For this community of the circumtropical species we may certainly advance in explanation the comparatively recent formation of the Isthmus of Panama. Besides the resemblance of the Crustacea on the east and west coasts of the isthmus, we have an actual identity of species in several cases, e.g. Pachycheles panamensis and Hippa emerita, and the same thing has been observed for the marine fish.
Another connexion, at any rate during early tertiary times, which probably existed between now isolated tropical coasts, was across the Atlantic from the West Indies to the Mediterranean and West African coasts. Numerous facts speak for this connexion. Species of Palinurus and Dromia occur in the West Indies and the Mediterranean, which only differ from one another in detail, and a connexion between these two regions has been urged from the minute resemblances of the late Cretaceous Corals of the West Indies with those of the Gosau beds of S. Europe, and also of the Miocene land-molluscs of S. Europe with those at the present time found in the West Indies.
To account, then, for the present distribution of littoral Crustacea we must imagine that great changes have taken place during comparatively recent times in the coast-lines of the ocean, but the guiding principle in both the past and present has been temperature, and this factor enables us, despite the immense changes in the configuration of the globe that must have taken place, to divide the coasts latitudinally into Arctic, Antarctic, and Circumtropical zones.
Pelagic Crustacea belong chiefly to the Copepoda (Calanidae, Centropagidae, Candacidae, Pontellidae, Corycaeidae), a few Ostracoda (Halocypridae and Cypridinae), and among Malacostraca a few Amphipoda (Hyperina), some “Schizopoda,” and among Decapoda only the Sergestidae, if we except the few special forms which live on the floating weeds of the Sargasso Sea, e.g. the Prawns Virbius acuminatus and Latreutes ensiferus, and the Brachyura Neptunus sayi and Planes minutus. Besides these Crustacea which are pelagic as adults, there is an enormous host of larval forms, both among Entomostraca and Malacostraca, which are taken in the surface-plankton.
In dealing with the Copepoda we have already mentioned the vast pelagic shoals of these organisms which occur at particular times of the year, and have an important influence on fishing industries. Anomalocera pattersoni (Fig. [27], p. 60) is a good instance of this. It is a large Heterarthrandrian, about 3 mm. long, with the body of a fine bluish green colour; it has a remarkable power of springing out of the water, so that a shoal has the appearance of fine rain upon the surface of the sea. It occurs in the open Atlantic and Mediterranean, but comes into the coasts during violent storms; the Norwegian fishermen hail its presence in the fjords as the sign of the approach of the summer herring.
It was Haeckel[[160]] who first clearly distinguished between “neritic” plankton, the species of which have their centres of distribution in shallow coastal waters and die out gradually as the open ocean is approached, and “oceanic” plankton which is habitually found in the open sea, and though it may invade the coasts is not dependent on the sea-bottom in any way. It appears that although these two kinds of plankton may get mixed up by currents and storms, they are always recruited by new generations from the neritic or oceanic stations proper to each kind.
Common oceanic species, found chiefly in the open Atlantic and in the North Sea, are Anomalocera pattersoni, Calanus finmarchicus, Centropages typicus, Metridia lucens, Oithona plumifera, etc. Common neritic species in the Channel and other coastal waters are Centropages hamatus, Euterpe acutifrons, Oithona nana, Temora longicornis, etc. It was found by Gough[[161]] that although the true oceanic species invade the Channel from the open Atlantic to the west, they become rarer and rarer as they advance up the Channel. Thus the plankton midway between the Lizard and Ushant at all times of year is about 70 per cent. oceanic, while at the line drawn from Portland to the Cap de la Hague it is about 35 per cent. Seasonal changes in the salinity of the Channel water, chiefly due to the influx of oceanic water from the Atlantic, as observed by Matthews,[[162]] do not clearly influence the distribution of oceanic and neritic forms. The influx of highly saline water from the Atlantic was most marked during the winter months up to February. From February to May the highly saline water receded, and during the summer months at the line drawn between Portland and the Cap de la Hague the salinity was rather low. This was increased in November by a patch of oceanic water being cut off from the main mass and passing up Channel, and it is noteworthy that during this month the highest percentage of oceanic forms was taken in the plankton of this region.
Calanus finmarchicus affords a clear instance of the way in which the plankton may be carried about for great distances by means of currents. This species has its home in the subarctic seas, but is carried down in the spring by the East Icelandic Polar stream to its spawning-place south of Iceland; the enormous shoals produced here are carried back, continually multiplying, along the coasts of Norway during the summer and autumn.
Besides these great migrations, the plankton organisms perform daily movements, the majority of the Crustacea avoiding the surface during the day, and often going down to as much as seventy fathoms or more, and only coming up to the surface at night. Others, however, e.g. Calanus finmarchicus, behave in the converse manner, preferring the sunlit surface to swim in.
Owing to their dispersal by means of oceanic currents the pelagic Crustacea do not offer any very striking features in regard to their distribution, and the possibility of always finding congenial temperatures by passing into the upper or under strata of water enables them to live in almost all seas. The tropical species of Sergestidae are mostly circumtropical, i.e. unhindered by the present barriers of land.