Deep.Sea Research, 1062, Vol. 8, pp. 245 to 250. Pergamon Press Ltd. Printed in Great Britain

Vertical zonation in the distribution of deep-sea benthic fauna in the ocean*

N I N A G . VINOGRADOVA

WITH the accumulation of information on the distribution of animal life in seas and oceans, differences in the inhabitants at different depths become increasingly evident. These differences are shown by the kinds of animals occurring at different levels, by their abundance (i.e., numbers) and by modification of such characteristics as colouring, size, feeding habits, breeding, etc. On the other hand, the dissimilarities among animals living at different depths are reflected in various schemes of vertical zonation on the ocean floor and in the whole water mass of the world ocean. Forty or more such schemes have been suggested by many authors.

Recent work on board the Soviet Research Vessel Vitiaz, the Danish Galathea and the Swedish Albatross have greatly added to our knowledge of the fauna at great depths. Living animals have been recorded as deep as 10,700 m. The material collected on these expeditions reveals some fundamental regularities in the distribution of deep-sea animals, which make it evident that the existing schemes of vertical zonation must now be revised. From an analysis of the differences in the distribution of benthic animals in the Kurile-Kamchatka trench, L. A. Z ~ V I T C H and J. A. BIRSTEIN (1955) delimited several levels of the deep-sea bottom fauna including a distinct ultra-abyssal zone in depths greater than 6000 m - the zone of trenches of BIRST~N, VINOORADOV, TSCmNDONOVA (1954) or the zone of superoeeanic depths of 7_~N~VITCH (1953). The existence of an endemic ultra-abyssal fauna is further corroborated by the collections of the Galathea (' hadal' fauna of BRvtn~, 1956, 1957, WOLFF, 1960) and Albatross expeditions.

The present paper is based on data obtained on recent expeditions as well as on material from earlier Russian and foreign expeditions that have collected deep-sea animals. From the published records of these expeditions together with other data on the vertical distribution of deep-sea invertebrates in depths exceeding 2000 mr, belonging to the following groups, Spongia, Coelenterata, Crustacea (Cirripedia, Isopoda and Decapoda), Pantopoda, Echinodermata (Crinoidea, Asteroidea, Echinoidea, Holothurioidea) and Pogonophora, a total of 1144 species are known. From these records, it appears that the number of species living between 2000 m and 6000 m decreases fairly rapidly. Thence down to the greatest depths investigated (I 1,000) they decrease much more slowly (FIG. 1). From the vertical distribution of individual groups of deep-sea invertebrates, it appears that there is a general increase in the total number of species in depths of roughly 3000 to 4000-4500 m. The depth of the maxima in these different groups tends to vary somewhat. For most groups - - Spongia, Isopoda, Spinulosa, Forcipulata, Holothurioidea - - these occur at depths of 2500-3000 m and 4000-4500 (FIGs. 2 and 3). In two other groups a maximum was

* R 6 s u n ~ o f a paper publislwA in Trudy Instituta Oceanologii, Akademiia Nauk, S.S.S.R., 27, 86-122. 1958.

1"For a bibl iography, see VINOOP.ADOVA (1958).

245

NINA G. VINOOItADOVA

observed at 3000-3500m without any other obvious maximum. In Cirripedia, Decapoda, Crinoidea and Echinoidea- there are apparently no maxima whatever (FIG. 4). Consequently no general increase in the total number of species was noted in depths of roughly 3000 and 4000-4500 m. However, in all the groups mentioned, an abrupt change in the taxonomic composition does exist at these depths.

Number of species

0 50 IO0 150 200 250 300 350 400 ,450 _50o

2000 / e~ e, - ~ o ""e

. . ..," . ~ . . . ~ o ~ "

Fig. 1. Vertical distribution of deep-water bottom invertebrates. (a) total member of ~ m = . ; (b) number of ~ whine upper limit of habitat lies in this layer; (c) number of species

whose lower limit of habitat lies in thl- layer.

o

IOOO

2o00

3000

40o0 E

5oo0

8OOO

go00

Fig. 2.

Number o f species

i0 20 30 40 50

. . . . D

. . . . C

Vertical distribution of deep-sea sponges which are found at the depths greater than 2000 m.

The depths at which these taxonomic changes occur differ somewhat in various groups. Thus in Spongia, Cirripedia, Isopoda, Decapoda, Forcipulata, Crinoidea and Holothurioidea these take place at 2500-3000 m and at 4000-4.500 m, but in Coelenterata, Phanerozonia, Spinulosa and Echinoidea they are at the 3000-3500 m

Vertical zonation in the distribution of Deep-Sea Benthic Fauns in the Ocean 247

and at 4500-5000 m (FIGS. 2, 3 and 4). In some groups (Spongia, Isopoda, Holothur- ioidea), there seems to be still another faunal change at 6000-7000 m, as indicated by some increase in the total number of species in depths greater than 6000 m. This is also true for the deep-sea Isopoda of the sub-order Asellota (WOL~, 1956). More detailed data are presented by N. G. VnqOGST, DOVA (1958) and for the ultra-abyssal zone by T. WOL~ (1960) in support of these observations.

Number of species

0 i0 20 30 40 50 60 70 8 0

i t

,, Fig. 3. Vertical distribution of deep-sea holothuriam which are found at the depths greater

than 2000 m. For an explanation of the curves, see Fig. 1.

IOOC

E zoo(

O 40OC

5OO(

6oo(

Fig. 4.

Number of qxlcles

I0 20 30 40 50 , , i i

o.+,.~ o,.,,,..

~ e . . ~ "~" ~ . . ~ . . ~ - ~ ' ~ " ~ j . ~ . _ . . . - - - ~

o

. . . . b

. . . . C

Vertical distribution of deep-eea . ~ a p o d s which are found at the depths greater than 2000 m. For an explanation of the curves, see Fig. 1.

The changes in the taxonomic composition of the bottom fauna are most pro- nounced in depths of about 3000 m. Here a great number of species and even whole groups of animals, widely distributed in the bathyal and even sublittoral zones of the ocean disappear and are replaced by many new species, and, still more important by genera and families characteristic only of oceanic depths greater than 3000 m. We believe, therefore, that this is the upper limit of the abyssal zone of the ocean.

248 NINA G. VINOGI~.~DOVA

Obviously it does not coincide exactly with the 3000 m isobath but rather a well defined transition layer exists between the bathyal and abyssal faunas in depths of 2500 to 3500 m. This limit is also indicated by recent investigations on Foraminifera (STSC~mNA, 1958; S IDN3VA, 1958) and on fishes (Am~AsI~V, 1958).

These regular, more or less abrupt changes in the fauna at certain depths in the ocean make it possible to subdivide the abyssal zone into two subzones, each charac- terized by its own, more or less discrete fauna : (1) an upper-abyssal subzone with an upper limit at 3000 m and a lower one at about 4500 m and (2) a lower-abyssal sub- zone in depths of 4500-6000 m. In the latter, the fauna has a predominance of species, rarely genera, characteristic only of depths greater than 4500 m. No higher taxonomic units restricted only to these depths are present. Many species, as well as characteristic genera and families of animals recorded only from depths greater than 6000m (ZEN~VITC~I, BELY~V, BntST~IN and FILATOVA, 1959; WOL~, 1960) (FIG. 5) delimit a distinct, well-defined ultra-abyssal zone. The number of these species may be expected to increase as more material is worked up. A preliminary estimate indicates that 58.5 per cent of the species of the abyssal zone (in depths ranging from 3000 to 6000 m) are endemic. Thus, BIRS~IN reports that 62 per cent of the invertebrate species in the ultra-abyssal zone are endemic, and WOLFF (1960) records 58 per cent of them as endemic.

0 2000 Ocean

6000 l 1©) 8000 ~)

500 450 400 350 300 250 200 ISO t00 50 0

Fig. 5. Scheme for the vertical zonation of the abyss. a) boundary of the abyssal zone. (b) upper-abyssal subzone. ((¢) lower-abyssal subzone. (d) ultra-abyssal zone.

It was found that the depths, where the most abrupt changes in the deep-sea fauna occur are similar in all oceans, the Atlantic, Pacific and Indian. Consequently, it may be assumed that certain common factors have affected and perhaps still affect the distribution of most of the deep-sea bottom fauna in the whole World Ocean. The boundaries which we have defined may vary in some regions depending on the configuration of the ocean floor and the distribution of certain environmental factors. It appears that the upper-abyssal and lower-abyssal subzones may shift upwards in closed basins and especially in marginal seas. Obviously, an ultra-abyssal zone does not occur in these areas.

The causes for the zonal distribution of deep-sea bottom animals are still not properly understood. It must be sought, however, in the ecological relationships of animals living in the greatest depths. Thus, the existence of the present deep-sea fauna may be correlated in part with a zonation of various environmental factors, as suggested by EK~O,N (1953) and in part with the geological history. M. N. SOKOt~3VA concluded that all the major changes in the ratio of the three main feeding types among deep sea bottom invertebrates (seston-feeders, detritus-feeders and carnivores) are related to depths of 3000, 5000 and 8000 m. The most definite limits to the distribu- tion of these groups lie at 3000 and 8000 m (SoKoLOVA, 1956, 1959a, 1959b). Her

Vertical zonation in the distribution of Deep-Sea Benthic Fauna in the Ocean 249

results agree well with the levels where the most pronounced faunal changes occur. These subsurface depths coincide with the steepest portion of the hypsographic curve of the earth's crust and with its inflections, i.e., at depths of 3000, 6000 and 4500 m (FI~. 2) where the environmental factors also seem to vary. These may be associated with changes in the general configuration of the ocean bottom. Hence, it too may directly influence the distribution of the bottom fauna. M.N. SOKOLOVA (1956, 1959a, 1959b) has suggested that the concentration of suspended matter in the bottom water-layer may be one such factor. The greatest convexity of the hypsographic curve coincides with a high concentration of particulate matter, favour- able to an abundance of seston-feeding animals. On the other hand the zone of greatest convexity coincides with a layer of rapid sedimentation and a high concentra- tion of organic carbon in the bottom deposits, favourable to all three feeding groups - seston-feeders, detritus-feeders and carnivores.

In the abyssal depths of the ocean a very important part is played by ancient groups of invertebrates, such as Spongia, Triaxonida, Porcellanasteridae, Asellota and others. It is the curves of their vertical distribution that exhibit the most pro- nounced deep-sea maxima and it is the abyssal fauna of these groups that is most sharply delimited from the fauna of the bathyal (VINOC3RADOVA 1958b; Z~VlTCI-I and Bms~n~, 1960). The zonation of the deep fauna suggests that the vertical distri- bution existing today may have been determined by the conditions existing in past geologic time. It may be assumed that the rate of migration to the great oceanic depths was not uniform and that the colonisation took place in several stages, as has been demonstrated by B F . ~ (1931) for the deep-sea Decapoda. This scheme for the vertical zonation of the abyssal zone of the ocean with some changes has been accepted as a part of the general scheme for the vertical zonation of the whole ocean proposed by some Soviet marine hydrobiologists (BELYAEV, BIRSTEIN, BOGOROV, VINOGRADOVA, VINOGRADOV, ZENr~VlTCH, 1959).

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250 NmA G. VINOORADOVA

STSCI~DmNA, Z. G. (1958) On the fauna of Foraminifera of the l~uril-Kamchatl~ trench. Trudy Inst. Okeanol., Akad. Nauk SSSR, 27, 161-179. (In Russian).

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