NEW RECORDS OF CHAETOCEROS EHRENBERG FROM WETLANDS OF COX’S BAZAR, BANGLADESH

Eleven species of brackish water Chaetoceros Ehrenberg newly recorded from Bangladesh have been illustrated and described in the present paper. The species are: Chaetoceros aequatorialis Cleve , C. constrictus Gran, C. decipiens Cleve, C. denicus Cleve, C. didymus Ehrenberg , C. diversus Cleve, C. pelagicus Cleve , C. pendulus Karsten, C. pseudobrevis Pavillard, C. seychellarus G.H.H. Karsten and C. tetrastichon Cleve. All of these species have been described here with citation of relevant references and collections examined.


Introduction
Chaetoceros Ehrenberg is one of the largest genera among marine phytoplankton and is represented globally by nearly 400 species (Tomas, 1997). In Bangladesh, so far 20 taxa of this genus were described, illustrated and published mostly from the northern Bay of Bengal (Islam and Aziz, 1975;1980). During a recent study on algal diversity in the coastal wetlands of Cox's Bazar, 11 taxa of the genus Chaetoceros were found to occur, which were not recorded earlier from Bangladesh. In this paper, these newly recorded taxa are described and illustrated.

Materials and Methods
The present study was carried out in two wetlands of Cox's Bazar, a tourism city of Bangladesh situated in the northern coasts of the Bay of Bengal. The studied wetlands were: Bakkhali river and Reju canal. This two wetland maintains the flow of entire watershed area of the city of Cox's Bazar. Bakkhali river estuary is located in the southernmost part of Cox's bazar. This river originated from south-eastern hill of Mizoram, India. This is the widest and longest river of Cox's Bazar. Length of Bakkhali river within Cox's Bazar district is about 67 km. Cox's bazar fish landing center is located in the bank of this river. City wastewater and all sorts of drainage discharges are dumped into it. Reju canal is another important river of Cox's bazar originated from north Arakan Mountain of Myanmar. This river produces huge fish and named famous for its marvelous scenario. Many eco-resorts are made in the bank of this river. Salinity of this river was lower than Bakkhali river. A total of 144 phytoplankton samples were collected from September 2018 to August 2020. Phytoplankton concentrates were collected with the help of sedimentation technique using Lugol's iodine (Wetzel and Likens, 2000). In a 1L capacity polystyrene bottle containing 1 ml Lugul's iodine was filled with the sample water and was transported to the National Professor A.K.M. Nurul Islam Phycology, Limnology and Hydrobiology Laboratory, Department of Botany, University of Dhaka for analysis. A random *Corresponding author: mujaddade@yahoo.com 1 Department of Water Resources Engineering, Bangladesh University of Engineering and Technology, Dhaka 1000, Bangladesh checking of the sedimented planktonic material was carried out under light microscope (Nikon Optiphot, UFX-11A microscope fixed with a Nikon FX-35WA camera, Japan) at a magnification of 100-400×. The species were imaged along with the measurement of taxonomic features particularly length and breadth of each cell, filament, etc.

Taxonomic enumeration
A total of 11 brackish water species of Chaetoceros have been identified as new reports from Bangladesh. The illustrated taxonomic descriptions of these taxa are given below.

Chaetoceros didymus Ehrenberg
Collection no. 6 (R3), 8 Sep. 2020, Cox's Bazar, Bangladesh. (Fig. 6) (Cupp 1943, 132, Fig. 87) Chain straight, not twisted, usually short, 3-5 cells found in chain, cells square in shape, setae arising from the corner of the cell, two types setae, one larger other shorter, setae twisted at the base or where they originated, small and thin setae more of less curved, often straight, and heavy or long setae almost cup shaped more curved and slightly thinner in ends, chloroplast more prominent and greenish brown in color. Apical axis 9.0-11.8 µm. Collection no. 5 (R2), 8 Oct 2020, Cox's Bazar, Bangladesh. (Fig. 7) (Subrahmanyan, 1946, 140, Fig. 234;Cupp 1943, 129, Fig. 81) Cells cylindrical, narrow, elongated cells straight connected to each other make a stiff chain, short, dumble shaped, uniform in diameter, two types setae, one upward and other downwards, greenish brown in color, cells 16.2 µm in broad, cells size may vary with season and nutrition.

Chaetoceros pelagicus Cleve
Collection no. 4 (R1), 8 Oct 2020, Cox's Bazar, Bangladesh. (Fig. 8) (Cupp 1943, 114, Fig. 69) Cells always solitary, cells up to 17 µm in width, chain of cells straight, cells rectangular, wide in shape, cells connectedly make a chain, valves unlike, valve center convex, apertures moderately wide, setae very long curved posteriorly, setae started thick but slowly it makes a thin and pointed end, chromatophores very small, distributed far out in the setae, very large in size.

Chaetoceros seychellarus G.H.H. Karsten
Collection no. 5 (R2), 8 Oct. 2020, Cox's Bazar, Bangladesh. (Fig. 11) (Cupp 1943, 108, Fig. 63) Straight chain of cells, cells rectangular, wide in shape, valve center convex, apertures moderately wide, two types of setae, one type long, wide, free other type of setae twisted to each other, curved posteriorly, setae started thick but slowly it makes a thin and pointed end, segmented disc like form found in setae, intercalary band present, cells 19-20 µm in width, robust in size.

Chaetoceros tetrastichon Cleve
Collection no. 5 (R2), 8 Oct 2020, Cox's Bazar, Bangladesh. Adding these 11 newly reported species of Chaetoceros in Bangladesh, the total number stands 31. There are still ample scopes of carrying out research on this most abundant marine phytoplankton in the vast pelagic region of the Bay of Bengal situated near the vicinity of Bangladesh.