Indian Journal of Marine Sciences Vol. 28, June 1999, pp. 163-168

Mass occurrence of bathypelagic chaetognath Eukrohnia fowleri from the Arabian Sea and Bay of Bengal

M.Terazaki

Ocean Research Institute, University of Tokyo, 1-15-1 Minamidai, Nakano-ku, Tokyo-I64, Japan

Received 5 December 1997, revised 19 February 1999

More than 250 individuals of a cosmopolitan bathypelagic chaetognath Eukrohnia fowleri were collected by ORI net from the Arabian Sea and Bay of Bengal. This high density is supported by high primary production and zooplankton biomass in both areas. Body length ranged from 13 mm to 27 mm Cav 21 mm) in· the Arabian Sea and from 11 mm to 30 mm Cav 18 mm) in the Bay of Bengal. Adults contributed 24% and 12% of total catch respectively from Arabian Sea and Bay of Bengal. Minimum body length of adult was 18 mm. The species inhabit in the oxygen minimum layer in the Arabian Sea and Bay of Bengal. The tropical northern Indian Ocean supports relatively higher population density of Eukrohniafowleri as compared to other oceans.

During the IIOE ( 1960-1965), zooplankton samples were collected vertically from 200 m to the surface with the Indian Ocean Standard Net (lOSN)1 from the Indian Ocean and adjacent seas2. Nair & Ra03

identified a total of 19 species belonging to 4 genera Eukrohnia, Krohnitta, Pterosagitta and Sagitta from 614 samples collected from the Arabian Sea. Recently, Pierrot-Bults & Nair4 reported 35 species including 5 genera from the entire Indian Ocean.

Eukrohnia fowLeri is a cosmopolitan species, abundant at depths below 800 m. It has been recorded from the Arctic to the Antarctic along the Atlantic,

Pacific and in the Indian OceanS-8. EukrohniafowLeri

has red or orange pigments and the pigment generally occurred in the gut but occasionally extended to the

h d d f· 9 S ·I & S . . 10 d S . . II ea an InS. I as f1mvasan an f1mvasan reported S77 specimen of E. fowLeri from 28 stations located in the Arabian Sea along the west coast of India. Nair & Rao 1 reported a total 7 individual s of E. fowLeri from two stations located off the southern coast of India . Breeding of E. fowleri is observed to be continuous throughout the year at ocean station P in the Subarctic Pacific8.

During the World Exped ition of R. V. Hakuho Maru (of the Ocean Research Institute, University of Tokyo) in 1990 E. fowleri were collected from di fferent parts of the world oceans. This paper presents the horizonta l and vertical distribution of E. fowle r; from the Arabian Sea and Bay of Bengal.

Materials and Methods Pelagic chaetognaths were collected by ORI net

(mouth area of 2 m2, mesh aperture of 0 .69 mm)12 from 16 stations during round the World Expedition of the RV Hakuho Maru (Fig. 1; Table 1). The net was towed obliquely at a ship speed of 2 knots and 2000 m wire was paid out. Sampling depth was recorded with a depth-time meter attached to the net. The filtered volume of water was estimated with a flowmeter mounted on the mouth of the net. Horizontal distribution of zooplankton in the epipelagic layer was investigated at same stations using a NORPAC net (0.33 mm mesh with 4S cm mouth di ameter)1J which was towed vertically at the speed of 1 m/s from 150 m to the surface. To investigate vertical distribution of E. fowLeri in detail , horizontal tows were made with a MTD net (56 cm mouth diameter and 0.33 mm mesh 14) from 11 depth layers between the surface and 1000 m, on February 3-4, 1990. Collections were preserved in 10% fo rmalin seawater solution neutralized with sodium tetraborate. Settling volume was measured to estimate net plankton biomass. In the laboratory, chaetognaths were segregated and all E. fowLeri were separated, counted, and measured for body length .

Hydrographic data from CTD casts were collected from all stations . Water samples were taken from 11 depths betwen 0 and 300 m, filtered with a What man GFIF glass microfiber filter and the pigments were ext racted In 90% acetone . Concentration of

164 INDIAN J. MAR. SCI. VOL. 28, JUNE 1999

PACIFIC OCEAN

Iscfw ar!w scfw 4r!W

2O·N

ATLANOC '

O·

• 2~W O· 2r!E. 4r!E st1E acrE • 14!1E IS0E 40W 120E

Fig. I-Track chart of the KH-89-2 cruise of the RV Hakuho Maru

Table I-Abundance of Eukrohnia fowleri, net plankton biomass (settling volu me) and chlorophyll a in the various waters of the world

SI. Area Lat. Long. Depth Sampling date Sampling Chi a* Settling No . (m) layer (m) (f1g!l) volume

(mill)

3 Off California 31-37N 160-59 W 3775 Nov. 241989 0-995 0.02-0.29 0.23 4 Off Mexico 16-44N 110-36W 338 1 Nov. 26 1989 0-1100 0.01-0.26 0.53 5 Off Costa Rica 9-59 N 95-03 W 3877 Nov. 29 1989 0-1 100 0.0 1-0.47 1.88 6 Caribbean Sea 18-08N 78-59 W 2832 Dec. 6 1989 0-1 121 0.09-0.27 0. 19 7 Gulf of Mexico 24-59 N 84-59 W 3384 Dec. 8 1989 0-1 100 0.19* 0.21 8 Western Atlantic 25-00N 59-59 W 6003 Dec. 16 1989 0-1347 0.07-0.23 0.10 9 Central Atlantic 26-44 N 46-56 W 3661 Dec. 19 1989 0-10 10 10.07-0.26 0.11 12 Eastern Atlantic 38-25 N 9-23 W 2958 Jan. I 1990 0~1160

13 Mediterranean 39-00 N 4-58 E 2603 Jan. 4 1990 0-1040 0.0 1-0.86 0.32 14 Mediterranean 36-46 N 17- 12E 3407 Jan. 13 1990 0-1204 0.20* 0 .1 2 17 Arabian Sea 14-58 N 59-58E 4 11 8 lan.24 1990 0-1600 0.01.0.79 1.80 18 Arabian Sea 14-59 N 64-59E 3856 Jan. 25 1990 0- 1150 0.75* t .23 19 Equator 00-00 79-59 E 4662 Feb. 3 1990 0-878 0.06-0.34 0.51 20 Bay of Bengal 15-03 N 85-07 E 3010 Feb. 7 1990 0-14 16 0.04-0.30 0.28 2 1 Bay of Bengal 14-55 N 89-53 E 2723 Feb.8 1990 0-1117 0.10* 0.64

SB-4 Andaman Sea 10-41 N 95-08 E 3106 Feb. I I 1990 0-1367 0.20* 0.52 22 South China Sea 15-03 N 115-05E 4274 Feb 23 1990 0-1015 0.03-0.35 0.30

*= The amount of chlorophyll a indicates the minimum and maximum values except values with an asterisk which were obtai ned from the surface water.

+ = less than I individual / 1000 m3.

E.fowleri (no/1000 m3)

+ + 2

+ + 2 2 0 0 37 5 + 10 40 I I I

...

TERAZAKI: MASS OCCURRENCE OF Eukrohniafowleri 165

chlorophyll a in the extracts was measured using a Turner fluorometer following f1uorometric method l5 .

Results

Hydrographic condition Temperature, salinity (PSU) and dissolved oxygen

(mill) ranges in the 750-1500 m layer were 4.8-1O.3°C, 34.98-35.52, 0 .20-1.28 at st 17, 4.2-8.2°C, 34.83-34.99, 1.16-2.25 at st 19, and 4.6-8.3°C, 34.84-34.99, 0.49-1.74 at st 20, respectively (Fig. 2).

Amount of inorganic phosphate-phosphorus (11M) and inorganic nitrate-nitrogen (11M) at sts. 17, 19 and 20, increased with increasing depth (Fig. 3). PJlOsphate-phosphorus and nitrate-nitrogen in the , 0-300 m layer at st. 17 ranged from 0.38 to 2.41 and from 1.0 to 28.7, respectively .

High concentration of chlorophyll a (> 0.5 Ilgll) in the 0-300 m layer was observed at sts 13 (western Mediterranean Sea), 17 and 18 (Arabian Sea; Table I).

Distribution of zooplankton biomass and Eukrohnia fnwleri

High zooplankton biomass (> 0.6 mllm3) was recorded at sts 17,18 (Pac ific, off Costa Rica) and 21 (Bay of Bengal). A total of 357 and 291 chaetognaths were collected at sts 21 and 17, respectively .

Eukrohnia fowleri was not found in samples at sts 13 and 14 (Mediterranean Sea). Density of Eukrohnia fowleri ranged from 0.1 individualllOOO3 to 40 indi vidualsl 1000 m3. High density was observed in the Arabian Sea, Bay of Bengal and Andaman Sea (Table I) .

Eukrohnia fowleri inhabited below 700 m and were represented in both day and night samples at st. 19 (Fig. 4) where range in temperature, salinity and dissolved oxygen were 6.5-8.3°C, 34.9 PSU and l.l6-1.37 mIll. But low concentration of dissolved oxygen < 1.0 mill was observed in the meso- and bathypelagic layer of the Arabian Sea and Bay of Bengal. Body length of Eukrohnia fowleri collected from the Arabian Sea and Bay of Bengal ranged from 13 mm to 27 mm (av 21 mm) and from 11 mm to 30 mm(av 18 mm) respectively. Adults contributed 24% and 12% of total chaetognath from Arabian Sea and Bay of Bengal with minimum adult length of 18 mm.

Discussion In the Pacific Ocean, Eukrohnia fowleri is

distributed below 500 m except at the Transition Zone between the Subarctic Water and Central Water (Fig. 5). Highest density of Eukrohnia fowleri were collected with the same ORI net, was

o 2 4 6 6 10 (' I iii i i ,I iii' I i I' i I I

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32 33 34 35 36 37 32 33 34 35 36 37 I ii' I I " " I Ii Ii I Ii if Iii" I I i I II Iii iii I Iii I II " I I i Ii I

o 30

200

400

600

600

1000

, , ",

Station 17

Solinity

(-O~ , , ,

.'

Station 19

Sal inity Temp Sal inity

Station 20

Fig. 2- Vertieal profiles of temperature (OC), salinity (PS U) and di ssolved oxygen (mlfl) at stns. 17,19 and 20 in the Indian Oeean

166 INDIAN 1. MAR. SCI. VOL. 28. JUNE 1999

Fig. 3-Vertical proJiJes of phosphate-phosphorus and nitrate-nitrogen at stns. 17,19 and 20 in the Indian Ocean

6 individual s/ 1000 m' in the Pacific Equatorial Water associated with upwelling. Therefore, the density of this spec ies in the Arabian Sea and Bay of Bengal is far large than density in other oceans . Temperature, sali nity and amount of dissolved oxygen along the distribution layer of Eukrohnia fowleri in the Pacific Ocean were reported in the range of 2.1-8.0°C, 34.20-34.62 PSU and 0,4-2.4 mI/l, respectivel/ . There is no significant difference in vertical distribution of temperature and salin ity between the Pacific and Indian oceans. Eukrohnia fowleri inhabit in the oxygen minimum layer (0.1 mill ) in the Arabian Sea and Bay of Bengal. According to Wyrtki l6, the isolation and stagnation of the north Indi an intermed iate water and the lack of substantial horizontal advection together wi th the high productivity of the northern Indian Ocean cause the development of a large layer of extremely low oxygen concentration in the Arabian Sea and Bay of Bengal. Owens et al. 17 reported high primary production (1391 mgClm2/d) near st 17 on 5 October, 1986.

High density of Eukrohnia f owleri might be supported by rich food organisms in the meso and bathypelagic layer which is related with high zooplankton biomass in the epipelagic layer.

The fertilized eggs of Eukrohnia fowleri are large,

about 900 11m in diameter, and 5 to 6 were included in each marsupial sac. The sacs hang from a trumpet shaped structure at the opening of the oviduct. The shape of the marsupial sacs is ovoid, 1. 1-1.3 mIn in the minor axis and 2.0-2.5 mm in the major axis. The young Eukrohnia fowleri after hatching were 3.0-3.5 mm in body length with 5 to 6 pairs or' hooks l8 .

Number of the young after hatching is very few compared with epipelagic species I9-21. Eukrohnia fowleri reaches maturity at a small s ize under high temperature as observed in Sagitta elegans at Plymouth22. The meso and bathypelagic layer in the Arabian Sea and Bay of Bengal provide a congenial environment to keep high population density of Elikrohniafowleri. It is suggested that low number of th is animal in the Pacific water off Costa Rica is caw:ed by the upwelling as Elikrohnia fowleri cannot adapt to the sudden change in environmental conditions.

Eukrohnia fowleri was di st ributed in the 1000-2000 m layer at ocean station P in the Subarctic PacificR, but occas ionally it appeared in the 500 to

1000 m stratum and length of adults ranged from 24 to 43 mm. M in imum body length of adu lts collected

' f

--

TERAZAKI: MASS OCCURRENCE OF Eukrohniafowleri 167

0

100

200 _

300 Day Night

400

] 500 .c;

Q. • 0

600

700

800

900

1000

L-J

5 inds.11000 m3

Fig. 4-Vertical distribution of EukT'ohnia. fowleri at st. 19 (0 = not collected)

from the Arabian Sea and Bay of Bengal were 20 mm and )8 mm, respectively. Temperature, salinity and dissolved oxygen in the water column at ocean station pi! were 2A-2.9°C, 34.34-34.50 PSU and OA5-0.72 ml/l. Water temperatures in the Arabian Sea (4.9-IOA°C) and Bay of Bengal (4.6-8SC) are higher than that observed at station p_

It appears the tropical northern Indian Ocean supports relatively higher population density of Eukrohnia fowleri as compared to the Atlantic and Pacific oceans. The rich food supply coupled with low competition in the meso-bathypelagic domain provide a congenial environment for the species to flourish .

Acknowledgement The author expresses his gratitude to

Dr. Vijayalakshmi R. Nair, National Institute of Oceanography, India for constructive comments on the manuscript. The author is grateful to the officers

sao

1000

1500

2000

Fig. 5-Vertical distribution of Eukrohnia fowieri in the various waters of the Pacific Ocean

and crew of the RV Hakuho Maru of the Ocean Research Institute, University ' of Tokyo, for their coo{ll!ration during the cruises.

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168 INDIAN 1. MAR. SCI. VOL. 28, JUNE 1999

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