The Evolution Theory, Vol. 1 of 2

by August Weismann

We shall take our first examples from the Animal Kingdom.

The partnership between certain sea-anemones (Actiniæ) and hermit-crabs (Paguridæ) had been noticed long before any particular attention was devoted to it. Many species of hermit-crab frequently carry a large sea-anemone about with them on the mollusc shell which they use as a protecting-house; indeed, two or three of these beautiful many-tentacled polyps are often attached to them, and this is not at all a matter of chance, but depends upon instinct on the part of both animals; they have the feeling of belonging to each other. If the sea-anemone be taken away from the hermit-crab and put in a distant part of the aquarium, the crab seeks about till it finds it, then seizes it with its claws and sets it on its house again. The instinct to cover itself with Actiniæ is so strong within it that it loads itself with as many of these friends as it can procure, sometimes with more than there is room for on the shell. The sea-anemone on its part calmly submits to the crab's manipulations—a fact very surprising to any one who is aware of the anemone's ordinarily extreme sensitiveness to contact, and knows how it immediately draws itself together on any attempt to detach it from the ground, and will often let itself be torn in pieces rather than give way. The mutual instincts of the two creatures are thus adapted to each other; but it does not at first sight seem as if any structural changes had taken place in favour of the partnership. This is true, indeed, as regards the hermit-crab, but not as regards the sea-anemone, although the nature of the adaptation on the sea-anemone's part only becomes apparent when the two animals are closely observed in their life together.

We owe our understanding of this adaptive change in the sea-anemone, and, indeed, our knowledge of this whole case of Symbiosis, to the beautiful observations of Eisig. Starting from the hypothesis that the mutual relations could only be the outcome of natural selection, Eisig pointed out that this partnership must offer some advantage not to one partner only, but to both; otherwise it could not have arisen through selection. The advantage to the sea-anemone is obvious enough; since of itself it can only move very slowly, and is usually firmly fixed in one place, it is easy to see that it would be useful to it to be carried about on the floor of the sea by the hermit-crab, and to get its share of the hermit-crab's food. But the service yielded to the hermit-crab by the sea-anemone in return is not nearly so apparent. Eisig made an observation in the Zoological Station at Naples which solved this riddle. He saw an octopus attack the hermit-crab and attempt to draw it out of its shell with the point of one of its eight arms. But before this had succeeded there sprang from the body of the sea-anemone a large number of thin worm-like threads which spread over the arm of the robber, who immediately let go his hold of the crustacean and troubled himself no further with it. These threads, called acontia, are thickly beset with stinging-cells, which must at least cause a violent smarting on the soft skin of the octopus. Thus we see that the Actinia instinctively defends its partner from attacks, and does it so effectively that we need not wonder how the instinct to provide itself with Actiniæ could have arisen in the hermit-crab. But the acontia seem to have been greatly strengthened in the course of the sea-anemone's association with hermit-crabs, for they do not occur in all forms, and they are most highly developed in those which live in Symbiosis with crustaceans.

Fig. 34. Hermit-crab (E), within a Gasteropod shell, on which a colony of Podocoryne carnea has established itself. From the common root-work (which is not clearly shown) there arise numerous nutritive polyps with tentacles (np), among which are smaller 'blastostyle' polyps with a circle of medusoid buds (mk), spine-like personæ (stp), and on the margin of the mollusc shell a row of defensive individuals (wp). F, antennæ. Au, eyes of the hermit-crab; slightly enlarged.

In this case the structural change, the transformation of the mesenteric filaments that occur in all Actiniæ into projectile acontia, is comparatively slight, but in another partnership between hermit-crabs and polyps the latter have undergone a much more marked adaptation. At Naples Eupagurus prideauxii is one of the commonest hermit-crabs. It lives at a depth of about a hundred feet, and is often brought to the Zoological Station by the fishermen in large quantities. Its borrowed mollusc shell often bears a little polyp, Podocoryne carnea (Fig. 34), which forms colonies of often several hundred individuals, arising from a common root-work of stolons which covers the shell. The polyp colony is composed of different kinds of individuals or personæ, illustrating the principle of division of labour: it includes (1) nutritive persons (np) which possess a proboscis, mouth, and tentacles on their club-shaped bodies; (2) much smaller blastostyles (bl), that is to say, polyps with degenerate mouth and tentacles, which are wholly given over to the production of buds (mk), which then develop into sexual animals, little free-swimming medusoids; and (3) protective personæ in the form of hard spines (stp), beneath the shelter of which the soft polyps withdraw when the mollusc shell is rocked about on the sea-floor by the rolling of the waves. In addition to these three different kinds of individuals or personæ there are also (4) defensive polyps (wp) of long, thread-like shape, thickly set with stinging-cells, but possessing neither mouth nor tentacles. It might at first be thought that these are for the defence of the colony, but this is not so; the fact is that they rather serve for the direct defence of the hermit-crab. This is indicated by the position they occupy in the colony; they are not regularly distributed over the surface, but are ranged round the edge, and, indeed, only on the edge which surrounds the opening of the mollusc shell. Here these defensive polyps stand in close array, sometimes spirally contracted, sometimes hanging loosely down over the hermit-crab like a fringe. Their function, like the acontia of Actiniæ, is to defend the crab when an enemy tries to follow it within the shelter of its domicile. This can easily be demonstrated by drawing out the hermit-crab from the Gasteropod shell, and, when the colony has settled down again, seizing the shell with the forceps and drawing it slowly through the water. The water-stream which then flows upon the shell mimics the attack of an enemy, and immediately all the defensive polyps, as at a given signal, strike from above downwards, and repeat this three or four times; they are scaring off the supposed enemy.

In this species of polyp a special form of individual has developed with a quite definite position in the colony, and furnished with a special instinct or reflex mechanism which is directly useful only to the crab, and has therefore, in a sense, arisen for its advantage. This can quite well be explained through natural selection, for indirectly these polyps are also of use to the colony, inasmuch as they protect their valuable partner, and thus render it possible for the hydroid colony to make the partnership of use to the hermit-crab as well as to itself.

This mutual arrangement thus satisfies the requirement which, from the selectionist point of view, must be made in regard to all that is new—that it must be useful to its possessor.

If it be asked what service the hermit-crab renders to the polyp colony in return, the answer is that, as in the symbiosis with sea-anemones, the hermit-crab carries the polyps to their food, which is also its own. Hermit-crabs eat all sorts of animal food, living or dead, which they find on the sea-floor, and the remains of their meal fall to the share of the polyps. Once, without special intention, I laid a hermit-crab with its polyp colony in a flat vessel of sea-water beside a bright green living sponge. After some time the majority of the polyps had become bright green; they had filled themselves with the green cells of the sponge.

I do not know how else we should picture to ourselves the origin of symbiotic instincts in such lowly animals except through the transmission and augmentation of variations in the instincts of the two partners—variations which made their possessors more capable of survival. Mollusc shells, ever since there were any, must have served as a foundation and point of attachment for polyp colonies; as a matter of fact, we find to-day on mollusc shells many kinds of polyp colonies which show no special adaptation to a life of partnership with hermit-crabs. From such indifferent associations a symbiotic one must gradually have been evolved in some instances, through the preservation and augmentation of every useful variation, both of instincts and reflex actions, as well as of form and structure. I shall not attempt to trace the course of this evolution in detail, but it is obvious that the development of defensive polyps, and of their instinct to defend the crustacean, can be interpreted neither as due to any direct influence nor as due to the effect of use, but only to the utility of this arrangement, the beginnings of which—polyps with stinging-cells—were already present. Their augmentation and perfecting must be referred entirely to natural selection. It is the same with adaptations which do not refer directly to the crustacean partner, but rather to the disposition of the polyps on the shell. The spinous personæ which protect the softer polyps from being crushed by being rolled about on the pebbles by the waves cannot possibly be regarded as the direct result of this crushing. But it is obvious that some such colonies must have had among their members some with a stronger external skeleton, and therefore less easily crushed than the rest, and this would lead to their more frequent survival.