The pteropod and heteropod molluscs formed an Important constituent in the zooplankton of the EEZ of India and the

adjacent areas. Between them the pteropods dominated numerically over the heteropads in all situations. The average

density of pteropods in the area investigated was 734 (expressed as number per 1000 m3 of water) and heteropoda was33

Antony, Geetha

Mathew, K J

Naomi, T S

Solomon, K

CMFRI Digital Repository

Proc. First Workshop Scient. Resul. FORV Sagar Sampada, 5 - 7 June, 1989 :155 -163 (1990) DISTRIBUTION AND ABUNDANCE OF PTEROPOD AND HETEROPOD MOLLUSCS IN THE EEZ AND ADJOINING WATERS OF INDIA K.J. MATHEW, T.S . NAOMI, GEETHA ANTONY AND K. SOLOMON Central Marine Fisheries Research Institute, Cochin - 682 031 ABSTRACT The pteropod and heteropjod molluscs formed an important constituent in the zooplankton of the EEZ of India and the adjacent areas. Between them the pteropods dominated numerically over the heteropods in all situations. The average density of pteropods in the area investigated was 734 (expressed as number per 1000 m' of water) and heteropods was 335. Both the groups wore more in the eastern Arabian Sea than in the Bay of Bengal. However, the difference in the rate of ocicurrence in the two sea areas was more pronounced in the case of heteropods, being 46.56% against 21.35% for the other group. Both the groups were relatively more in the shelf waters, the increase over the oceanic area being more than one and a half times for the pteropods which sometimes exhibited some kind of population explosion in the shelf waters. For the heteropods theinceaseinthe shelf area was of the order of 87.88%. The pteropods had a trimonthly abundance, say, during February, May, August and November. November and December were the favourable months for the heteropods. A seasonal variation was not very significant for both the groups. However, pteropods were comparatively more during the premonsoon season off the west coast (826) and the east coast (729). With regard to heteropods, while an increase (449) was noticed in postmonsoon season in the eastern Arabian Sea, in the Bay of Bengal the increase was during the premonsoon period (373). In the eastern Arabian Sea there appeared to be a gradual reduction in f)opulation of both the groups from south to the north. However, in the Bay of Bengal the situation prevailed was exactly the opposite. The observed reduction in population was effected through the various seasons starting from premonsoon througji monsoon to the postmonsoon. A population outburst was noticed for pteropods in the shelf waters of the southernmost latitudinal region of the eastern Arabian Sea during the pcrmonsoon. In the same region off the east coast a similar phenomenon occurred during the nwnsoon season. The day-night variations in the two groups was striking in that both had an overall increase by 27% in the night samples. There was considerable difference in regard to diurnal variations during different months for pteropods and heteropods. INTRODUCTION Pteropods and heteropods are two groups of pelagic marie gastropod molluscs found abundantly in the zooplankton. The former is a sub-order be-longing, to the Sub-class Euthyneura, Order Ophist-hobranchiata. The latter comes as a section of Sub-order Taenioglossa which belongs to the Sub-class Streptoneura and Order Aspidobranchiata. These two groups from the IndianOcean havebeen studied in the past from the taxonomic point of view (Ref: various expeditions which visited the IndianOcean). Somestudies have been made in the Indian Ocean on the spatial distribution and quantitative abundance of some species of pteropods and heteropods. Sak-thivel (1969) made a preliminary study on the distri-bution and relative abundance of euthecosomatous pteropods in general and the seasonal variation of species of the genus Limacina. The seasonal and diurnal variations and also the spatial abundance of L. inflata was studied, again, by Sakthivel (1973a). The biogeographical change in the latitudinal bound-ary of a species of pteropod was also investigated by him ( Sakthivel, 1973b). Aravindakshan (1969) has given a preliminary report on the geographical dis-tribution of some species of heteropods of the Indian Ocean. Later he (Aravindakshan, 1973) made a study on the distribution and ecology of one species of piterotrachea. Another study made by him was on the distribution of the species of the family Pterotra-cheidae in the Indian Ocean (Aravindakshan, 1977). The above studies being highly generalised for the Indian Ocean as a whole and supported by relatively less number of samples, they cannot be considered conclusive. For the geographical distri-bution, the average values for 5° squares only were worked out which again tell upon the inadequacy of samples. The present study pertaining mainly to the EEZ of India based on a relatively large number of samples has permitted a microlevel analysis of the spatial distribution on a half-degree basis and of other aspects of abundance. The material and methods have been detailed in the paper dealing with the zooplankton biomass KJ.MATHEWetfli. given elswhere in this volume (Mathew et fl/.,1990) The unit biomass is always expressed as number per 1000 m^ of water. RESULTS AND DISCUSSION While sorting zooplankton collected onboard FORV sugar sampada it was found that the planktonic gastropods formed a major constituent. Among them the pteropods and heteropods had a major share quatitatively and hence were taken up for a common consideration to facilitatea rathercomparativestudy. Some species of the pteropods are known for their swarming habit in the shelf waters during some months when the food is plentiful. In view of this, the pteropods were considered separately from the heteropods. In all the situations the pteropods surpassed theheteropods in number. Thus while the pteropods had an average density of 734 in the entire area of study, the heteropods had an average concentration of 335 only. The former group occurred at a rate of 790 in the eastern Arabian Sea and at a rate of 651 in the Bay of Bengal. The heteropods also had a domi-nance off the west coast with 384 individuals against 262 in the Bay of Bengal. The abundance of pteropods in the continen-tal shelf waters was the most striking. While they were present at a rate of 1,292 in the shelf waters, the oceanic area had them at a rate of 489 only. Such abundance in the shelf waters could be due to the swarming of pteropods in certain months. Such a difference in abundance was not noticed with the heteropods whose density in the two sea areas was 496 and 264 respectively. Both the groups exhibited difference in the diurnal occurrence also, being more in night samples. An interesting point noted was that the percentage of increase in night samples was ex-actly the same for both the groups (27%) which may suggest theirequal ability to perform diurnal vertical migration. Distribution in space The pteropods had a cosmopolitan distribu-tion in the area investigated, of course, in varying densities and this is shown in Fig. 1. Very high-density areas where the number of pteropods ex-ceeded 5,000 per 1000 m' of water were found off Ratnagiri, Kanyakumari, Madras and south of Cal-cutta. High-density areas where the range was be-tween 2,000 and 5,001 occurred off Paradcep, around Andamans, north of Cochin, in the Lakshadweep seas snd southwest of Veraval. The spatial distribution of heteropods is pre-sented in Fig.2. They also enjoyed a universal distri-bution but in smaller quantities. Area of maximum density exceeding 5,000 was observed north of Co-chin in the shelf waters, besides other pockets in the shelf and oceanic areas as seen from Fig. 2. Monthly variations In the case of pteropods, November accounted for the maximumabundance with 1,541 which can be considered a swarm (Fig. 3). The least abundance was in October with 328 specimens. On the whole the April-October period was comparatively less productive with the exception of May and August. It was interesting to note that from February onwards, once in every three months there was an unusual increase in population. Wi th regard to heteropods also, November ac-counted for the maximum abundance with 752 speci-mens. December also had a proportionately high density. Any significant variation in other months was not noticed (Fig. 4). September with 186 indi-viduals represented the month of least abundance. On a consideration of the coastwise monthly abundance of the two groups, it was found that in most of the months the pteropodsoccurred in greater numbers in the eastern Arabian Sea (Fig. 5). In the Bay of Bengal they were relatively more abundant in April, August and January. Off the west coast, while November with 1,541 pteropods stood first, off the east coast it was August which yielded themaximum number, of course, of a low magnitude (992). In regard to heteropods also, abundance in the Bay of Bengal was noticed in February, April and October (Fig. 6). In the eastern Arabian Sea they had the greatest abundance in June. Seasonal variations A further consideration of the abundance on the basis of three seasons, viz., premonsoon (Febru-ary - may), monsoon (June - September) and post-monsoon (October - January) in the two sea areas separately, revealed that no significant difference was observed in the case of pteropods during the three seasons either in the eastern Arabian Sea or in the Bay of Bengal. The differences between the seasons of maximum and minimum abundance were 46 in the former sea area and 209 in the latter. As far as the heteropods also are concerned, there was no 156 6^ si" 66* 67' U1 68* 6^ 7Cf 7(* 72* 7S* 7< TS* 7«* TT 78* TT 80* 81* 82* 83* 84" 8S" 86* 87* 88* 89* 90* »f 92* 9S" Fig. 1 Spatial distribution Ptorpoda m the EEZ of India and adjacent seas. 94* 95* 96* 2 H ; m iO ' -3 iO :> m JO O -3 o o tn O a > z m m N 00 2 > m 82* M 8< 85* a^ UT' 88* 8 ? 9tf 91 SZ" » ? sV 95" Fig. 2. Spatial distribution of Heteropoda in the EEZ of India and adjacent seas. ON THE PTEROPODS AND I lETEROPODS OF THE INIDAN EEZ 15M 1000 o °500f N 6 Z 0, F M A M J J A S O N O J Months Rg. 3. Monthly abundance of I'tcropoda in the area investigated. substantial seasonal difference off the west and east coasts. Monthly distribution in the shelf and oceanic areas As seen from Figs. 7 and 8, the ptcropods and heteropods were more aburidantly present in the shelf area than the oceanic. When the shelf area con-tribiuted pteropods at the rate of 1,292, the share of the oceanic area was only 489. Its maximum density in the shelf was observed during March, June and December. Aninterestingpointnoted wasthatsome kind of population outbursts occurred in the shelf waters in those months during which the density went up sharply from 1,091 in may to 4,454 in June and from 380 in November to 4,824 in December. However, similar outbursts in population never IOOOT o o o 500-o z F M A M J J A S O N D J Fig.4. Monthly abundaiiceol I loloro|H)..l.i m tliearea investigated. I6OO1 , 1200-800 400 I [ Eastern Arabian Seo H B a y or Bengal X no observotion F M J T M J J A S 0 N D J Months Fig. 5. Monthly abundance of Pteropoda in the eastern Arabian Sea and the Bay of Bengal. occurred in the oceanic waters even though ups and downs in population size took place in different months. In the case of heteropods, a sudden increase in population took place in December only, of course, in a moderate way (Fig. 8). In the oceanic areas vast variations were not noticed in different months. Coast-zvise latitudinal abundance With regard to both the groups there was a gradual reduction in population from the southern to the northern latitudes in the eastern Arabian Sea (Figs. 9 & 10). While in the southernmost region 1,126 specimens of pteropods were taken, the contri-bu tion of the northernmost region was at a rate of 390 specimens only. Similarly the heteropods from a maximum of 496 in the southern latitudinal region i200n o o o 800-400-—I \_\ Eastern Arabian Sea HlBay Of Bengi X no otjservation flDm h F M A M J J A S O N D J M O N T H S Fig. 6. Monthly abundance of 1 leteropoda in the eastern Arabian Sea and the Bay of Bengal. 159 K.J.MATHEWe/fl/. OCEANIC 1553 J J A S MONTHS Fig 7. Monthly abundance of I'tcropoda in the shelf and oceanic waters. thinned down to 355 in the northernmost region. In the Bay of Bengal the situation was almost the opposite: the two groups were more in the north-em latitudes (Figs. 9 & 10). Latitudinal seasonal abundance off the two coasts Figs. 11 and 12 show the seasonal variationsof pteropods and heteropods in the four latitudinal regions in the eastern Arabian Sea and the Bay of Bengal. In all the latitudinal regions, the density of pteropods during different seasons was almost dia-metrically opposite in the two sea areas (Fig. 11). While in the southernmost region in the eastern Arabian Sea there was a gradual reduction in popu-lation from the premonsoon through monsoon to 2500 OCEANIC 1126 • EASTERN AR/«IAN SEA • BAY OF BENGAL 1000-, m £ o o o 500-.aol-i^ ^ ^ > ,#JIgfN ^' 23N Fig. 9. Latitudinal abundance of I'teropoda in the eastern Arabian Sea and the Bay of Bengal. lOOOn j — j Eastern Arabian Sea en i H Bay Of Bengal o o o 500-Fig. 8. Monthly abundance ot I lotoropoda in the shelf and oceanic waters. ^*30;- 10'- 1 5 \ 20 \ tON 15N 20N 23N Fig. 10. Latitudinal abundanccof I leteropoda in the eastern Ara-bian Sea and the Bay of Bengal 160 ON THE ITEROPODS AND HETEROPODS OF Tl IE INIDAN EEZ 1200-r "1 Eastern Arabian Sea I I I Boy Ot Bengal X no observotion PRE MON POST PRE MON POST PRE MON POST PRE MON POST C 3 0 ' - 10'N lO" - 15'N 15' - 20'N 20" — 23 'N Fig. 11. Seasonal abundance of Pterof)oda in the various latitu-dinal regions of the eastern Arabian Sea nd the Bay of Bengal. postmonsoon, in the Bay of Bengal in the same region an increasing trend was noticed from premonsoon onwards. In the second region the density was the least during premonsoon in the eastern Arabian Sea, which gradually increased to the postmonsoon sea-son. At the same time in the Bay of Bengal the pteropod population decreased from a premonsoon maximum to a postmonsoon minimum. Almost the same was the trend in the third and fourth regions also. In the case of heteropods, the trend of vari-ation of populations in the various latitudinal re-gions of the two sea areas during different seasons was noticed in an opposite way in the two southern latitudinal regions (Fig.'12). In the third and the fourth legions of the eastern Arabian Sea and the Bay of Bengal, the seasonal fluctuations followed the same trend, either increasing or decreasing. plEoslern Arabion Sea • e a y Of Bengal PRE MON POST i'sa - IO'N PRE MON POST 10* - I^ N PRE MOi POST l5 - 2(JN PRE MON POST 2lJ - ZSH Fig. 12. Soasonal abundance of Hctcropoda in the various latitu-dinal regions of the eastern Arabian Sea and the Bay of Bengal. Abundance in the shelf and oceanic waters of the four latitudinal regions of the west and east coasts dur-ing different seasons A still finer analysis of the data was made to understand the variations in the abundance in the shelf and oceanic areas of the various latitudinal regions in the two sea areas and the results obtained are given in Fig. 13 for pteropods and in Fig.l4 for heteropods. One significant observation made was that the pteropods experienced an explosion of the population in the shelf waters of the southernmost latitudinal region of the eastern Arabian Sea during the premonsoon. Off the east coast three such popu-lation explosions occurred in the shelf waters but of comparatively less intensity during the monsoon in the first latitudinal region and during the premon-soon in the second and the fourth regions. In general the populationsof pteropods were significantly more in all the regions in the shelf water. 3000-, Bay Of Bengol ^2000 r~\ o o ^KX» r - i I Jill..lLri.lift[ill l| tooo-3000 Eastern Arabian Seo Qshett ^Oceonic X no observatian 1000 PI Jlllililillilii^^ PRE MON POST PRE MON POST PRE MON POST PRE MON POST 4 3 0 ' - 1 t f N - 10'—15'N 15" — 20"N 20* — 23"N Fig. 13. Seasonal abundance of Ptcropoda in the shelf and oceanic waters in the different latitudinal regions of the eastern Arabian Sea and the Bay of Bengal. With regard to heteropods also population density was more in almost all latitudinal regions and seasons in the shelf waters but population out-breaks were not as intense as in pteropods. How-161 K.J. MATHEWe/ay. ever, very high numbers were noticed in the shelf waters of the northernmost region in the two sea areas (Fig. 14). BAY OF BENGAL KOOn SOO Q shelf H oceonic X no observation J lift .Jl^HBlr^ftll ,.J EASTERN A R A B I A N SEA PRE MON POST PRE MQN POST PRE WDN POST PRE HON POST fc 30' - ION VS - 1 5 N 15 - 20 N 20* - 2 / N Fig.l4. Seasonal abundance of 1 leteropoda in the shelf and oceanic waters in the different latitudinal regions of the eastern Arabian Sea and the Bay of Bengal. Day -night abundance There was a clear difference in the day-night occurrence of both pteropods and hetcropods (Figs. 15 & 16) with more numbers in the night samples. The overall increase in the night samples amounted to about 27%. When considered on a monthly basis itwasfoundthatinFebruary, March and August, the Ddoy • nigtit 1000--5C» 23S1' V,5t H F M A M J J A S O N O J M o n t h s Fig. 15. Monthly day - night variations in the abundance of Pteropda. 162 • doy 1000-1 • n i g N 122, o5oa ^TflUm M A M J J A S O N D J M O N T H S Fig. 16. Monthly day - night variations in the abundance of 1 leteropda. day samples had relatively more pterOpods. The diurnal variation was least noticed in June for this group. The night time abundance was phenomenal in May and November when 3-5 times increase was observed. When compared to pteropods, the monthly diurnal variations among heteropods were not very striking except in June and November. In the former month, more than 100% increase was noticed in the day samples. The other months which exhibited a day time abundance though of low magnitudes were February and December. In general the monsoon months had more number of heteropods in the night samples. ACKNOWLEDGEMENTS We are thankful to Dr. P.S.B.R. James, Direc-tor, CMFRI for his keen interest shown in this work. Our thanks are also due to Dr. K. S. Scariah and Shri P.P Pavilhran for the help rendered in the computer analysis of the data. REFERENCES ARAVINIJAKSIIAN, I ' .N . 1969. Freliminary report on the geographi-cal distribution of the species of Carinariidae and Ptero-tracheidae(Hctcropoda,Mollusca) from the International Indian Ocean Expedition. Proc. Symp. Indian Ocean, Part 2: 575-584. National Institute of Sciences of India. ARAVINOAKSHAN, I'.N. 1973. Distribution and ecology of Pterotra-cheacoronata {PoTska\). In: The Biology of the Indian Ocean. Bernt Zeit/..schcl (Ed.), .389-400. Springer Verlag, Berlin. ARAVINI)AK.SI[A.N, P.N. 1977. Ptcrotracheidae (Metcropoda, Mol-lusca) of the Indian Ocean from the International Indian Ocean Expedition. Proc. Warm Water Zoo-pl., Spl. Publ., UNESCO/NIO, 137-145. ON THE PTEROPODS AND HETEROPODS OF THE INIDAN EEZ MATHEW, K.J., T.S. NAOMI, GEETHA ANTONY, D . VINCENT, R. SAKTHIVEL, M. 1973 a. Studies on Limacina inflata d' Orbigny ANIUCUMAR AND K. SOLOMON 1990. Studies on zooplank- (Thecosomata: Gastropoda) in the Indian Ocean. In: The ton biontass and secondary & teritary production of the Biology of the Indian Ocean. Bernt Zeitzschel (Ed.), 383-397. EEZ of India. Proc. First Workshop Scient. Resul FORV Springer Verlag, Berlin. Sagar Sampada, 5-7 June, 1989, Cochin, 59-69. SACTHIVEL, M. 1969. A preliminary report on the distribution and relative abundance of Euthecosomata with a note on the SACTHIVEL, M . 1973 b. Bio-geographical change in the latitudinal seasonal variation of Limacma species in the Indian ocean. boundary of a bisubtropical pteropod Stylioh subula Proc. Symp. Indian Ocean, Part 2: 700-717. (Quoy et Gaimard) in the Indian Ocean. Ibid., 401-404. 163 

Distribution and abundance of pteropod and heteropod molluscs in the EEZ and adjoining waters of India

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Distribution and abundance of pteropod and heteropod molluscs in the EEZ and adjoining waters of India

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