The Zoologist/4th series, vol 3 (1899)/Issue 701/Editorial Gleanings
EDITORIAL GLEANINGS.
Dr. A. Alcock, the Superintendent of the Indian Museum, Calcutta, has just published, in the 'Journ. Asiat. Soc. Bengal,' a very interesting account of a new Hermit-Crab (Chlænopagurus andersoni) exhibiting adaptive commensalism with a Sea-Anemone.[1] The Hermit-Crab is noteworthy (1) in having for its refuge, not the usual mollusc-shell, but a sheet or blanket formed by the cœnosarc of a colony of Sea-Anemones; (2) in being—as far as the male is concerned—symmetrical; aud (3) in having the appendages of the 3rd-5th somites of the male, and of the 2nd-5th segments of the female, present on the right or left side indifferently.
"There is nothing unusual in the fact that the protective covering of the abdomen is not a mollusc-shell, for in these seas[2] alone there are several well-known instances of Hermit-Crabs making use of other convenient receptacles. For instance, Pylocheles miersi is found impacted in hollow twigs of sunken drift-wood; Troglopagurus, according to Messrs. Thurston and Henderson, lives in small cavities in coral; and I have myself seen a large Cœnobita, on the island of Minnikoy, holding the empty shell of a small coco-nut over its abdomen. Again, in other parts of the world, Gryllopagurus lives in burrows of its own construction; Pylocheles Agassizii was found concealed in a cavity in a piece of sandstone, and another specimen was taken from the gastral chamber of a siliceous sponge; Xylopagurus rectus, like our Pylocheles miersi, was discovered in a lodging in drift-wood; Ostraconotus and Tylaspis are both believed to have some special protective shield, other than a shell; and Porcellanopagurus lives free among seaweed.
"Again, the association of our new form of Hermit-Crab with a Sea-Anemone is nothing strange: indeed, commensalism between Crustacea and Sea-Anemones is one of the most familiar facts of zoology, and a large number of instances of it have been described. In most cases, however, the facts seem to be that an individual of a definite species of Crab and an individual of a definite species of Sea-Anemone have both at once taken possession of the same mollusc-shell, which they continue to inhabit for their mutual advantage,—the Crab acting as locomotive to the Sea-Anemone, and the Sea-Anemone in return acting as a defence and warning-post, and possibly also as a decoy, for the benefit of the Crab. But, though the mutual advantage of the association is plain enough, the absolute and essential necessity of it is not so plainly seen, and it is reasonable to imagine that when in the course of growth the Hermit-Crab has to seek a new and larger shell, the partnership with the Sea-Anemone can be dissolved by simple withdrawal, without dangerously affecting the life of either individual—at any rate until such time as each can find a new partner of suitable size. In other words, there is no adaptation of either animal to the other, and each seems capable of existing apart from the other. In the present case there is no shell to act as introduction to and bond between the two animals; and the Sea-Anemone, which is a colonial form with a spreading cœnosarc, merely forms a sheet, which the Crab simply tucks under its telson by one end and pulls over its back by the other end—the polyps seeming to have no power of adhesion, and to depend on the Crab for a fast hold.
"The nearest approach to this state of affairs is found in Parapagurus pilosimanus, which, when full-grown, lives in a cavity hollowed out of the coenosarc of a colony of a large species of Epizoanthus. But in this case the individual Hermit-Crab and Sea-Anemone start their partnership with an empty mollusc-shell, which in course of time, as the occupants increase in size, becomes absorbed, so that at last the Crab is entirely dependent on the polyp-colony for the protection of its soft abdomen. But even here, though the association seems to have become much more intimate and permanent, there seems to be no essential adaptation of either animal to the other, nor does it appear to be beyond the bounds of possibility that each might exist—though its existence might not be so complete and secure—apart from the other.
"In the case of the new form of Hermit-Crab, now described, there is no evidence of the intervention of a shell, or other adventitious support, at any stage. Captain Anderson dredged 205 specimens, of both sexes and all ages, and in every observable instance the parent polyp of the protective colony appears to have settled on the hinder end of the abdomen of the Crab, and to have gradually spread by budding as the latter increased in size; so that the intimate and immediate connection between the two animals appears to be, from the first, a necessary one. In other words, the peculiar interest of the case is that the two animals seem to have become directly adapted to one another, and to be incapable of a separate and independent existence."
In August last there was published at St. Petersburg the first number of the 'International Review of Fisheries and Fishculture,' of which the contents are printed either in the English, German, or French languages. Among much that is both interesting and valuable may be found an article by Dr. Einar Lönnberg, of Sweden, on "A short comparison between the Capsian and the Baltic Seas." In the first, animal life is much richer than in the second, and we are given a summary of the principal features of the faunas of these seas.
"Passing on to draw an incomplete sketch of the fauna of these seas, I think, we can omit the Seals, three species in the Baltic and one (of northern origin) in the Caspian Sea, although they are destructive to the fish. The fish-fauna has many characteristics in common. Firstly we see a whole lot of freshwater fishes being common to both seas. Nearly all the Baltic freshwater fishes are also found in the Caspian Sea, but the latter is inhabited by a great number of very important foodfishes which are entirely wanting in the Baltic. Among those I think the Belorybitza (Luciotratta), the different species of Sturgeons and the Caspian Pikeperch (Stizostedium caspium) must be ranked first, not forgetting the Caspian Herrings and others. The Baltic has, in addition to its freshwater fishes, some marine fishes which may have entered through the Sound and the Belts, but of these the Plaice and Turbot are of commercial value only in the southern parts, the Flounder up to the neighbourhood of Stockholm, but the Cod still further north to the islands Ulföarne near Hernösand, although of less importance north of Aaland. The Baltic Herring yields the largest quantities and the anadromous Salmon and katadromous Eel are the best paid fishes in the market. Among the fishes which belong to the Baltic relict fauna, only Cottus quadricornis is used for food, but of course being a small fish it is of little value. The fishes of the Caspian Sea seem mostly to belong to the freshwater fauna or to that of brackish water; true marine types are scarce. The Belorybitza being closely related to the "White Salmon" of the Arctic Sea, seems to point to a northern origin, as do the Caspian Seal and some of the lower animals. The Sturgeons are also, at least partly, inhabitants of the Black Sea. But the Mediterranean fauna, which has taken possession of the Black Sea, does not seem to have been able to enter the Caspian Sea." Comparisons of the lower animals are of "great interest, because they show (as is also done by many species of fish) that hardy forms can endure to live and thrive well both in the Caspian and the Baltic Sea, in spite of all differences between these seas. But it must not be forgotten that the greatest part of the Caspian fauna is endemic and characteristic, for that region and the lower fauna of the Baltic is partly hardy marine forms which mostly have entered through the sounds in the south-west, although some are relict forms, and partly freshwater species."
This work was published before January 1, 1929, and is in the public domain worldwide because the author died at least 100 years ago.
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