The Sea Shore

These interesting crabs have been spoken of as the monkeys of the sea, and the comparison will certainly be tolerated by anyone who has watched the creatures as they climb among the corallines and sea firs in an aquarium. Among such growths they are quite at home; and although their movements do not often suggest the extreme agility of the monkey tribe, yet the ease with which they seize the branches of the submarine forest with their long ‘arms’ and pull their bodies from one tree-like structure to another is decidedly monkey-like. Their comparison with the long-legged spiders is also a happy one as far as their general form and movements are concerned, but it must be remembered that they have not the same reputation for cruel, predaceous habits, for they are more truly the scavengers of the deep, subsisting mainly on the decomposing bodies of their dead associates. The movements of most spider crabs are so slow and deliberate that one can hardly imagine them capable of anything of the nature of violent action; yet, when occasion requires it, they will sometimes strike at the object of their wrath with a most vigorous snap of their claws.

In these crabs, too, we find most interesting instances of protective resemblance to their surroundings. Some of the small, slender-legged species are not to be recognised without a careful search when they are at rest among clusters of sea firs, their thin appendages and small bodies being hardly discernible in the midst of the slender, encrusted branches, and their peculiar forms are still more concealed by their colouring, which generally closely resembles that of the growths among which they live. Further, the carapace of spider crabs is in itself a garden on which thrive low forms of both animal and vegetable life. Minute Algæ, and occasionally some of moderate size, are rooted to the shell, often securely held by the aid of the rough hairs and tubercles that are so characteristic of the exo-skeletons of these creatures; and patches and tufts of animal colonies that have found a convenient settlement on the moving bed still further serve to obscure the nature of the living mass below—a mass that is always in danger of becoming the prey of the fishes which inhabit deep water. It is probable, therefore, that this association is one that is beneficial to both sides as far as the animal life is concerned, the lower species serving to disguise the true nature of the crab, thus protecting it from its numerous enemies, while they in return are conveyed, carriage paid, to the feeding-grounds, where they can freely partake of the fragments that become diffused in the surrounding water.

Our illustration on p. 288 shows three species of spider crabs, all of which are common on parts of our shores. The Scorpion Spider Crab (Inachus dorsetensis) derives its specific name from the fact that it was first found off the coast of Dorset; but it is abundant off many of our shores, both in the south and north, and may frequently be seen entangled among the fishermen’s nets. It may be distinguished from other and similar species by the four spines arranged in a line across the front portion of the carapace, and the five large, pointed tubercles behind them. This species is undoubtedly a favourite food of the cod, for several specimens may often be taken from the stomach of a single fish.

The next species—The Slender-beaked Spider Crab (Stenorhynchus tenuirostris)—is seldom missing from the dredgings hauled in off the south-west coast, and is fairly common in other parts. Its legs are extremely slender, and bear spines on the inner side, and its body, where free from the incrustations so often covering the carapace of spider crabs, is of a fresh pink colour.

The other one shown in the same illustration is Arctopsis lanata, sometimes known as Gibb’s Crab, the carapace of which is pointed behind, bears a large pointed tubercle on each side, and is completely covered with a thick clothing of stiff hairs. It is also common on many parts of our coasts, more especially the coasts of Devon and Cornwall.

The pea crabs are all small, and they are parasites, living within the shells of bivalve molluscs. One species—the Common Pea Crab (Pinnotheres pisum) is frequently found in the Edible Mussel; the female, which is much larger than the male, being much more commonly found. Another species—the Pinna Pea Crab (P. veterum), infests the Pinna and Modiolus.

On Plate VI. is a drawing of the Angular Crab (Gonoplax angulata) mentioned above, the striking form and delicate colouring of which can never be mistaken. We would, however, call particular attention to the broad and square front of the cephalothorax, with its two sharp spines, and to the length of the eye-stalks. Unfortunately for the amateur, this pretty crab is only to be found in deep water, off the coasts of Devon and Cornwall, so that here, again, the aid of the fisherman is valuable; but, as observed in the case of other deep-sea dwellers, may also be looked for in the stomachs of cod and other bottom fishes. The sex figured is the male, in which, when fully grown, the front legs are much longer than in the female.

Another abundant and well-known species is the Edible Crab (Cancer pagurus), which is as familiar an object in town as on the sea coast. Unlike the common lobster, its natural colour is not considerably changed by boiling, being only turned from a dull to a brighter red.

The finest specimens of this crab are to be caught beyond low-water mark, the usual snare being the basket or pot, baited with fish refuse, but large numbers live among the stones and rocks left exposed at low tide, and sometimes include specimens of considerable size. They should be looked for under large stones that are loosely piled together, or in the narrow chinks of rocks.

It is very interesting to compare the habits of the two common crabs just mentioned. The former, when molested, will run off in great haste, but always retreat with its front to the enemy, and its sharp and powerful pincers far apart and wide open, ready for immediate use in its own defence if necessary. The latter species, on the other hand, though strongly built and provided with formidable claws, seldom runs far, and hardly ever attacks one in the act of pulling it out of its hiding-place; but, on the contrary, doubles all its ten legs under its body as if endeavouring to approach, as nearly as possible, the form of a ball, and will allow itself to be rolled about without showing any signs of life.

The genus Xantho contains two or three species that are common on the Cornish and Devon coasts, and which may be known by their depressed and deeply-grooved carapace and the presence of three or four prominent tubercles on the latero-anterior margins. The abdomen of the female has seven joints, while that of the male has only five. One of these (Xantho florida), shown on Plate VI., is a powerfully built crab, as may be seen when, after being disturbed, it pushes its way among the loose stones of the beach, often lifting masses many times its own weight.

On the same plate is also a figure of the pretty Velvet Crab (Portunus puber), also known as the Lady Crab and the Violet Fiddler. The first of these popular names has its origin in the dense covering of close hairs that clothe the carapace, and the last refers to the beautiful violet colouring of parts of the front legs, and, to a lesser extent, of the remaining legs. This is, perhaps, the most ferocious of all our shore crabs, and its attacks, when disturbed, are of such a determined nature that the catching of the larger specimens is quite a lively sport. Though it can hardly be described as an abundant species, yet it sometimes occurs locally in such numbers that it may be found under nearly every stone of any size. In fact, we have searched two or three localities on the south-west coast where this crab is not only extremely numerous, but is at the same time almost the only species to be found; and it seems not unlikely that the pugnacious Lady has been the means of driving the less formidable species from its favourite haunts.

When you disturb a Velvet Crab it will immediately raise itself in a menacing attitude, stretching its brightly coloured pincers as wide apart as possible, and then it will either retreat backwards, or even make a firm stand, ready to strike as soon as it is threatened with an attempted touch. Try to grasp it, and its two powerful weapons of defence are brought together with lightning-like rapidity giving one a decidedly smart blow, possibly followed by a grip of great tenacity for a creature of its size; but, should it miss its aim, its pincers strike together with a sharp click, only, however, to extend at once in preparation for the next attempt.

It will be observed that the walking legs of this crab are all flattened, and that while the first three pairs terminate in sharp, lance-like claws, the last pair are broad and fringed with hairs, thus showing their close relationship to the swimming crabs. In fact, the same genus contains British species which are popularly known as Swimming Crabs.

One of the swimmers is represented in fig. 4 of Plate VI. It is generally known as Henslow’s Swimming Crab or the Nipper, the scientific name being Polybius Henslowii. The carapace of this species is quite smooth, thus enabling the crab to move through the water with less resistance, and the walking legs, particularly the last pair, are flattened and fringed for use as paddles. It is said that this crab can raise itself from the bottom to the surface of moderately deep water by means of the swimming feet, and that it preys on fishes which it pursues with some vigour.

Other crabs than those briefly described will reveal themselves to the sea-side collector, but we have not the space to introduce them here. Sufficient information has been given, however, to enable the reader to broadly classify his specimens—a matter of more importance to the young naturalist than the mere naming of species.

Leaving the crustaceans now, and passing for a moment to the Arachnoidea—the second great division of the arthropods—we shall briefly describe the Shore Spider (Pycnogonum littorale), which is the only representative of the class likely to be met with by the sea-shore collector.

It will be seen by our illustration that this creature by no means resembles a typical spider. The powerful jaws, really modified antennæ, that are such formidable weapons in the latter, together with other appendages of the head, are undeveloped in the shore spider, and the head is prolonged forward to form a rigid beak with the mouth at the summit, and the head and thorax together form a cephalothorax of four distinct segments, each of which bears a pair of legs. Further, the cephalothorax forms almost the whole of the body, for the abdomen, usually so large in spiders, is here represented by a mere tubercle. The shore spider is unable to swim, but crawls about among the weeds and stones of the bottom, clinging firmly by means of the curved claws of its eight thick legs, and is protected by its dull grey colour which closely resembles that of the encrusted stones among which it spends the greater portion of its existence. It may sometimes be found hiding under stones near low-water mark, but is far more commonly seen among the ‘rubbish’ hauled in by the trawl.

The head of an insect is furnished with a pair of compound eyes, very similar in structure to those of a crustacean, and often, in addition, a cluster of simple eyes; also a pair of antennæ, usually composed of many joints. These antennæ are important organs of touch, and are employed, at least by many forms, as a means of communication between one insect and another. In them are also located the organs of hearing, and, possibly, those of other senses.

The mouth varies very considerably in different insects, but is often supplied with a pair of mandibles or biting jaws, and, below them, a pair of maxillæ or chewing jaws, both pairs being jointed to the head in such a manner as to be capable only of horizontal movements. Above and below these jaws are, respectively, the upper lip or labrum, and the lower lip or labium, the latter having appended to it a pair of jointed feelers called the labial palpi, and an additional pair of palpi are also frequently attached to the maxillæ, and therefore called the maxillary palpi.

These organs of the mouth of an insect are modified in various ways according to the functions they are called upon to perform. Thus, in bees, the upper lip, as well as the mandibles, are adapted for chewing, while the maxillæ and the labium are grooved in such a manner that when brought together they form a tube through which fluids may be sucked into the mouth. Also, in the butterfly and the moth, the maxillæ are not constructed for chewing, but consist of two channelled rods which, when approximated, form a long tube or proboscis employed for suction; and in these insects the labial palps are large for the protection of the proboscis, which is retracted and closely coiled between them when not in use. Further, in the bugs, the labium is long and tubular, while the mandibles and maxillæ are often modified into sharp, stiff bristles that work within the tube, the whole thus forming a combined piercing and sucking arrangement.

Insects are developed from eggs, but in their young state they are segmented larvæ, with strong jaws, antennæ, simple eyes, and usually three pairs of legs attached to the first three segments next to the head.

As regards internal structure, we need only mention here that the body is traversed by numerous branching tubes (tracheæ) that open at the exterior and constitute the respiratory apparatus; that the insect is provided with a contractile, tubular heart by means of which the blood is propelled through a system of blood-vessels; that the nervous system consists of a chain of ganglia, connected by a nerve cord, sending nerve filaments to all parts of the body; and that the digestive tube is often a complicated structure, especially in the case of those insects that feed on herbivorous matter.

However, one would hardly expect to find similar forms of life abundant in the water that washes our shores, the disturbing action of the waves, even in calm weather, being more than such fragile creatures could withstand. And this is really the case, for there are but few insects that may be described as marine in the strictest sense of the word; and of these the species that have been observed are mostly inhabitants of warmer seas.

It is noteworthy that all the insects which exhibit marine tendencies are small, and they seldom, if ever, live permanently below the surface. But few of them can swim. A few run on the surface of the water, supporting themselves on the surface film after the manner of water-gnats, whirligig beetles, &c., without ever being wetted; and these are said to feed on different kinds of floating matter, and occasionally to dive below the surface.

A rambler on the sea shore in the summer time will always meet with plenty of insect life, but the number of species observed may not be large: and omitting all those which show no decided preference for the coast, but are found in inland districts as well, we find that by far the larger proportion live at or near the high-water mark, where they feed on the refuse washed up by the waves. Some species, however, live among the stones, or burrow into the sand, between the tide-marks; and these, as a rule, are not driven inland by each advancing tide, but allow the sea to wash over them, having at first protected themselves from disturbance by burrowing or seeking other suitable shelter.

These latter, like many of the insects that inhabit fresh water, are well adapted to withstand prolonged immersion. Their bodies are not capable of being wetted, a covering of short hairs effectually preventing the water from coming into actual contact with the body. The openings of the breathing tubes (spiracles) are also guarded by closely set hairs which prevent the water from entering; and, in some cases, the creatures are provided with special air-sacs in which a supply of air is stored for use while the insect is shut off from the external atmosphere.

The lowest order of insects includes the so-called Bugs (Rhynchota), which are parasitic on plants or animals. Quite a number of these are to be found inhabiting fresh water, but only one is truly marine in its tendencies. This one is a small insect, only about an eighth of an inch in length, and named Æpophilus (fig. 222). It has never been seen except between the tide-marks, and occurs so near low-water level that it is submerged during the greater part of its existence. But little is known of this peculiar creature. Even its food has not been ascertained. As with the other Rhynchota, but little change of form takes place during growth, the young being very much like the adult in appearance. It has been observed that the larvæ live crowded together under the protection of stones.

The reader is probably acquainted with those fresh-water bugs that are popularly known as ‘boatmen’ on account of the oar-like action of their long, fringed hind legs; and although none of these may be described as marine, yet certain species may often be seen in salt and brackish water, living in company with creatures that are decidedly inhabitants of the sea.

Allied to the bristle-tails, and usually grouped with them in the same order, are the little Spring-tails, some species of which may often be seen huddled together on the surface of the water of a rock pool. They are so small that, unless closely examined, they may be mistaken for particles of floating inorganic matter which have been blown into a sheltered corner of the pool, and this idea may be strengthened by the fact that these minute creatures are driven by the wind into such sheltered spots. But when we disturb them their true nature immediately becomes apparent, for they may then be seen to move about on the surface of the water, sometimes creeping on the surface film, and clambering on the adjacent rock or weed, or leaping more or less vigorously, in which latter case their bodies do not become wetted, the surface film remaining unbroken by their exertions. And even when the rising tide drives the spring-tails into crevices where they remain submerged, perhaps for hours together, their bodies still remain dry, the water being kept off by numerous short bristles and prominences with which they are furnished.

When we examine a spring-tail by means of a lens we observe that it has no traces of wings, but that each of the three segments representing the thorax bears a pair of short legs, and that the abdomen consists of only five or six segments. The head is furnished with a pair of jaws, and the antennæ, which are short and thick, are composed of but few joints—never more than six in number.

Some spring-tails live among the refuse washed up on the beach, where they may be seen jumping about in company with the sandhoppers when the material is disturbed. Such is the case with Isotoma maritima, the illustration of which shows the forked tail that enables the little animal to jump about so vigorously. But some of the marine spring-tails are not so true to their name, since they are not provided with this characteristic jumping organ, and have to content themselves by creeping about slowly with the aid of their short legs. One of these springless spring-tails (Anurida maritima) is one of the commonest of the group, and is distributed over almost every part of our coast.

Passing over several orders of insects which do not seem to have any marine representatives, we come to the Diptera or two-winged insects, of which the familiar house-fly is a type, and here we have to deal with those troublesome creatures that literally swarm in the neighbourhood of the matter washed up to the highest level of the tide during the whole of the summer months. But although these insects are so very numerous, we do not find among them a particularly large number of species, their abundance being due more to the extreme prolificacy of those that occur.

In this order, which includes all gnat-like creatures, as well as those insects that are generally known as flies, the first pair of wings are well developed, while the second pair are rudimentary, and represented merely by a pair of scales, or by two little pin-like bodies called the balancers or halteres. Some are provided with piercing organs by means of which they can inflict a small wound and then extract the juices of their victim, as does the female gnat, but the majority have a proboscis adapted for suction only. The larvæ of the Diptera are generally limbless maggots, gifted with a pair of jaws, and they are usually very voracious feeders, devouring decomposing animal or vegetable matter in enormous quantities.

If we turn over a fermenting mass of the miscellaneous matter thrown up on the beach quite beyond the reach of the tides, we may observe a multitude of little maggots which feed on the moist, odorous portion that was protected from the direct rays of the sun, together with a number of dark-coloured pupæ that lie at the very bottom of the heap or buried in the sand below. These are two stages of the black fly (Cœlopa frigida) that is so attentive to us when we rest on the dry sand above high-water mark. This fly is very like the common house-fly in general appearance, though its body is rather smaller. Other species of the same genus often accompany them, all being very similar in general appearance and habits, and none of the larvæ seem adapted to a life in the water. They are always found beyond the reach of the tide, and are drowned if submerged for any length of time.

Another species belonging to the genus Actora will often be seen in the same company, and this is readily distinguished by their lighter greyish colour and its superior size. Also, along the water-line, we often meet with species of the family Dolichopodidæ, so called on account of the length of their legs, and noted for the beautiful metallic colours which adorn their bodies. These flies are carnivorous in habit, deriving their food from living as well as from freshly killed animals, and their short, fleshy proboscis contains a piercing bristle by which they can puncture the skins of the animals that provide them with food. Most of the flies of this group live on trees, walls, fences, &c., where they pursue and attack their prey, but certain species follow the line of breakers on the sea shore, as before indicated, and obtain their food from the various marine animals that are stranded on the beach. A peculiar feature of the family is the nature of the abdomen of the males, which is bent under the body and furnished with a number of appendages.

Another marine dipterous insect is a gnat-like fly closely allied to Chironomus, which we have described in a former work[*] of this series dealing with fresh-water life; and it will be sufficient to mention here that Chironomus is commonly known as the window-gnat on account of the frequency with which it may be seen flying on the windows of our dwellings; also that the larva, known popularly as the bloodworm, is truly aquatic in habit, being able to swim by rapidly looping its body in opposite directions, and being provided with a breathing apparatus adapted for the absorption of the oxygen gas contained in solution in water. The larva of the marine species referred to above may sometimes be seen in rock pools, where it shelters itself among the sediment at the bottom. It is much like the bloodworm in appearance and structure, but its body is greenish instead of red.

The last order of insects calling for notice here is the Coleoptera or sheath-winged insects, popularly known as beetles, and characterised by the hard and horny nature of the front pair of wings (elytra), which are modified into sheaths and serve to protect the second pair; the latter are thin and membranous, usually adapted for flight, and lie folded beneath the former when not in use.

One large section of beetles is known as the Geodephaga or Ground Beetles—a group of very predaceous insects that burrow into the soil and attack almost every living thing that comes in their way, and well represented by numerous species that may be found in our gardens, and, in fact, almost everywhere.

A considerable number of these insects show a decided preference for salt marshes and the sea shore, where they hide under stones, or burrow into the sand or mud in search of their prey. They are not marine in the strictest sense of the word, for they are not adapted for a life of submersion in water, either in the larval or in the perfect condition; yet they are often found below high-water level, and some species burrow into the sand of the beach as the tide advances, allowing the water to cover them for hours together.

One interesting family of the ground beetles (the Bembidiidæ) includes several small species, all of which frequent salt and wet places, such as salt marshes, the mouths of rivers, and the sea shore. We give enlarged illustrations of a few of these, the actual size being denoted at the side of each.

Bembidium biguttatum may be identified by its brilliant bronze-green colour, and the two distinct impressions on the elytra which have suggested the specific name. B. pallidipenne is, as its name implies, a pale-winged species, the elytra being of a light yellowish colour. B. fumigatum is so called on account of the smoky tint of the elytra; and the last species of the same genus figured (B. quadriguttatum) may be known by the four conspicuous spots on the deep violet-coloured outer wings.

The same family contains an interesting little beetle—Cillenium laterale—only about one-sixth of an inch in length, that lives among the refuse washed on the beach, where it feeds on the sandhoppers; and although the latter are so much superior in size, the beetle has no difficulty in holding and killing its prey, always seizing it on the ventral side of the body, which is less protected by the hardened skin. This species, which is of a copper colour, does not confine its ravages to that portion of the beach which is above high-water mark, but often allows itself to be covered by the advancing tide, remaining submerged for a considerable time. Another species—Aëpus (Æpys) marinus—is even more aquatic in its habits, for it searches out its prey among stones, chiefly at the mouths of rivers, below high-water level, and is often submerged for hours together. It is even provided with air-sacs to enable it to withstand such prolonged submersions.

There is another section of beetles which has elytra so short that they cover only a small portion of the abdomen; but although so short, these elytra completely cover the long membranous wings, which are folded up beneath them in a wonderfully compact manner. The section referred to is termed Brachelytra, from the feature just mentioned, and includes a few species that are more or less marine in their habits. One of them—Micralymma brevipenne—lives under stones below high-water level, and apparently passes through all its stages within reach of the waves. Another of the Brachelytra (Bledius) burrows into the sand or mud near high-water mark, throwing up the débris as it proceeds. Both these beetles are carnivorous, and the latter is in turn preyed upon by a ground beetle of the genus Dyschirius, which hunts and devours it within its own home.

The reader will have observed that the sub-kingdom Arthropoda is not only a very extensive one in the sense that it contains a vast number of animal forms, but also that its members exhibit a very great variety of form and structure; and the beginner will probably find no little difficulty in locating his specimens in their correct position in the scale of life. The following table, however, will serve to show the general classification of the group at a glance, and thus form a basis for a more detailed study at any future time:—

SUB-KINGDOM ARTHROPODA

CLASSIFICATION