CHAPTER LXV

ASSOCIATION OF ANIMALS-MESSMATES OR
COMMENSALS (COMMENSALISM)

Messmates are organisms of different species which are more or less closely associated, to the benefit of at least one partner in the concern. Cases of this sort are grouped under the head of Commensalism. Mutualism (Symbiosis) is a much more intimate kind of relation between two organisms, to the advantage of both, as already described. The best examples of such Mutualism or Symbiosis involve a partnership for certain cases where plants. and animals are thus associated (see p. 75). It is doubtful whether any two kinds of animal live together in this intimate fashion. Parasites are animals which live on or in other animals, at their expense, and to their detriment. Parasitism also includes cases where one organism concerned is a plant (see p. 76).

In a broad sense all the animals which live and feed together in the same place may be regarded as messmates, and the relations between such species may be very complex. It will, however, be well to restrict the term to cases where the connection is of closer and more constant nature, involving the interests of definite species. But it must not be forgotten that this kind of association has no doubt gradually arisen from relations which were originally of more casual kind. So many instances of Commensalism are known that it will only be possible to describe a few of the more striking examples.

FISHES (PISCES) AS MESSMATES

Some extraordinary cases have been described where small bony fishes take up their quarters within the digestive organs of lower animals, sallying forth from these peculiar refuges as circumstances dictate. The most familiar instance of this is

afforded by a slender form (Fierasfer) living in the gullet of a kind of Sea-Cucumber, which does not appear to gain anything by way of return for its hospitality. Some of the giant seaanemones living on the Great Barrier Reef of Australia harbour gaily-coloured little fishes belonging to the Perch family, a given species of anemone being the home of a particular species of fish. The fish-guest (Amphiprion percula) of one such obliging zoophyte (Discosoma Kenti) is orange-red in colour, marked by three cross-bands of pearly white, these and the fins being edged with black. An allied anemone (D. Haddoni) entertains a little fish (A. bicinctus) which differs from its relative in possessing two bands only, while the black edging is absent.

The same anemone also extends its hospitality to a red-andwhite Prawn (Palamon). In these cases the fishes not only find a secure shelter, the stinging properties of which ward off attack, but also probably filch some of the food of their living homes. On the other hand, it is possible, as suggested by Saville Kent, who has described the associated animals, that the bright tints of the guests serve as "lure colours", enticing animals which serve as food for the anemones.

The relations just described are occasionally reversed, as when a fish serves as a moving home to zoophytes. Alcock describes a Rock Perch (Minous inermis, fig. 1124), native to the Indian Ocean, as being always more or less encrusted with small polypes (Stylactis minoi), which, being of course carried about from place to place, have a better chance of getting abundant food than if they were attached to a stone or sea-weed. The fish may perhaps derive some protection from the stinging properties of its guests.

MOLLUSCS (MOLLUSCA) AS MESSMATES

A marine snail (Pleurotoma symbiotes, fig. 1125), living in the deep water of the Indian Ocean, always has its shell more or less encrusted with colonial sea-anemones (Epizoanthus). Both animals are no doubt benefited, for the mollusc is protected, while the anemones are carried about.

A number of small Bivalve Molluscs are associated with burrowing Sea-Urchins or Crustaceans. One such bivalve (Montacuta ferruginosa), native to South Devon, lives in the dwelling which a Heart-Urchin (Echinocardium cordatum) excavates in muddy sand. The circulation of sea-water which takes place

within the burrow (see vol. iii, p. 357) ensures a constant supply of food by which the mollusc benefits. In places where the sand is loose and wet the Heart-Urchin is in the habit of coming to the surface, along which it makes its way, but the lodger is not thereby left behind, for it spins byssus threads that attach it to its partner.

A rare little British Bivalve (Lepton squa

Fig. 1125.-An Indian Sea mosum) inhabits the burrow of a prawn-like Crustacean (Upogebia stellata), and, having an exceedingly flat shell, does not interfere with the movements of its protector. A similar partnership exists on the coast of Florida between two species related to the preceding, while on the shores of Oregon and California a third association of the sort is more intimate, for here the Lepton attaches itself to the abdomen of the Upogebia. A burrowing Australian prawn (Axius plectorhynchus) harbours two species of a kind of bivalve (Ephippodonta), which is never found elsewhere. The flatness, so necessary to allow of the restless movements of the prawn, is here produced by the valves of the mollusc opening to their fullest extent. This particular prawn appears to be a specialist in the matter of providing lodgings, for four other bivalves (one species of Kellia and three of Mylitta) find a commodious home in its burrow, which also contains an orange-coloured sponge. The last possibly serves as a protection to the crustacean, but the arrangement would appear to be quite one-sided so far as the molluscs are concerned.

As with Fishes (see p. 171), Bivalve Molluscs are not always lodgers in the case of partnerships, but may afford shelter to weaker creatures. A well-known instance is that of the little rounded Lodger-Crabs (Pinnotherida), in which the eyes have undergone great reduction. Among bivalves which provide them with homes may be mentioned Horse-Mussels (Modiola), Oysters (Ostrea), Pinnas, and Tridacnas, while some crabs of the kind take up their quarters within ascidians or sea-cucumbers. Van Beneden thus speaks of these little lodgers (in Animal Parasites and Messmates):-"It is not a taste for voyaging which tempts them, but the desire of having always a secure retreat in every place. The pinnothere is a brigand who causes himself to be followed by the cavern which he inhabits, and which opens only at a well-known watchword. The association redounds to the advantage of both; the remains of food which the pinnothere abandons are seized upon by the mollusc [or, rather, some of its remains may be carried by ciliary action into the mouth of the mollusc]. It is the rich man who instals himself in the dwelling of the poor, and causes him to participate in all the advantages of the position. The pinnotheres are, in our opinion, true messThey take their food in the same waters as their fellowlodger, and the crumbs of the rapacious crabs are doubtless not lost in the mouth of the peaceful mussel. There is no doubt that these little plunderers are good lodgers, and if the mussels furnish them with an excellent hiding-place and a safe lodging, they themselves profit largely by the leavings of the feast which fall from their pincers. Little as they are, these crabs are well furnished with tackle, and advantageously placed to carry on their fishery in every season. Concealed in the bottom of their living dwelling-place (a den which the mussel transports at will) they choose admirably the moment to rush out to the attack, and always fall on their enemy unawares. Some of these pinnotheres live in all seas, and inhabit a great number of bivalve molluscs." The habits of these curious little crabs attracted attention in remote times, and have been the subject of much curious speculation. Stebbing (in A History of Crustacea) makes the following remarks upon the ancient views, and discusses the origin of the commensal habit:-"The name Pinnoteres means one that watches or guards the Pinna, and there can be little doubt that it was the form used by Aristotle

mates.

[and not Pinnotheres, meaning one that 'hunts the Pinna'], seeing that he also speaks of Pinnophylax, a word of precisely the same meaning. Not only Aristotle, but many succeeding writers of renown, such as Cicero, Pliny, and seemingly Linnæus himself, accepted the opinion that there was a compact between the mollusc and the crustacean for their mutual benefit. Whenever little fishes swam in between the expanded valves of the mollusc, it was supposed that its companion gave it a little friendly nip, upon which the valves snapped together, the prey was secured, and shared between the confederates. A similar policy was pursued to exclude the intrusion of a dangerous foe. The great antiquity of the belief is attested by the fact that the Egyptians in their hieroglyphics made use of the pinna and crab to symbolize the helplessness of a man without friends. That the belief was untenable was pointed out by many naturalists, from Gesner down to Cuvier, on the ground that molluscs do not feed on little fishes, and that the residence of the crabs within the valves was sufficiently explained by the prevailing softness of the carapace in this family. This indeed applies chiefly to the females, and it is the females that appear to be most frequently found thus domiciled. It is so much the nature of crustaceans to take refuge in any sort of cleft or cranny that the first entrance of the Pinnotheres into any sort of bivalve can be easily understood. When the residence proved to be peculiarly secure, the shell of the crab would by degrees lose a hardness that was no longer especially necessary. That the crab may at times be useful to the mollusc seems after all not so very improbable, for at the approach of an enemy so nervous a creature as a crab would no doubt begin to scuttle about, and in this way communicate its terror to its more apathetic companion, which would then naturally close its doors against the danger.'

JOINTED-LIMBED ANIMALS (ARTHROPODA) AS
MESSMATES

We are here especially concerned with Insects and Crabs, regarding which groups there is a great wealth of material from which to select, so that only a few examples can be here given, supplementing, for the latter animals, what has just been said about Pinnotheres.