CHAPTER X

CRUSTACEA AS PARASITES AND MESSMATES

The life of every animal is in more or less intimate relation with that of all the living creatures which surround it. Some serve for its food, or supply it with shelter or foothold; others prey upon it, or compete with it for the necessaries of life; and others, again, influence it for good or evil in countless ways more subtle than these, but equally important. There are some a.s.sociations of a closer and more enduring nature, to which the names of Symbiosis, Commensalism, and Parasitism, are applied, and it is with examples of these that the present chapter is concerned.

The term Symbiosis is strictly applied to an intimate physiological partnership, such as we find in some of the lower animals and plants, and in this sense there are no truly symbiotic Crustacea. The word, however, is sometimes used, in its literal sense of a "living together,"

to embrace all cases of animals living together for mutual advantage.

Commensalism means, literally, "sitting at the same table," and ought to be applied only to cases where two or more animals, living together as "messmates," partake of the same food; but it is sometimes used more loosely to include instances where one of the animals does not actually share in the food-supply of the other. Parasitism, again, implies that the parasite lives permanently at the expense of its host, by sucking its juices or otherwise, and in this case also there are innumerable degrees and varieties of dependence, which defy inclusion in a strictly logical scheme of cla.s.sification. Even such typical parasites as Tape-worms, for example, might strictly be regarded as commensals, sharing in the host's food only after it has entered the alimentary ca.n.a.l. Finally, in all these kinds of interrelation, we find cases where the a.s.sociation is temporary, intermittent, or almost accidental, and where there are no perceptible adaptations of structure directed to its maintenance in either of the partners. From these we may trace a series of gradations leading to cases where the a.s.sociated organisms are never found apart, and where the structure of both is profoundly modified in adaptation to the particular form of a.s.sociation.

Perhaps the simplest form of a.s.sociation between two animals is found where one utilizes the other as a means of transport. The little Gulf-weed Crab, previously mentioned, is very often found clinging to the carapace or skin of large marine Turtles. It is not a parasite, since it can hardly derive any food from the Turtle itself; neither is it a commensal, for there is no evidence that it shares in the Turtle's meals. It probably takes to a Turtle, when it can find one, as giving it a wider range of operations than is afforded by its usual drift-log or tuft of sarga.s.so-weed. A somewhat similar case is afforded by some of the Barnacles that are found on the skin of Whales. The species of _Conchoderma_, for instance, are often found on certain Whales, but they may also occur on inanimate floating objects. Other Whale-infesting Cirripedes, however, are specially adapted to their habitat, and never occur elsewhere. For example, _Coronula_ (Plate XXVIII.) is a genus of sessile Barnacles in which the sh.e.l.l is elaborately folded, forming a series of chambers into which prolongations of the Whale's epidermis grow, securely fixing the sh.e.l.l.

_Tubicinella_ is even more effectively protected against dislodgment, for its sh.e.l.l is sunk in the thickness of the Whale's skin, with only the opening exposed. Other genera of sessile Barnacles (_Chelon.o.bia_, etc.) are found adhering to the sh.e.l.l of Turtles. The increased food-supply made available by the host's movements through the water is probably the chief advantage that the Barnacles gain in such cases. This is indicated by the fact that certain small stalked Barnacles (_Dichelaspis_, etc.), found on large Crabs and Lobsters in tropical seas, generally cl.u.s.ter on the mouth parts of their hosts, near the entrances to, or even within, the gill chambers, profiting no doubt by the respiratory currents and the food particles they carry.

[Ill.u.s.tration: _PLATE XXVIII_

GROUP OF BARNACLES, _Coronula diadema_, ON THE SKIN OF A WHALE.

j.a.pAN. (REDUCED)]

A great variety of Crustacea find shelter and defence in a.s.sociation with Sponges, Corals, and other more or less sedentary animals. Sponges are not eaten by many marine animals, the needle-like spicules which often form their skeleton no doubt helping to render them distasteful, and many small Crustacea, Amphipods, Isopods, Prawns, etc., profit by their immunity from attack, and take up their abode in the internal channels and cavities of the Sponge. The beautiful siliceous Sponge known as "Venus's Flower-basket" (_Euplectella_) very often contains imprisoned within it specimens of a delicate little Prawn (_Spongicola venusta_) or of an Isopod (_aega spongiophila_). As these Crustacea share with the Sponge the food particles drawn in by the currents of water pa.s.sing through the pores in its walls, they are in the strict sense commensals.

The Corals and various other animal organisms commonly known as "Zoophytes," forming together with the Jellyfishes the group Coelentera, are very effectively protected against the attacks of most predatory animals by the possession of "stinging cells," and this protection is shared by many other animals which shelter among them. Thus, the branching Coral stocks which grow in great luxuriance on tropical coasts support a rich and varied a.s.semblage of animals, some of which may actually prey upon the Coral polypes, but all of which profit by the fact that few enemies venture to pursue them in their retreats.

Innumerable prawn-like animals of the Alpheidae and other families, and many kinds of Crabs, are found among living Corals. The Crabs of the family Trapeziidae are especially characteristic of such habitats, and their thin, flat bodies seem to be adapted to slip into slits and crannies of the Coral blocks. The most highly specialized of all Coral Crabs, however, are the species of the family Hapalocarcinidae, which modify in various ways the growth of the corals on which they live. In some of the more delicately branched kinds of Coral there may sometimes be found hollow bulbous growths, each of which contains imprisoned within it a little Crab--_Hapalocarcinus marsupialis_ (Fig. 66). It seems that the female Crab (the habits of the male are not definitely known) settles down among the branches of the Coral, and that the irritation of its presence causes the branches to grow up and surround it, coalescing with each other to form a kind of cage, and ultimately leaving only one or two small openings. Through these openings water can enter to enable the Crab to breathe, and no doubt food particles find their way in, but it is not possible for the Crab to leave its prison.

The production of these abnormal growths of the Coral is closely a.n.a.logous to the formation of "galls" on plants as a result of the irritation set up by the presence of insect larvae or other parasites, and it is not inappropriate, therefore, to speak of them as "Coral galls."

[Ill.u.s.tration: FIG. 66--TWO BRANCHES OF A CORAL (_Seriatopora_) SHOWING "GALLS" INHABITED BY THE CRAB _Hapalocarcinus marsupialis_. ON THE RIGHT THE FEMALE CRAB, EXTRACTED FROM THE GALL AND FURTHER ENLARGED]

The Medusae, or Jellyfishes, like other Coelentera, are provided with poisonous stinging cells, which, in the larger species of our own seas, are powerful enough to cause discomfort to bathers who come in contact with them. The protection thus afforded is no doubt of advantage to the little globular Amphipods of the genus _Hyperia_ (Fig. 67), which are almost always to be found sheltering under the bells of the larger Medusae. In what way the Amphipods escape injury from the stinging cells of their host is not known.

[Ill.u.s.tration: FIG. 67--_Hyperia galba_, FEMALE. ENLARGED. (After Sars.)]

In all the cases mentioned, the advantages of the partnership seem to be all on one side, but there are numerous instances in which both partners seem to reap some benefit. A species of Hermit Crab very common in moderately deep water on many parts of the British coasts, _Eupagurus prideauxi_, is always found to have a Sea-anemone (_Adamsia palliata_) attached to the sh.e.l.l which it carries. The Anemone has a broad base which is wrapped round the sh.e.l.l, the mouth, surrounded by the tentacles, being on the under-side next the opening of the sh.e.l.l. There seems no reason to doubt that the presence of the Anemone does afford some degree of protection to the Hermit, and that, on the other hand, the Anemone benefits by being carried about, and shares in the crumbs from the Hermit's meals. It is stated that, when the Hermit removes to a new sh.e.l.l, it detaches the Anemone from the old sh.e.l.l with its pincers and places it in position on the new one. It appears, however, that it is not always necessary for the Hermit to remove to a larger sh.e.l.l as it grows, for the enveloping Anemone, as it increases in size, extends beyond the mouth of the sh.e.l.l, and so enlarges the shelter. Further, the Anemone in course of time dissolves the sh.e.l.l almost entirely away, and the Hermit is enveloped only by the soft fleshy mantle which it forms.

In a similar way the deep-sea Hermit Crab _Parapagurus pilosima.n.u.s_ (see Plate XVI.) is always found lodged in a fleshy ma.s.s formed by a colony of Sea-anemones (_Epizoanthus_), within which, when it is cut open, may be found the remains of the sh.e.l.l which the Hermit first inhabited. A further development of the same habit is given by _Paguropsis typica_, found in deep water in Indian seas, which does not inhabit a sh.e.l.l at any time, but carries a fleshy blanket formed by a colony of Anemones.

In dredging off the British coasts, we often find smooth rounded lumps of a Sponge (_Suberites ficus_), generally yellowish-grey in colour, having a round opening in which the claws of a small Hermit Crab (_Eupagurus cuanensis_) may be seen. On cutting open the Sponge, the body of the Hermit is seen to be lodged in a spiral cavity, and at the apex may be found the remains of a sh.e.l.l that has been corroded away by the Sponge which settled on and replaced it. Other species of Hermit Crabs constantly have their sh.e.l.ls covered with a h.o.r.n.y crust formed by Hydroid zoophytes (_Hydractinia_, etc.), and in this case also the extension of the Hydroid colony beyond the lip of the sh.e.l.l relieves the Hermit from the necessity of so frequently changing to a larger sh.e.l.l as it grows.

A number of other animals are found a.s.sociated with Hermit Crabs, without, as far as we can see, rendering any service in return for the house-room. The Whelk-sh.e.l.ls inhabited by _Eupagurus bernhardus_ (see Plate VII.) often contain one of the bristle-footed worms (_Nereilepas fucata_), which may sometimes be observed to protrude its head from the sh.e.l.l when the Crab is feeding, and to s.n.a.t.c.h away fragments of the prey from the very jaws of its host. It is thus, in the strict sense of the word, a commensal. Species of Copepods, Amphipods, Porcelain Crabs, and even a Mysid, have been found sharing the lodging of Hermit Crabs in a similar way, and in addition there are various parasites, presently to be mentioned, found on the Crabs themselves, so that each Crab forms the centre of a whole community of widely diverse organisms all more or less directly dependent on it.

A habit similar to those of some Hermit Crabs is that of the Crab _Dromia_ (see Plate IX.), mentioned in a previous chapter, which carries, as a cloak, a ma.s.s of living sponge, holding it in position by means of the last two pairs of legs. Even the "masking" habit of the Spider Crabs, already described (p. 96), may be regarded as a kind of symbiosis, since the sponges, zoophytes, etc., which grow on the Crabs no doubt benefit by being carried about in return for the protection they give.

[Ill.u.s.tration: FIG. 68--A, THE CRAB _Melia tessellata_ CLINGING TO A BRANCH OF CORAL, AND CARRYING IN EACH CLAW A LIVING SEA-ANEMONE; B, ONE OF THE CLAWS FURTHER ENLARGED TO SHOW THE WAY IN WHICH THE ANEMONE IS HELD. (After Borradaile.)]

One of the strangest habits is that of certain little tropical Crabs, of which _Melia tessellata_ (Fig. 68) is the best known, which carry in each claw a living Sea-anemone and use it as a weapon. The claws or chelipeds are in this case of small size, so that they would be of little use by themselves for attack or defence; but the fingers are provided with recurved teeth, enabling them to take a firm hold of the slippery body of the Anemone. Particles of food caught by the tentacles of the Anemone are removed and eaten by the Crab, which uses for the purpose the long walking legs of the first pair. The same limbs are also used in the process of detaching the Anemones from the stone on which they may be growing. The Anemones do not appear to suffer from the rough treatment to which they are subjected, but whether they can reap any benefit from the partnership is very doubtful.

[Ill.u.s.tration: FIG. 69--THE COMMON PEA CRAB (_Pinnotheres pisum_), FEMALE. NATURAL SIZE.]

From remote antiquity it has been known that a little Crab (Fig. 69) is frequently found living within the sh.e.l.ls of bivalve Molluscs, such as Oysters, Mussels, and especially the large mussel-like _Pinna_, which is common in the Mediterranean. Ancient writers regarded this as a case of a.s.sociation for mutual advantage, believing that the _Pinnotheres_ warned the _Pinna_ of the approach of enemies or of the entrance of prey between its gaping valves. It is even stated that the Pinna and Crab were depicted in Egyptian hieroglyphics to symbolize the dependence of a man on his friends.

As a matter of fact, however, there is no reason to believe that the Molluscs which harbour species of _Pinnotheres_ and allied genera benefit in any way by the presence of the Crabs. The latter probably feed, as their hosts do, on particles brought in by the current of water entering the mantle cavity. They are therefore strictly "commensals,"

though it is usual, and perhaps equally correct, to speak of them as "parasites." The case is, indeed, an example of the difficulty of defining these two terms. At all events, the Pinnotherid Crabs show one of the characteristics of parasites in being to some extent degenerate in their structure. The carapace and the rest of the exoskeleton, no longer needed for protection, have become soft and membranous, and the eyes and antennules, the chief organs of sense, are very minute. As in many parasites, also, the eggs produced by the female are very numerous, and the abdomen is very broad and deeply hollowed for their reception.

While most of the Pinnotheridae live in bivalve Molluscs, some species are a.s.sociated with other invertebrate animals. _Pinnaxodes chilensis_ is found in a species of Sea-urchin (_Strongylocentrotus gibbosus_) on the coast of Chili. On opening the sh.e.l.l of the Urchin, the Crab is found enclosed in a thin-walled bag formed by enlargement of the terminal part of the host's intestine.

It did not escape the notice of Aristotle that a little Shrimp sometimes occurred in the _Pinna_ in place of the Crab. This is _Pontonia custos_, and other species of the same and allied genera have similar habits.

The order Isopoda includes a very large number of parasitic species. The extensive family Cymothoidae presents a whole series of gradations in habits and structure between actively swimming predatory species and others which in the adult state are permanently fixed to their host, usually a fish, and are incapable of movement. At one end of the series are the species of _Cirolana_, which have powerful biting jaws. Of _C.

borealis_ (Fig. 70), Mr. Stebbing remarks that "it is a good swimmer, tenacious of life, a savage devourer of fish, and not to be held in the human hand with impunity." The species is not uncommon in British seas, and numerous individuals will sometimes attack a Cod or other large fish, perhaps after it has been caught on a hook, and gnaw their way into its body, so that when brought to the surface the fish consists of little more than skin and bone.

[Ill.u.s.tration: FIG. 70--_Cirolana borealis._ ABOUT TWICE NATURAL SIZE.

(After Sars.)]

The little _Eurydice achatus_, belonging to the same subfamily, Cirolaninae, is commonly taken in the tow-net in sandy bays on our own coasts. It is said sometimes to attack bathers, and to "nip most unpleasantly."

More definitely parasitic are the species of _aega_ and allied genera, which have piercing and suctorial mouth parts, and suck the blood of fish. They are usually found adhering closely to the skin of their victim by means of the strong hooked claws of the anterior pairs of legs; but they have not lost the power of locomotion, and, as females bearing eggs are never taken on fish, it would appear that they drop off after gorging themselves with blood, and probably seek a retreat at the bottom of the sea, where they may hatch their young in safety. The digestive ca.n.a.l of _aega_ dilates into a large bag, which becomes distended with a semi-solid ma.s.s of blood. This ma.s.s, when extracted and dried, is the "Peter's stone" of old Icelandic folklore, to which magical and medicinal virtues were attributed. The species _aega spongiophila_, already mentioned, differs in its habits from all the other species of the genus, since it lives, not on fish, but in the interior of a sponge.

[Ill.u.s.tration: _PLATE XXIX_

_Cymothoa oestrum_, AN ISOPOD PARASITE OF FISH (SLIGHTLY ENLARGED)

_Sacculina carcini_ ATTACHED UNDER THE ABDOMEN OF A COMMON Sh.o.r.e-CRAB (REDUCED)]

The most completely parasitic members of the Cymothoidae are found in the subfamily Cymothoinae, including the typical genus _Cymothoa_ (Plate XXIX.) and many others. The adult animals are found clinging to the skin of fishes, the legs being provided with strong hook-like claws that give them a very firm hold. Some species, especially common on Flying-fishes, cling to the tongue of the fish, and almost prevent it from closing its mouth. When young, the Cymothoinae swim freely, and the shape of the body is not unlike that of the Cirolaninae; but after they have settled on a host the body often becomes distorted and unsymmetrical. A still more remarkable change occurs in the reproductive organs in some, if not in all members of this subfamily. Each individual, when it first attaches itself to a host, presents the characters of the male s.e.x.

Later it becomes a female, develops a brood-pouch, and produces eggs.

The animals are, in fact, hermaphrodite; but it is to be noted that the hermaphroditism is of a different kind from that presented by the Cirripedia, since the organs of the two s.e.xes are successively, not simultaneously, developed. Where, as in this case, the male phase comes first in the life-history of the individual, the condition is known as "protandrous" hermaphroditism.

Another large group of parasitic Isopods is the suborder Epicaridea, all the species of which are parasitic on other Crustacea. It is not uncommon to find specimens of the common Prawn (_Leander serratus_) which have a large swelling on one side of the carapace. If the lower edge of the carapace be raised, it will be seen that this swelling is due to the presence in the gill cavity of an Isopod parasite (_Bopyrus squillarum_). A closely similar form, found on Prawns of the genus _Spirontocaris_, is _Bopyroides hippolytes_, represented in Fig. 71.

Other allied species are found on Hermit Crabs and other Decapods. When extracted, the parasite is seen to have a flat and curiously distorted body, with extremely short legs ending in hooked claws. The under-side is generally occupied by a relatively enormous ma.s.s of eggs, which is only partly covered in by the small brood-plates. The mouth parts form a short piercing beak with which the parasite sucks the blood of its host. On the under-side of the abdomen may usually be found the minute male, attached, like a secondary parasite, to the body of the female.

[Ill.u.s.tration: FIG. 71--A, FRONT PART OF BODY OF A PRAWN (_Spirontocaris polaris_), FROM ABOVE, SHOWING ON THE RIGHT SIDE A SWELLING OF THE CARAPACE CAUSED BY THE PRESENCE OF THE PARASITE _Bopyroides hippolytes_ IN THE GILL CHAMBER; B, THE FEMALE PARASITE EXTRACTED AND FURTHER ENLARGED; C, THE MALE PARASITE ON SAME SCALE AS THE FEMALE. (After Sars.)]

The species of Epicaridea are very numerous, and they infest Crustacea belonging to nearly all the chief groups of the cla.s.s, a few even being parasitic on other Epicaridea. Many of them differ greatly from the _Bopyrus_ just described, and in some cases it would be impossible to guess from the structure of the adult animals that they were Isopoda, or even Crustacea at all. The life-history is not yet completely known.

When hatched from the egg, the free-swimming larvae have a short and broad body, and, as in other Isopod larvae, have only six instead of seven pairs of legs. A later larval stage, just before attachment to the final host, has a long narrow body and the full number of legs. It has lately been shown, however, that, in all probability, between these two free-swimming stages there intervenes a stage in which the larvae is temporarily parasitic on certain Copepoda. Further, some of the Epicaridea, like the Cymothoinae described above, are protandrous hermaphrodites, developing the male organs when in the last larval stage, and pa.s.sing into the female phase after they have become attached to the host. In _Bopyrus_ and many other genera, however, there is no evidence that the males ever develop into females.

Some of the most remarkable Epicaridea are those belonging to the family Entoniscidae, which are parasitic on Crabs. In these the parasite penetrates from the gill chamber into the interior of the body of the host, remaining enveloped, however, by a delicate membrane which grows in with it from the wall of the gill chamber. The body is distorted in an extraordinary fashion, so that at first sight it seems impossible to trace any resemblance to the form of a typical Isopod.

Among the Amphipoda there are a few species belonging to various families of the Gammaridea which have suctorial mouth parts, and lead a semi-parasitic existence; but the only completely parasitic forms are the Whale-lice, forming the family Cyamidae (see Fig. 23, p. 55) in the suborder Caprellidea. Although differing greatly in the broad, flattened shape of the body from the slender, thread-like Caprellidae, they closely resemble them in structure, particularly in having the abdomen reduced to a mere k.n.o.b. The fourth and fifth pairs of thoracic limbs have disappeared, although the gills corresponding to them are very large; and the last three pairs of legs have long curved claws which enable the Whale-louse to cling firmly to the skin of its host. The mouth parts are adapted for biting, not for sucking blood, and the animals seem to live by gnawing the skin of the Whales. In one respect the Whale-lice are unique among Crustacean parasites: they have not the power of swimming at any period of their life-history. The young settle down near their parents, and ma.s.ses of many hundred individuals of all sizes are found clinging close together on the skin of the host.

No group of Crustacea exhibits more numerous or more varied examples of parasitism than the Copepoda. Every grade of transition between a free predatory habit of life and the most complete dependence upon a host may be traced in various families of the subcla.s.s. Only a few examples can be mentioned here.

[Ill.u.s.tration: FIG. 72--A FISH-LOUSE (_Caligus rapax_), FEMALE. 5.

(After Wilson.)]

The commonest "fish-lice" are the numerous species of the family Caligidae, many of which, belonging to the genera _Caligus_ (Fig. 72), _Lepeophthirus_, etc., are found on marine fishes on our own coasts. In these the body is broad and flat, but in many of them the resemblance, even in general form, to the free-living Copepoda is easily traceable.

The maxillipeds form powerful hooked claws, by means of which the animals cling to the skin of the fish they infest, and in _Caligus_ the basal segments of the antennules have a pair of suckers which aid in adhesion. The mouth parts are adapted for piercing, and are enclosed in a suctorial proboscis.

When the young Caligid, after pa.s.sing through the free-swimming larval stages, first becomes attached to a fish, it adheres by means of a thread-like process issuing from the front of the head, and formed by the secretion of a gland. At this stage, formerly described as an independent species under the generic name of _Chalimus_, the parasite is unable to detach itself from its host; but later, in many species, it re-acquires the power of swimming, and specimens of _Caligus_, for instance, are commonly found free in tow-net gatherings.

[Ill.u.s.tration: FIG. 73--STAGES OF DEVELOPMENT OF _Lernaea branchialis_. F IS SLIGHTLY, THE OTHER FIGURES GREATLY, ENLARGED. (After A. Scott.)