How did mutualisms between individuals of different species
develop? These associations take many forms, but some symbioses evolve so that dependence
results; the best known examples being lichens, where fungi and algae live
together to mutual advantage.
A symbiosis is found between a species of hermit crab, Pagurus prideaux and a sea anemone Adamsia palliata (pictured below, the Adamsia having pink spots),
although it is not essential for the survival of each [1]. There are many
species of hermit crabs, but all are characterised by having a soft abdomen
that is held within a snail shell left behind after the gastropod has died.
This is a remarkable association between a living animal and the remains of
another and, as an individual crab grows, it leaves one shell and searches for
another that is more suitable for its increasing size. The aperture of shells
occupied by P. prideaux usually has an
individual of A. palliata attached to
it, located the entrance in a position where it doesn't have a marked effect on
the centre of gravity of the shell.
The advantages of symbiosis between sea anemones and hermit
crabs have been reviewed by Gusmão
and Daly [2], with the sea anemones benefitting from an increase both in dispersal
and in the chance of encountering food; and the hermit crabs being protected
against predators by the stinging cells of the sea anemone. So how does the
sedentary sea anemone move to the snail shell occupied by the crab? The
locating behaviour was described by Philip Henry Gosse, the great
nineteenth-century Natural Historian. Here is his observation [3]:
Carefully taking up the shell with
the aquarium-tongs, and bringing it close to the surface, but not out of water,
I gently dislodged the Adamsia with
my fingers, and allowed it to fall prone upon the bottom. I then released the
shell with its tenant, and drove the latter towards the spot where the zoophyte
lay.
No sooner did the Crab touch the Adamsia than he [sic] took hold of it with his claws, first with one, then with
both, and I saw in an instant what he was going to do. In the most orderly and
expert manner he proceeded to apply the Adamsia
to the shell. He found it lying base upward, and therefore the first thing was
to turn it quite round. With the alternate grasps of the two pincer-claws,
nipping up the flesh of the Adamsia
rudely enough, as it seemed, he got hold of it so that he could press the base
against the proper part of the shell, the inner lip. Then he remained quite
still, holding it firmly pressed, for about ten minutes; at the end of which
time he cautiously drew away first one claw, and then the other; and, beginning
to walk away, I had the pleasure to see that the Adamsia was once more fairly adhering..
Long-term attachment is aided by the secretion of chitin,
this being a feature of several sea anemones and not just Adamsia. However, the chitin produced by Adamsia forms an extension of the snail shell and, in the following
description of the process [4], the reference to chitine and a horny membrane
come from the observations of Henry Gosse:
..sea anemones of the genus Adamsia living on gastropod shells
inhabited by hermit crabs may extend the shell's lip by secreting what is variously
called a cuticle, "chitine", a horny membrane, or solidified mucus.
This is what Gosse wrote in Actinologia Britannica [5]
Very frequently, there is found
intervening between the Adamsia and
the shell to which it is affixed, a film of membrane, of a horny texture,
somewhat brittle, of a translucent dark greenish-brown colour. After death this
film is found adherent to the surface of the shell, from which, however, it
easily peels when dry.. ..From several specimens.. .. I have been able to learn
the nature and object of this membrane.. ..it appears to me manifest that the
membrane is a provision for the support of the growing Adamsia.. .. it is composed mainly of chitine, having no calcareous
element.. .. The membrane is not invariably present.
Gosse goes on to say [5]:
Pagurus Prideauxii seems to be as dependent on the Adamsia, as the latter is on it.. ..Why
one species of Soldier-crab must needs seek the companionship of this Anemone,
while other Soldier-crabs are able to live alone; and why this species of
Anemone must needs associate with the Soldier-crab, while others kinds of
Anemone are solitary, I can by no means answer.
As described above, we now have information of the mutualism
and how it benefits both partners. For Henry Gosse (whose beautiful
illustration is shown below), the association of the hermit crab and gastropod
shells and the mutualism between Pagurus prideaux
and Adamsia palliata were yet more
wonderful examples of the complexity of God's Creation.
For those of us that believe in evolution, it is fascinating
to speculate on the various steps that must have taken place to allow the association
between the crab and the shell, and then the crab and the sea anemone. The secretion
of chitin by the sea anemone was a pre-adaptation and may have originally been
a means of ensuring that broken, or friable, substrata could be colonised. It
now results in the secure attachment to snail shells and gives the hermit crab the
advantage of changing shells less frequently. How did the
crab learn to transplant a sea anemone on to a new shell; and why this one
species?
Evolution fills one with a sense of wonder, doesn't it?
[1] R. M. L. Ates (1995) Pagurus
prideaux and Adamsia palliata are
not obligate symbionts. Crustaceana
68: 522-524.
[2] Luciano C. Gusmão
and Marymegan Daly (2010) Evolution of sea anemones (Cnidaria: Actinaria:
Hormathiidae) symbiotic with hermit crabs. Molecular
Phylogenetics and Evolution 56: 868-877.
[3] Philip Henry Gosse (1865) A year at the shore. London, Alexander Strahan.
[4] Daphne Fautin Dunn and Martin H. Liberman (1983) Chitin
in sea anemone shells. Science 221:
157-159.
[5] Philip Henry Gosse (1860) Actinologia Britannica: a history of the British sea-anemones and
madrepores. London, Van Voorst.
