Fig. 237.—Macrocyclis laxata Fér., Chili.
CHAPTER XII
DISTRIBUTION OF MARINE MOLLUSCA—DEEP-SEA MOLLUSCA AND THEIR CHARACTERISTICS
Marine Mollusca may be divided roughly into Pelagic and non-Pelagic genera. To the former division belong all Pteropoda and Heteropoda, and a large number of Cephalopoda, together with a very few specialised forms of Gasteropoda (Ianthina, Litiopa, Phyllirrhoe, etc.). Pelagic Mollusca appear, as a rule, to live at varying depths below the surface during the day, and to rise to the top only at night. The majority inhabit warm or tropical seas, though some are exceedingly abundant in the Arctic regions; Clione and Limacina have been noticed as far north as 72°.[380]
The vertical range of Pelagic Mollusca has received attention from Dr. Murray of the Challenger, Professor Agassiz of the Blake and Albatross, and others. Agassiz appears to have established the fact that the surface fauna of the sea is limited to a comparatively narrow belt of depth, and that there is no intermediate belt of animal life between creatures which live on or near the bottom and the surface fauna. Pelagic forms sink to avoid disturbances of various kinds, to depths not much exceeding 150 to 200 fathoms, except in closed seas like the Gulf of California and the Mediterranean, where the bathymetrical range appears to be much greater.[381]
Non-Pelagic Mollusca are, from one point of view, conveniently classified according to the different zones of depth at which they occur. Thus we are enabled to distinguish Mollusca of (a) the littoral, (b) the laminarian, (c) the nullipore, or coralline, and (d) the abyssal zones. It must be borne in mind, however, that these zones cannot be exactly defined, and that while the littoral zone may be understood to imply the area between tide-marks, and the abyssal zone a depth of 500 fathoms and upwards, the limits between the laminarian and the coralline, and between the coralline and abyssal zones can only be fixed approximately.
The difficulty of assigning special genera or species to special ‘zones of depth’ is increased by two important facts in the phenomena of distribution. In the first place, it is found that species which occur in shallow water in northern seas often extend to very deep water in much lower latitudes. This interesting fact, which shows the importance of temperature in determining distribution, was first established by the dredgings of the Lightning and Porcupine off the western coasts of Europe. In the second place, a certain number of species seem equally at home in shallow and in abyssal waters, in cases where a great difference of latitude does not occur to equalise the temperature. Thus the Challenger found Venus mesodesma living on the beach (New Zealand) and at 1000 fath. (Tristan da Cunha); Lima multicostata in ‘shallow water’ (Tonga and Port Jackson) and at 1075 fath. (Bermuda); Scalaria acus from 49 to 1254 fath. (N. Atlantic); and S. hellenica from 40 to 1260 fath. (Canaries). The Lightning and Porcupine found, or record as found,[382] Anomia ephippium at 0 to 1450 fath., Pecten groenlandicus at 5 to 1785 fath., Lima subauriculata at 10 to 1785 fath., Modiolaria discors at 0 to 1785 fath., Crenella decussata at 0 to 1750 fath., Dacrydium vitreum at 30 to 2750 fath., Arca glacialis at 25 to 1620 fath., Astarte compressa at 3 to 2000 fath., and Scrobicularia longicallus at 20 to 2435 fath. Puncturella noachina has been found at 20 to 1095 fath., Natica groenlandica at 2 to 1290 fath., Rissoa tenuisculpta at 25 to 1095 fath. In many of these cases we are assured that no appreciable difference can be detected between specimens from the two extremes of depth.
In spite, however, of these remarkable vagaries on the part of certain species, we are enabled roughly to distinguish a large number of genera as ‘shallow-water’ and ‘deep-water’ respectively, while a still larger number occupy an intermediate position. Among shallow-water genera may be named Patella, Littorina, Nassa, Purpura, Strombus, Haliotis, Mytilus, Cardium, Solen; while among deep-water genera are Pleurotoma, Scissurella, Seguenzia, Dentalium, Cadulus, Limopsis, Nucula, Leda, Lima, and Axinus.
Theories on the geographical distribution of marine Mollusca have been revolutionised by the discoveries of recent exploring expeditions. The principal have been those of Torell (Swedish) (1859–61) on the coasts of Greenland and Spitzbergen; of the Lightning and Porcupine (British) in 1868–70, in the N.E. Atlantic, off the Scotch, Irish, French, and Portuguese coasts, and in the Mediterranean; of the Challenger (British), under Sir C. Wyville Thomson, in 1873–76, in which all the great ocean basins were dredged or sounded; of the Blake (American), under Alexander Agassiz, in 1877–80, in the West Atlantic, Gulf of Mexico, and Caribbean Seas; of the Travailleur (French) in 1880–83, off the west coasts of France, Portugal, and Morocco, Madeira, the Canaries, and the Golfe du Lion; of the Talisman (French) in 1882, off the west coast of Africa from Tangier to Senegal, the Atlantic Islands, and the Sargasso Sea; of the Albatross (American) in 1891, off the west coast of tropical America; of several other vessels belonging to the U.S. Fish Commission and Coast Survey, off east American shores; and of the Prince of Monaco in the Hirondelle and Princesse Alice at the present time, in the N. Atlantic and Mediterranean.
The general result of these explorations has been to show that the marine fauna of very deep water is much the same all the world over, and that identical species occur at points as far removed as possible from one another. The ocean floor, in fact, with its uniform similarity of temperature, food, station, and general conditions of life, contains no effectual barrier to the almost indefinite spread of species.[383] To give a few instances. The Challenger dredged Silenia Sarsii in 1950 fath., 1100 miles south-west of Australia, and also in 2650 fath. off the mouth of the Rio de la Plata; Semele profundorum in 1125 fath. near the Canaries, and in 2900 fath. mid N. Pacific; Verticordia deshayesiana in 155 fath. near Cape York, and in 350 fath. off Pernambuco; Arca pteroessa in 2050 fath. mid N. Pacific, in 1000–1675 fath. west of the Azores, and in 390 fath. off the West Indies; Arca corpulenta in 1400 fath. off N.E. Australia, in 2425 fath. mid-Pacific, and in 1375 fath. near Juan Fernandez; Lima goliath in 775 fath. off S. Japan, and in 245 fath. off S. Patagonia; Pleurotoma engonia in 700 fath. north-east of New Zealand, and in 345 fath. off Inoshima. A surprising range was occasionally found even in shallow-water species; thus Petricola lapicida was discovered by the same expedition in the West Indies and N. Australia, Cardita calyculata off Teneriffe and in Bass Strait, Arca imbricata off Cape York and in the West Indies, Modiolaria cuneata at Port Jackson and Cape of Good Hope, Lima squamosa at Teneriffe and the Philippines. In these latter cases it is not improbable that the species lives in deep water as well, from which it has not yet been dredged.