar to those of a lobster, and his two little black eyes are not mounted on stalks, but are flush with the surface of the head. There are two quite distinct kinds of sand-hopper which live in crowds together on our sandy shores. They are not very different, but still are distinguished by naturalists from one another; one is called Talitrus (Fig. 20, a), the other Orchestia (Fig. 20, b). They are very similar in appearance and structure to a fresh-water creature common in weedy streams, which has no English name (except the general one of "fresh-water shrimp"), and is called by naturalists Gammarus.

Fig. 20.—a, Talitrus locusta, b, Orchestia littorea,
the two common kinds of "sand-hopper." Of the
natural size. c, A kind of small lobster which
burrows in the sand, Callianassa subterranea.
About two-thirds the natural size, linear.

In the open sea there are many hundreds of kinds of small crustaceans resembling the sand-hoppers in their compressed (not flattened) shape of body and in the details of their legs and the grouping of the joints of the body. Many of the smallest crustaceans which swarm in the surface waters of the sea and form part of that floating population, mostly of small transparent or iridescent and blue creatures, which we call the "plankton," or "surface-floating" population, and may be gathered by towing a very fine net behind a boat on a quiet day, can produce flashes of light which are vivid enough when seen at night. They contribute, together with jelly-fish and the teeming millions of minute bladder-like Noctiluca, and other unicellular animalcules, to produce that wonderful display seen from time to time on our coasts, and called "the phosphorescence of the sea." These minute crustaceans produce flashes of light by suddenly squeezing from pits or glands in the skin a secretion which is chemically acted on (probably oxidized) by the sea-water, the chemical action setting up light-vibrations, but not the usual excess of heat-vibrations to which we are accustomed when light accompanies ordinary "burning" or "combustion."

Fig. 21.—A Phosphorescent Shrimp (Euphausia pellucida).

The lamp-like phosphorescent organs are numbered 1 to 6. There is another on the outer edge of the stalked eye, making seven in all on each side of the animal. g, points to the hindermost gill, enlarged.

Other crustaceans of several kinds, of an inch and more in length—transparent, delicate creatures, resembling small prawns in appearance—also produce light. Some of them are known by names referring to this fact, such as Lucifer (light-bearer) and Nyctiphanes (night-shiner). They possess special lantern-like knobs scattered about on the body, which have transparent lenses, and resemble small bull's-eye lanterns. Some have a row of seven lanterns on each side of the body (Fig. 21), but one kind has as many as 150 dotted about. These lanterns were only a few years ago thought to be eyes, and their elaborate microscopic structure was described as that of an eye. Of course, this was due to the fact that dead preserved specimens were studied, and not the living animal. Some twenty years ago I witnessed a most impressive exhibition of these phosphorescent shrimps at the house of my friend Sir John Murray, of the "Challenger," at Millport, on the Clyde. He had obtained them (the kind called Nyctiphanes) in great quantities at a depth of ninety fathoms in the great Scotch fiord, and amongst other curious facts about them had shown that they enter Loch Fyne in vast numbers, and are the special nourishment of the celebrated Loch Fyne herrings. It had been noticed that the intestine of the plump, well-fed herrings is full of a deep-black substance, and Sir John Murray showed that this was the black, indigestible pigment of the eyes of the hundreds of phosphorescent shrimps swallowed by these favoured fish, which owe their fine quality to their special opportunity for feeding in the depths of the loch on the exceptionally abundant and nutritious light-producing crustaceans! At night my friend showed me a large glass vessel holding four or five gallons, in which were a hundred or so of the phosphorescent shrimps swimming around. We turned out the lamps of the room, and all was dark. Then a gentle tap was given to the jar, and each little crustacean lit up, as though by order, a row of seven minute lamps on each side of its body, swimming along meanwhile, and reminding one of a passenger steamer, as seen from the shore, as it glides along at night with its lights showing through a row of cabin windows. The shrimps' lights shone steadily for a minute or so, then ceased, and had to be lit up again by again signalling their owners by knocking on the glass. These little lamps, with their bull's-eye lenses, are far more elaborate structures than the glands which in other cases cause a flash by discharging a luminous secretion into the water. They are even more elaborate than the internal permanent phosphorescent structure of the glow-worm (an insect, not a crustacean), which has no condensing lens.