Part 11 (1/2)
CHAPTER X
CRUSTACEA AS PARASITES AND MESSMATES
The life of every animal is increatures which surround it Some serve for its food, or supply it with shelter or foothold; others prey upon it, or coain, influence it for good or evil in countless ways more subtle than these, but equally important There are so 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 terical partnershi+p, 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 soether,”
to eether for e
Co at the saht to be applied only to cases where two or ether 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 Parasitisain, implies that the parasite lives per its juices or otherwise, and in this case also there are innurees and varieties of dependence, which defy inclusion in a strictly logical scheme of classification Even such typical parasites as Tape-worarded as co in the host's food only after it has entered the alimentary canal Finally, in all these kinds of interrelation, we find cases where the association is temporary, intermittent, or almost accidental, and where there are no perceptible adaptations of structure directed to its maintenance in either of the partners Fro to cases where the associated organisms are never found apart, and where the structure of both is profoundly modified in adaptation to the particular form of association
Perhaps the simplest form of association between two animals is found where one utilizes the other as a means of transport The little Gulf-weed Crab, previouslyto 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 sargasso-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 theyobjects Other Whale-infesting Cirripedes, however, are specially adapted to their habitat, and never occur elsewhere For exaenus of sessile Barnacles in which the shell is elaborately folded, forations of the Whale's epider the shell
_Tubicinella_ is even ment, for its shell is sunk in the thickness of the Whale's skin, with only the opening exposed Other genera of sessile Barnacles (_Chelonobia_, etc) are found adhering to the shell of Turtles The increased food-supply h the water is probably the chief advantage that the Barnacles gain in such cases This is indicated by the fact that certain se Crabs and Lobsters in tropical seas, generally cluster on the mouth parts of their hosts, near the entrances to, or even within, the gill cha no doubt by the respiratory currents and the food particles they carry
[Illustration: _PLATE XXVIII_
GROUP OF BARNACLES, _Coronula diadema_, ON THE SKIN OF A WHALE
japAN (REDUCED)]
A great variety of Crustacea find shelter and defence in association with Sponges, Corals, and other es are not eaten by many marine animals, the needle-like spicules which often for 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 speciicola venusta_) or of an Isopod (_aega spongiophila_) As these Crustacea share with the Sponge the food particles drawn in by the currents of water passing through the pores in its walls, they are in the strict sense coanisether with the Jellyfishes the group Coelentera, are very effectively protected against the attacks ofcells,” and this protection is shared bythereat luxuriance on tropical coasts support a rich and varied assee 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 fa 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 hly specialized of all Coral Crabs, however, are the species of the farowth of the corals on which they live In some of the more delicately branched kinds of Coral there rowths, each of which contains imprisoned within it a little Crab--_Hapalocarcinus66) It seems that the female Crab (the habits of thethe 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 for only one or two ss 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 abnorous to the foralls” 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 thealls”
[Illustration: 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 thee to the little globular A 67), which are aler Medusae In ay the A cells of their host is not known
[Illustration: FIG 67--_Hyperia galba_, FEMALE ENLARGED (After Sars)]
In all the cases es of the partnershi+p 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 urus prideauxi_, is always found to have a Sea-anemone (_Adamsia palliata_) attached to the shell which it carries The Anemone has a broad base which is wrapped round the shell, theon the under-side next the opening of the shell There seems no reason to doubt that the presence of the Aneree of protection to the Hermit, and that, on the other hand, the Ane carried about, and shares in the crumbs from the Hermit's meals It is stated that, when the Hermit removes to a new shell, it detaches the Anemone from the old shell with its pincers and places it in position on the new one It appears, however, that it is not always necessary for the Herrows, for the enveloping Anemone, as it increases in size, extends beyond the es the shelter Further, the Anemone in course of time dissolves the shell almost entirely away, and the Hermit is enveloped only by the soft fleshy mantle which it forurus pilosied in a fleshy mass formed by a colony of Sea-anemones (_Epizoanthus_), within which, when it is cut open, may be found the remains of the shell which the Hermit first inhabited A further developuropsis typica_, found in deep water in Indian seas, which does not inhabit a shell at any time, but carries a fleshy blanket for off the British coasts, we often find senerally yellowish-grey in colour, having a round opening in which the claws of a surus cuanensis_) e, the body of the Hered in a spiral cavity, and at the apex may be found the ree which settled on and replaced it Other species of Hermit Crabs constantly have their shells covered with a horny crust formed by Hydroid zoophytes (_Hydractinia_, etc), and in this case also the extension of the Hydroid colony beyond the lip of the shell relieves the Herer shell as it grows
A number of other animals are found associated with Her any service in return for the house-roourus bernhardus_ (see Plate VII) often contain one of the bristle-footed worms (_Nereilepas fucata_), which may sometimes be observed to protrude its head fro, and to snatch 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 foranisms 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, ait in position by ” habit of the Spider Crabs, already described (p 96), es, zoophytes, etc, which grow on the Crabs no doubt benefit by being carried about in return for the protection they give
[Illustration: 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 knohich 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 theers are provided with recurved teeth, enabling them to take a firm hold of the slippery body of the Aneht 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 sa the Ane The Aneh treatment to which they are subjected, but whether they can reap any benefit from the partnershi+p is very doubtful
[Illustration: FIG 69--THE COMMON PEA CRAB (_Pinnotheres pisum_), FEMALE NATURAL SIZE]
Fro 69) is frequently found living within the shells of bivalve Molluscs, such as Oysters, Mussels, and especially the large mussel-like _Pinna_, which is coarded this as a case of association forthat the _Pinnotheres_ warned the _Pinna_ of the approach of ene 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 ”coh it is usual, and perhaps equally correct, to speak of them as ”parasites” The case is, indeed, an exa these two terms At all events, the Pinnotherid Crabs show one of the characteristics of parasites in being to soenerate in their structure The carapace and the rest of the exoskeleton, no longer needed for protection, have become soft and ans of sense, are very s produced by the female are very numerous, and the abdomen is very broad and deeply hollowed for their reception