On the geographical distribution of corals

860 ORDINARY

MEETING.

FRIDAY, MAY 5TH, 1882. W. H. HUDLESTON, ESQ., M.A., F.G.S., PRESIDENT, in the Chair. The donations to the Library since the last meeting were announced, and the thanks of the Association returned to the donors. The following paper was then read : ON THE GEOGRAPHIOAL DISTRIBUTION OF CORALS. Bz STUART O. RIDLEY, M.A., F.L.S.

The Corals ought to be interesting above most other groups to a Geological Association from their ancient occurrence, the large extent to which their structures are capable of preservation in the fossil state, and their actual immense abundance in the seas of many past ages, placing them among the chief contributors to the rocks of the globe. In no country are these facts more prominently brought before our notice than in our own. The study of the Geographical Distribution of Corals is one which has found but few workers, and is at the present time rather behind that of the distribution of most other large groups of the Invertebrata. This is mainly due to the fact that very little has been done lately to incorporate the information acquired during the last twenty years with the results published by Esper in 1791-7, by Dana in 1839, by Darwin in 1842, by Milne-Edwards and Haime, 1857 -60. Since these older writers, A. E. Verrill, of America, has done more than anyone else to extend, systematize, and render precise our knowledge of their distribution, by his works on the Corals of the Atlantic and Pacific Oceans; but the information published by him is all contained in separate papers. Studer, a German naturalist, has done good service by his descriptions (1878-9) of the collections formed by a recent German circumnavigatory voyage, that of the "Gazelle." Up to Verrill's time, however, it was almost exclusively the shallow water which had yielded up its treasures to the explorer of the sea. M. and G. O. Sal's, of Norway, had, indeed, brought up some strange and novel forms from considerable depths off the Norwegian coast from 1849 and onward, and the Bay of Biscay, the Shetland Seas,

s, O. RIDLEY ON THE GEOGRAPHICAL DISTRIBUTION OF CORALS. 361 and some other localities had been explored at depths beyond 200 fathoms, with success, but not in a manner sufficiently systematic to give a thorough insight into the fauna at abyssal depths. But in 1869-70 the vessels " Porcupine" and "Lightning" were despatched by our Government to explore the deep sea in the Atlantic, and series of soundings and dredgings were taken, giving results which leave no room for doubt as to the broad features of, at any rate, the Coral fauna at depths of 300 to 700 fathoms. Of these results, those embodied in Professor Duncan's report on the Corals are of the highest interest, no less to the palreontologist than to the zoologist, showing, as they do, the existence of links between the past and present races of Corals which had previously been unknown. The younger Agassiz and Pourtales in America have, during the last ten years, treated the West Indian seas in the same manner, and with results of similar import. Lastly, our own "Challenger" expedition, bringing material from a far larger area and from far greater depths than any previous deepsea explorations, has, by the reports which it has enabled Professors Moseley (on the Deep-Sea Madrepores and some other groups) and Kolliker (in the Pennatulida, or Sea-pens) to produce already, both confirmed previous knowledge, and extended the depthlimits at which Zoophyte life was known to be possible, besides illustrating the ancient character of some of the groups and of individual species, and adding many striking new types to our lists. In consi-dering the subject of Distribution, I propose to treat of it under two heads, viz.: 1, The Facts of Distribution; and, 2, The Causes at work in producing the Distribution as we now see it. I include under the term Corals on this occasion only those members of the three groups, ZOANTHARIA, ALAGONARIA, and HYDROCORALLINlE, which secrete a calcareous skeleton, omitting from the second of these groups the Pennatulidce, as being freeliving organisms. For the definitions of the groups I must refer members to some remarks which I made on this subject before the Association on a former occasion. In the generalizations which are here formed as to the proportions in which groups are represented in certain localities, allowance must be made for the fact that it has been fonnd impossible to embody every individual locality recorded for each species, but the general results lllay be taken as approximately correct.

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In dealing with the first head the Facts of Distribution, I shall consider the Coral fauna of different marine districts in succession, and commence with (A) the ATLANTIC OCEAN, and in particular with it s most northern division, the Arctic Sea. (c.] As many as twelve species of Corals are known to inhabit these inhospitable waters, illustrating the fact that the temperatnre of the Arctic waters is no barrier to the well-being of a very large number of Invertebrata (especially Polyzoa and Foraminifera), or even of a group of them which reaches its highest development in tropical seas. These twelve species comprise exclusiv ely Alcyonaria, viz.: seven Alouoni ida: (Alcyonium, A mmothea, Rhieoxenia, Eunephthya, Gersemia); two Primnoidce l Primnoa, Lithoprirnnoa); two Isididce i Isidellay ; one of the Brial'eacece (Briareum). A stony coral has been recorded, but its occurrence cannot be accepted as genuine. (b.) In the North Temperate regions of the Atlantic, as elsewhere, an important distinction exists between the deep and shallow-water faunas. In this district the deep fauna is better known than in any other part of the world. Near our own and th e American coasts we find seven species of A lcyonium, some small genera of the same family, containing one or two species each (Rltizo xenia, Anthopodium, Anthomastus), and a few stray Gorqoniido: Among Stony Corals (Zoantharia) Caryophyllia predominates (but in deep water), and several allied forms appear. Sphenotrochus here represents Flabellum of the Indo-Pacific region, L ophohelia prolifera and Amphihelia are attractive branched forms of wide distribution in moderately deep water. The Hydrocorallinre, the third great division of Corals, are also represented here, by species of Allopora and a Stylaster gemma8cens. Passing to (c) the SUbtropical Atlantic, we find (a) the West African islands and the neighbouring coasts to exhibit more of th e Porous and colonial forms of Stony Corals than in the more northern seas; M adraoie asperula, e.g. extending across to the WeRt Indies. Dendrophullia and Balanophyllia appear to be quite at home, judging from th e abundanc e in which they are found. Caryophyllia is still largely represented in the deep sea. Flabellum here begins to appear, replacing the northern Sphenetrochus. Lophohelia prolifera extend s even beyond here far into th e South Atlantic. A mong Alcyonaria the Prec ious Corals

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(Coralliidro) here first appear, the Red and a White form being found. Leptogorgia is attractive and well represented here. The Mediterranean fanna ((3) has the same general facies as the contiguous portions of the Atlantic, bnt has some species peculiar to it, as Cladocora ccespitosa and a number of Paracyathi (Duncan), but most of its species extend into the North Atlantic. It has but few Gorqoniido, several fleshy Alcyonoids, the Red Coral (Corallium), numerous simple Madrepores, and the almost ubiquitous compound Lophohelia prolifera, but not a single true Reef Coral, except Astrocomia phareneis, a lately discovered member of an otherwise fossil genus, not elsewhere found. These facts should be borne in mind, as in dealing with the Red Sea, separated from the Mediterranean by only about 100 miles of sandy and rocky land, we shall find a reef-coral fauna, perhaps the richest in the world, but no species of any Coral which is found in the Mediterranean. In (e) the tropical Atlantic, but little is known of the Corals of the Equatorial Coast of West Africa (a) ; but here, as in the north, Eunicella is a prominent Gorgoniid, Leptogorgia In mid-ocean ({3) Lophohelia is abundant, as in the Pacific. prolifera and nearer to Brazil M adracis are still found at considerable depths, and the widely ranging Solenoemilia uariabilis, and a species of Stephanotrochue, a detached form, cross the ocean here, while Deltocpathue italicus (shown by Mr. Moseley to be identical with an Italian fossil) comes south from Massachusetts. Caryophyllia extends from the north by its species C. maculate, also found in the Pacific. These species, it should be observed, are all of wide range, and inhabit great depths. The West Indies (oy), however, are the great focus of Corals in the tropical Atlantic. This was to have been expected from the great varieties of depth presented by the bottom, from the high temperature of the water, and the absence over most of the area of considerable accessions of fresh-water from rivers. Of Alcyonaria, we have here a few Xeniidce (Ammo thea and an endemic genus, El'ythropodium), a few genera from temperate seas (Primnoa, Briareumy, and several genera (Eunicea, Riisea, Acis "') of the same family quite peculiar to this area, Eunicea alone contributing about twenty species to its fauna. Pleoiaura, though also

*

Since recorded from the Indian Ocean.

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found in the Pacific, has several species here, and Pteroqorqia attains a very great development in size. Of Zoantharia, Paracyathus, Rhisotrochue, Desmophsjllum, and numerous other genera represent the Turbinoiiida, Oculinidce are weH represented by Aeohelia, Arnphihelia, and Oculina, Of Astl'a!aCete, deep-sea rather than shallow-water forms occur, e.g. Antillia (the only recent species); Manicina and Lithophyllia seem also to be confined to this region. Diploria and Ma!andrina, Siderina and Drbicella are well represented here, though not confined to the Atlantic. The Cladocoracean genus Astranqia is found on both sides of the isthmus; the large genus, Porites, though chiefly found in the Pacific, is just represented here. Of other Porosa, Balanophpllia is quite at home here, as in the Atlantic generally; Madrepora has several species, but its true home is the Indo-Pacific region. On the aspect of the West Indian Coral fauna in general, Pourtales remarks (Catalogue Mus. Compo Zoo!' Osmbr., Massachusetts, viii., pp. 33-50) that the existing West Indian deep-sea coral fauna agrees much more closely with the corresponding Recent and Tertiary European fauna than with the West Indian fossil fauna. I have already drawn some comparisons between the West Indian and Pacific Coral faunas; it seems of some interest to look at Verrill's comparison of the two sides of the Isthmus of Panama with respect to this subj ect. It should be noticed that the species enumerated form but a small proportion of those occurring in the tropical regions of the two districts, and represent only those known to Verrill himself at the time of writing (1866). According to Verrill (Proc. Boston Soc. Nat. Rist. x., p. 323), the following are the main aspects of the two faunas : At A8pinwall (West Indian side). 1. Coral Reefs, with the same features and same species of Corals as Florida and West Indies.

At Panama (Pacific side). 1. Nv true Goral Reef' here as on the West Coast of Central America, a totally different Bet of families except Porites panamensis and Stephanocora,

2. Chief Zoantharia, Porites astraloides, clava,"ia, .Madrepora palmata, cervicornis. prolijera, Mceandrina clivosa,labyrinthica, Manicina a"eoZata, sinuosa, Sidel'nia rtuiiata, galaxea, Or,i. cella caverno.a, annularis, &c.

2. Stephanocor« stellata, Astrangia Haimei, pulchella, concinlla, deniata, costata, Phyllangia dispersa, Ulallf1ia Bradleyi.

DISTRIBUTION OF CORALS. 3. Gorgtmiidce: Agree in general with West Indies. Pterogorgia, Xiphigorg ia, Pleeaura, Plezaurella,

Eunicea.

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3. None of the West Indian genera. appear to occnr, Gorgonia. ramulus, aUI,QI1ttiaca, L eptogorgia rigida, lfhipido. gorgia Agassizi, media, stenobraohis, .Muricea acervata, hi,pida, echinate; hebes, robusra, appressa, Echinogorgia arbiucula, Sympodium pac1jicum.

The Brazilian fauna (8) has a good deal in common with that of the West Indies. There are a few reefs at some distance from the coast eAbrohlos Is., Bahia, &c.), but the fresh waters of the Amazon and Orinoco appear to have kept them in check. Besides some Alcyonaria already mentioned from the West Indies, we may notice, as found here, two genera peculiar to the district, Filigella and Phyllogorgia. Of the Zoantharia not much is known, except concerning the deep-water forms, of which we find here a Bathycyathus, a Flabellum, and a Sphenotrochus, which appear not to be found elsewhere. The rare and remarkable Haplophyllia, with a tetrameral arrangement of its septa, extends down from the West Indies. On going further south of the Equator, we find a remarkably rich locality (£) off the mouth of the Rio de la PIal a, in deep water, probably out of the reach of the influence of the fresh water, owing to the inferior density of that element, otherwise it would appear to be an unlikely locality for such essentially marine forms. But it is chiefly HydrocorallinaJ here, whose occurrence is remarkable i of these the genera Sporadopora and Spinipora are peculiar to this spot, and the species Allopora profusula and S tyl aster d ensicaulis i Errina labiata also occurs here and off Tristan Da Cunha. The coast fauna is little known. In Patagonia we come in contact with the Pacific region. H ere Desmophyllum ingens, an immense Turbinoliid, of a Mediterranean and West Indian genus, and, identical perhaps, with a Sicilian tertiary fossil, is the most striking Zoantharian j a fine Flabellum also occurs here, an Astrangia, and a few (lorqoniidce, On the opposite side of the Atlantic, the Cape produces several fine GOl'goniidaJ and Briareidai (e.g. Lophoqorqia palma and Solanderia vel'rucosa). The coast is not adapted for the growth of shallowwater Stony Corals in any abundance. n, The next region is the INDO-PACIFIC, which must be taken as undoubtedly a true faunal region, New Zealand, the Red Sea, and Japan being perhaps separated as sub-regions,

en

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Beginning with (a) the North Temperate part of this area, we find Japan possessing a fauna which, so far as it is known, but slightly resembles th at of the North Atlantic. Thus of Alcyonaria we find here a Primnoa, and a form allied to Isie , also a D esmophyllum, a B alanophyllia, and quite a number of Flabella. But here and off China we miss Alcyonium and Eunicella, Oaryophyllia and Paracyathus, and have instead Oalyptrophora, Atopsella, Oyathohelia, Eupsammia, The Sandwich Islands ({3), in a slightly more southern latitude, have a true reef-coral fauna, and in the Bonin and Loochoo Islands (y), between the parallels of 30° to 25° N., we have already the reef Corals Turbinaria (probably) and several Madrepora, this genus being unknown in the Wes$ African islands of about the same latitude, but probably exclusively tropical in the Atlantic. The Stylasterid Endohelia japon ica is peculiar to Japan, and off its coast the deep water presents quite a focus of Pennatulida. The Pacific Euphqlliacea: are already represented in this subtropical region by two EuphyllilJJ. Flabellum continues to be largely represented, but the Porous and Astrreid Corals now assert th eir supremacy, a supremacy which th ey maintain over the oth er Aporous Corals (TurbinoliidlJJ and Oculinidce, &c.) throughout the whole Indo-Pacific region, except iu the South Temperate districts, whose lower temperature keeps out Thus in the South Chinese seas (0) these tropical forms. Eupsammia, Heteropsammia, and Rhodopsammia, replace the externally very similar Oaryophyllia, Turbinolia, and Trochocpathus of the Atlantic, and to th e Reef forms already menti oned are added Lophoseris, P orites, Montipo1'a, P sammocora, Stephanoseris, Diaseris, Plesiastraa, Pocillopora (one of the T abulata), mostly unknown out of the Indo-Pacific region. These forms are more or less replaced in the tropical parts (f) of the Pacific and Indian Oceans (e.g., Malay Archipelago and Pacific Islands) by Gala xea, Mussa, (keloria , Hydnophora, Acanthastraa, Favia; and numerous A strreacese, as Prionostrcea, &c., her e also attain th eir maximum, showing that here the Astrreidre attain their highest development. Herpetoliltra, Fungia Leptoeeris, &c., represent the Funqiida: The Red Sea «() fauna is, in spite of its higher latitude, perhaps quite as rich in genera and species of these groups, and abounds in the softer Alcyonaria (Alcyonium, Spon godes, Ammoth ea), and thus pres ents a striking contrast to the neighbouring Mediterranean waters. Perhaps the most attractive part of the tropical superficial fauna to the

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travelling naturalist's eye must be that of the Alcyonaria; here, we have Anthelia, Xenia, Sympodium, Telesto, Sarcophqtum, Spong odes, Siphonogorgia, species of Alcyonium, Heliopora ccerulea, Tubipora, and several Melitluea. Most of these are of striking form and colour, and all peculiar to the region, and are said to give these reefs and shores a marvellously beautiful appearance. On the Australian coast we find a continuation of the Tropical fauna for some distance southwards. New Zealand (1]) has a small number (in the present imperfect state of our knowledge of this district) of mostly endemic species, of Rhipidogorgia, of Suberia, amongst the Alcyonaria j and of (kenopsammia, Desmophyllum, (lonocpathu«; and Cylicia, amongst the Zoantharia. Sub eria and Conocyathus are peculiar, as genera, to these seas. Tnrning to the second heading, The Causes of Distribution, I find one important cause to be the depth of the water, which presents a barrier to forms which are suited only to certain depths. The distribution of Zoantharia in the Atlantic should be studied in the light of the facts of the bathymetry of this region. Thns we have a long stretch of water of or beyond 3,000 fathoms in depth, extending from about 36 0 N. lat. and 50° W. long. southwards nearly to Trinidad, although not very broad; a band of similar depth lies off the Guinea coast, and extends for about 12° north and south, while midway between Brazil and South Africa a long belt of this deep water reaches from the Equator to about 32° S. lat. At these points, then, the passage across the ocean is barred to all Corals, except possibly a few of the more bathymetrically ubiquitous, as Bathyactis " and if any ordinary deep-sea form is to effect a crossing from one side to the other, it must be by way of some of the passages of shallower But even water which intervene between these abyssal tracts. outside this deep area, between the United States and West Indian seas and our own is interposed a broad band of water of 2,000 fathoms and upwards in depth j hence probably the extremely few and doubtful cases of specific identity of forms found on the two sides of the North Atlantic. Another cause of distribution is temperature. The Isothermal line of 68° Fahr. in the surface waters, in the winter, constitutes the limit to the thermal conditions under which reef-corals can live. This line passes in the North Atlantic from about the Canary Islands on the east across to Carolina, in the United States, on the west. The Bermudas are

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close to this line, which leaves the true West Indian Islands In the South well within the area of beneficial warmth. Atlantic it passes from near the south of Brazil rapidly northwards to within about 10° of the Equator. A few coral reefs occur off the Braziliau coast j those of the West Indies are too well known to need mention. Neither North or South Africa boast of any reefs worthy of the name, although, as we see, the temperature admits of it over a large extent of the coast. In the North Pacific the line runs from near the apex of the peninsula of Lower California to the islands off the south of Japan, passing just to the north of the Sandwich Islands and In no part of the world is the intimate relation of Hawaii. temperature to coral life better illustrated than here, for at all the four points mentioned reef-corals are well developed, though not found further north. In the South Pacific the line begins just south of the Galapagos Islands, off Peru, bends southwards, and passes along the Tropic of Capricorn, to the south of the Society and all the Polynesian Islands, and, leaving _New Zealand a little to the south, reaches the Australian coast at about the latitude of Port Jackson. The Polynesian Islands are distinguished for their coral reefs j New Zealand has none. The vast barrier reef of the East Australian coast ceases just north of the point where the isotherm of 68° reaches the coast. In the Indian Ocean the line runs approximately parallel to the Tropic of Capricorn, well to the south of the numerous coral-island groups which stud the seas to the north and north-west of Madagascar and to the south of India. The possible influence of geological conditions has been touched on while discussing the relations of the West Indian fauna. Although the shallow-water faunas of the West Indies and the Pacific are very different as regards species, yet there is so much agreement between them in genera as to lead to the belief that at no very distant period a communication existed between the two seas, probably in one of the earlier tertiary periods. Other causes have probably co-operated in producing the present distribution j the above appear to be the chief ones. After the reading of this paper some remarks were made by the President on the geological features of the district which the Association were about to visit on the following Whit-Monday and Tuesday.