Study of Dental Fluorosis in Subjects Related to a Phosphatic Fertilizer
Indian Journal of Geo Marine Sciences Vol. 46 (07), July 2017, pp. 1371-1380
Diversity and abundance of epipelagic larvaceans and calanoid copepods in the eastern equatorial Indian Ocean during the spring inter-monsoon
Kaizhi Li, Jianqiang Yin*, Yehui Tan, Liangmin Huang & Gang Li
Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China
*[E-mail: [email protected]]
Received 20 August 2015; revised 29 September 2015
This study investigated the species composition, distribution and abundance of larvaceans and calanoid copepods in the eastern equatorial Indian Ocean. In total, 25 species of larvaceans and 69 species of calanoid copepods were identified in the study area. Although the average diversity and evenness indexes of larvaceans were lower than those of calanoid copepods, the abundance of larvaceans was higher than that of calanoids with means of 40.1±14.9 ind m-3 and 28.4±9.1 ind m-3, respectively. Larvacean community was numerically dominated by Oikopleura fusiformis, Oikopleura longicauda, Oikopleura cophocerca, Fritillaria formica and Fritillaria pellucida, accounting for 83% of total larvacean abundance. The calanoid community was dominated by the following five species, represented 61% of calanoid copepods: Clausocalanus furcatus, Clausocalanus farrani, Acartia negligens, Acrocalanus longicornis as well as the copepodite stage of Euchaeta spp. This study highlights that the importance of larvaceans in the eastern equatorial Indian Ocean.
[Keywords: Appendicularians, Calanoids, Water mass, Monsoon, Indian Ocean]
Introduction It has been clear that small organisms of marine Clausocalanus) and the cyclopoid genera (such as zooplankton have historically been under-sampled by Oithona, Oncaea and Corycaeus1). Most calanoid coarse-mesh nets1,2. Larvaceans and copepods are copepods are considered as ‘herbivorous’ zooplankton important constituents of the mesozooplankton, and with a preference for larger particles. Small calanoid play pivotal roles from the microbial ecosystem to copepods often form a dominant zooplankton group in higher trophic levels in the tropical oligotrophic tropical pelagic environments11. They have a great waters. Larvaceans (or Appendicularians), an influence on the efficiency of trophic coupling important group of zooplankton, appear to show little between the primary producers and other larger taxonomic diversity with only 65 species currently pelagic carnivores1,12. Although the important roles of described due to their general frailty2. In recent years, larvaceans and copepods in marine food webs are it has become obvious that the remarkable filtration well studied, little is known about their diversity, systems and rapid population growth of larvaceans abundance or distribution, especially in tropical may be contributing to their population sizes equaling oligotrophic waters. or exceeding those of copepods3-8. Larvaceans play a Monsoons play a vital role in determining the pivotal role, from the microbial ecosystem to upper various physical and chemical features of the northern trophic levels, and their elaborate ‘houses’ enable Indian Ocean. The circulation of the upper layers them to be efficient filter feeders of picoplankton and undergoes drastic seasonal variations, with the nanoplankton9,10. southwest monsoon current occurring during May- Copepods, with a length < 1 mm, are the most September, and the northeastern monsoon current abundant metazoans on Earth, including adults and during December-February13. Another important copepodites of the calanoid genera (such as current is the Wyrtki Jet, located between 2°S and Paracalanus, Pseudocalanus, Acartia, and 2°N in the equatorial Indian Ocean. The Wyrtki Jet is
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an intensive and narrow eastward surface current that (unit: m3). The filtered water volume ranged from occurs twice a year during the monsoon transition 38.70 to 79.98 m3. Trawl winch speed was about 0.5– periods in spring (April-May) and fall (October- 1m/s. The samples were preserved in a 5% formalin- November)14. The Wyrtki jet and the equatorial seawater solution for later analysis. A subsample, undercurrent may advect the Arabian Sea’s salty (10% of the sample), was counted under a water eastward along the equator and thereby change stereomicroscope?after fraction with a stempel- the upper ocean salt and nutrient levels in the eastern pipette. All specimens for larvaceans and copepods Indian Ocean13-15. For example, two distinct water were identified to species level when possible, based masses have been identified below the subsurface on the status of taxonomic information currently waters located north and south of the equator, with the available19-24. first consisting of low-salinity waters from the Bay of Bengal and the second consisting of high-salinity and nitrate deficient waters from the Arabian Sea16. The distribution and abundance patterns of mesozooplankton of the Bay of Bengal and the Arabian Sea have been described, with the former being closely influenced by fresh water inflow and cold eddies17, and the latter being influenced by seasons, upwelling, and oxygen concentrations18. However, little information is available on the mesozooplankton from the eastern equatorial Indian Ocean, especially for larvaceans and copepods, which
are important intermediates between the classical and microbial food webs. Fig. 1—The location of sampling stations in the eastern equatorial Therefore, this study investigated the species Indian Ocean. The triangles represent the location of stations on the latitude (5°N-5°S) scale, and circles represent the location of composition and abundance of larvaceans and stations on the longitude (80°-95°E) scale. calanoid copepods in the eastern equatorial Indian Ocean, along with the environmental conditions Vertical profiles of temperature and salinity were influencing their distribution. measured using a CTD (SBE911 Plus, USA) at each station, as described in detail by Xuan et al.15. To Materials and Methods determine chlorophyll a (Chl a) concentration, Sampling and laboratory procedures seawater from 7 depths (0 m, 25 m, 50 m, 75 m, 100 The survey region was located between 5°N and m, 150 m, and 200 m) at each station was filtered 5°S along the longitude from 80° to 95°E, and through a 0.70 μm cellulose Whatman GF/F filter. For included 26 sampling stations (Fig. 1). The cruise was determination of surface picophytoplankton cell supported by the National Science Foundation of fractions, prefiltered samples (3 μm pore-size China, and conducted by the Shiyan 1 scientific polycarbonate filter) were filtered. Detailed research vessel at the South China Sea Institute of information on collection and measurement of Chl a Oceanology.Stations I1-I5 were located around samples has previously been documented by Li et Sumatra Island, 16 stations (I6-I21) lay along the al.25. equator, and the remaining 5 stations (I22-I26) were located between the region south of Sri Lanka and the Data analysis equator. Sampling stations were located at 1° Species richness (S) was calculated as the number latitudinal intervals between 5°S and 5°N as well as of taxa observed in a given sample. The Shannon- 1°longitudinal intervals between 80° and 95°E , Wiener diversity index (H′) and Pielou’s index of except for I1-I5. evenness (J′) were calculated in accordance with Zooplankton were collected at each station once s with a planktonic net (50 cm mouth diameter, 160 μm Ma26: H PlnP , J H , where p is the i i lnS i mesh opening) towed vertically from 200 m depth to i1 the surface during 12–24 April 2011. The net mouth proportion of individuals from a sample unit was equipped with a flowmeter (Hydro-Bios) to belonging to species i. The three biodiversity determine the volume of filtered water in each tow parameters (S, H′ and J′) were assessed for spatial
LI et al.: LARVACEANS AND CALANOIDS IN THE EASTERN INDIAN OCEAN 1373
differences for both larvacean and calanoid copepod Species diversity species. The Shapiro-Wilk test was used to examine A total of 25 species of larvaceans were identified differences in physical and biological parameters with belonging to two families and five genera (Table 1). respect to the latitude (combined I1-I5 and I22-I26) and longitude stations (I6-I21), using a significance level of P<0.05. Pearson’s correlation analysis was used to determine which environmental factors (surface sea temperature, salinity, and Chl a) may have been influencing larvacean and calanoid copepod diversity indexes and abundance (after being log-transformed). The distribution of species and stations in relation to physical and biological factors was explored by canonical correspondence analysis (CCA).
Results Environmental conditions Results on hydrography and Chl a concentration previously published in Xuan et al.15 and Li et al.25 have been briefly redescribed here. Warm water with Fig.2—Variation in temperature (°C) (a-b), salinity (PPT) (c-d), a temperature higher than 28°C was present in the and chlorophyll a concentration (mg m-3) (e-f) at surface (0 m) upper 50 m layer (Figs. 2a-b), and a strong vertical and 50 m, 100 m, 150 m, and 200 m depth. temperature gradient was located at depths of 50-150 m. The temperature at the 100 m layer decreased Three other taxa (Oikopleura spp., Fritillaria spp. sharply from 2°N to 3°S (Fig. 2a), and from 80° to and Euchaeta spp.) were also identified as genera 95°E (Fig. 2b). The salinity distribution was much because of their damaged or immature morphological more complex than that of temperature due to the characteristics. Fritillaria formica comprised two influences of different water masses. Low-salinity subspecies: Fritillaria formica digitata, and surface water was observed south of Sri Lanka (I22- Fritillaria formica tuberculata, with the former being I26) and off Sumatra Island (I1-I10), and extended more abundant than the latter. Similarly, Fritillaria southward from the Bay of Bengal (Figs. 2c-d). pellucida consisted of F. pellucid typical and F. Arabian Sea high-salinity water (> 35.0) was carried pellucida omani, also with the former being more eastward along the equator to around 95°E by the abundant than the latter with the former dominant. Wyrtki Jet and the equatorial undercurrent. It also Sixty-nine species of calanoid copepods were spread toward the region south of Sri Lanka (north to identified belonging to 20 families and 32 genera 3°N). Chl a concentration varied greatly with (Table 1). Calanoids accounted for 56% of copepod increasing depth, with surface values being less than species during the survey. 0.10 mg m-3 in most cases, except at I6 and I21(Figs. The species richness of larvaceans ranged from 10 2e-f). It increased to about 0.15 mg m-3 within the (I23) to 21 (I15) at each station, while the species depth range from 50 m to 100 m, and then decreased richness of calanoids ranged from 18 (I3) to 32 (I24) to less than 0.01 mg m-3 near the 150 m or 200 m (Fig. 3a). Average diversity and evenness indexes of layers. Depth-integrated Chl a concentration ranged calanoids were higher than those of larvaceans from 12.7 to 26.8 mg m-2, and the <3 μm Chl a (calanoid average diversity: 3.51±0.43 and calanoid concentration accounted for 80–93% of total Chl a, average evenness: 0.74±0.08; and larvacean average except at I6 and I7 where the cyanobacterium diversity: 2.56±0.44 and larvacean average evenness: Trichodesmium hildebrandtii was numerically 0.67±0.10). The same patterns of distribution for dominant25. Significant differences between diversity and evenness indexes were observed for latitudinal and longitudinal distribution were found in larvaceans and calanoids (Figs. 3b-c). The difference the surface temperature (P=0.023), surface Chl a in biodiversity parameters between the latitudinal and (P=0.002), integrated Chl a (P=0.005) and <3 μm Chl longitudinal distribution was not significant, with the a (P=0.002), with high values in the equatorial exception of the calanoid copepod evenness index section.
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(P=0.045). The species numbers of both groups with depth-integrated Chl a concentration (Table 2). were positively correlated with surface temperature, Larvacean richness was also positively correlated and their evenness indexes were negatively correlated with surface and <3 μm Chl a concentration.
Table 1— Alphabetical list of larvacean and calanoid copepod species with average abundance, arranged by Family and Genus, collected in the eastern equatorial Indian Ocean during the spring inter-monsoon period. Family Genus Species Density (ind m-3) Oikopleuridae Megalocercus Megalocercus huxleyi Ritter in Ritter &Byxbee, 1905M 0.37±0.48 Oikopleura Oikopleura albicans (Leuckart, 1854)S 0.01±0.03 Oikopleura cophocerca (Gegenbaur, 1885)L 3.23±3.28 Oikopleura fusiformis cornutogastra Aida, 1907M 0.25±0.50 Oikopleura fusiformis Fol,1872L 17.45±8.18 Oikopleura gracilis Lohmann, 1896 S 0.25±0.68 Oikopleura longicauda (Vogt, 1854)L 7.71±4.05 Oikopleura parva Lohmann, 1896 M 0.61±0.71 Oikopleura rufescens Fol, 1872M 0.72±0.70 Oikopleura spp. 0.73±1.42 Stegosoma Stegosoma magnum (Langerhans, 1880) M 0.40±0.46 Fritillariidae Fritillaria Fritillaria abjornseni Lohmann, 1909 M 0.16±0.24 Fritillaria borealis intermedia Lohmann, 1905 M 0.62±0.63 Fritillaria borealis sargassi Lohmann, 1896M 1.93±1.28 Fritillaria formica Fol, 1872L 2.77±2.59 Fritillaria fraudax Lohmann, 1896S 0.01±0.06 Fritillaria gracilis Lohmann, 1896S 0.04±0.11 Fritillaria haplostoma Fol, 1872M 0.49±0.65 Fritillaria megachile Fol, 1872M 0.37±0.35 Fritillaria pacifica Tokioka, 1958S 0.02±0.09 Fritillaria pellucida (Busch, 1851)L 1.95±1.72 Fritillaria tenella Lohmann, 1896S 0.03±0.09 Fritillaria venusta Lohmann, 1896M 0.16±0.48 Fritillaria spp. 0.23±0.27 Tectillaria Tectillaria fertilis (Lohmann, 1896)M 0.11±0.33 Arietellidae Arietellus Arietellus aculeatus (Scott T., 1894)S 0.01±0.03 Augaptilidae Augaptilus Augaptilus longicaudatus (Claus, 1863) S 0.01±0.05 Haloptilus Haloptilu saustini Grice, 1959S 0.01±0.03 Haloptilus longicornis (Claus, 1863) M 0.17±0.27 Haloptilus ornatus (Giesbrecht, 1893) M 0.03±0.07 Heterorhabdidae Heterorhabdus Heterorhabdus papilliger (Claus, 1863)S 0.19±0.21 Lucicutiidae Lucicutia Lucicutia clause (Giesbrecht, 1889) S 0.01±0.05 Lucicutia flavicornis (Claus, 1863) M 0.92±0.87 Lucicutia ovalis (Giesbrecht, 1889)M 0.05±0.09 Metridinidae Pleuromamma Pleuromammaa bdominalis (Lubbock, 1856) M 0.02±0.06 Pleuromamma gracilis Claus, 1863 M 0.47±0.64 Pleuromamma robusta (Dahl F., 1893)M 0.15±0.56 Pleuromamma xiphias (Giesbrecht, 1889) M 0.02±0.06 Acartiidae Acartia Acartianegligens Dana, 1849 L 2.54±1.79 Candaciidae Candacia Candacia catula (Giesbrecht, 1889)M 0.13±0.13 Candacia curta (Dana, 1849) M 0.06±0.13 Candacia pachydactyla (Dana, 1849) S 0.01±0.04 Candacia truncata (Dana, 1849) M 0.11±0.20 LI et al.: LARVACEANS AND CALANOIDS IN THE EASTERN INDIAN OCEAN 1375
Centropagidae Centropages Centropages calaninus (Dana, 1849) M 0.07±0.12 Centropages furcatus (Dana, 1849)M 0.03±0.08 Centropages gracilis (Dana, 1849) 0.005±0.02 Centropages orsinii Giesbrecht, 1889 0.005±0.02 Fosshageniidae Temoropia Temoropia mayumbaensis Scott T., 1894 M 0.05±0.10 Pontellidae Calanopia Calanopia minor Scott A., 1902M 0.12±0.29 Labidocera Labidocera detruncata (Dana, 1849) M 0.01±0.04 Pontellina Pontellina plumata (Dana, 1849) M 0.03±0.07 Temoridae Temora Temora discaudata Giesbrecht, 1889S 0.05±0.15 Calanidae Canthocalanus Canthocalanus pauper (Giesbrecht, 1888)M 0.26±0.33 Cosmocalanus Cosmocalanus darwinii (Lubbock, 1860)M 0.82±0.84 Mesocalanus Mesocalanus tenuicornis (Dana, 1849)M 0.55±0.66 Neocalanus Neocalanus gracilis (Dana, 1852) M 0.17±0.16 Neocalanus robustior (Giesbrecht, 1888) S 0.02±0.07 Undinula Undinula vulgaris (Dana, 1849)M 0.19±0.26 Paracalanidae Acrocalanus Acrocalanus gracilis Giesbrecht, 1888M 0.92±0.91 Acrocalanus longicornis Giesbrecht, 1888L 1.16±1.02 Acrocalanus monachus Giesbrecht, 1888 M 0.58±0.49 Calocalanus Calocalanus contractus Farran, 1926 M 0.06±0.11 Calocalanus gracilis Tanaka, 1956 M 0.09±0.18 Calocalanus pavo (Dana, 1852) M 0.57±0.54 Calocalanus pavoninus Farran, 1936 M 0.28±0.40 Calocalanus plumulosus (Claus, 1863) M 0.38±0.46 Calocalanus styliremis Giesbrecht, 1888M 0.61±1.02 Parvocalanus Parvocalanus crassirostris (Dahl F., 1894) M 0.39±0.58 Eucalanidae Pareucalanus Pareucalanus attenuatus (Dana, 1849) M 0.20±0.29 Subeucalanus Subeucalanus crassus (Giesbrecht, 1888) M 0.02±0.06 Subeucalanus subcrassus (Giesbrecht, 1888)S 0.13±0.30 Subeucalanus subtenuis (Giesbrecht, 1888) M 0.22±0.44 Rhincalanidae Rhincalanus Rhincalanus rostrifrons (Dana, 1849)M 0.25±0.42 Aetideidae Aetideus Aetideus armatus (Boeck, 1872)S 0.01±0.04 Aetideus giesbrechti Cleve, 1904 S 0.07±0.13 Euchirella Euchirella amoena Giesbrecht, 1888 S 0.02±0.06 Euchirella messinensis (Claus, 1863)S 0.01±0.04 Gaetanus Gaetanus minor Farran, 1905 S 0.02±0.06 Gaetanus pileatus Farran, 1903 S 0.01±0.04 Clausocalanidae Clausocalanus Clausocalanus arcuicornis (Dana, 1849) M 0.14±0.38 Clausocalanus farrani Sewell, 1929L 2.77±4.53 Clausocalanus furcatus (Brady, 1883)L 8.46±3.91 Clausocalanus mastigophorus (Claus, 1863) M 0.03±0.13 Euchaetidae Euchaeta Euchaeta concinna Dana, 1849 M 0.02±0.09 Euchaeta indica Wolfenden, 1905 M 0.12±0.16 Euchaeta media Giesbrecht, 1888 S 0.01±0.04 Euchaeta rimana Bradford, 1974 M 0.53±0.60 Euchaeta spp. (Copepodite)L 2.36±1.52 Phaennidae Phaenna Phaenna spinifera Claus, 1863 S 0.03±0.08 Scolecitrichidae Scolecithricella Scolecithricella abyssalis (Giesbrecht, 1888) M 0.14±0.32 Scolecithricella bradyi (Giesbrecht, 1888) M 0.03±0.09 1376 INDIAN J. MAR. SCI., VOL. 46, NO. 07, JULY 2017
Scolecithricella longispinosa Chen & Zhang, 1965 M 0.02±0.08 Scolecithricella minor (Brady, 1883) M 0.13±0.27 Scolecithricella tropica Grice, 1962 0.005±0.02 Scolecithrix danae (Lubbock, 1856)M 0.36±0.65
Fig.3—Distribution of species richness (a), diversity index (b) and evenness index (c) of larvaceans and calanoid copepods at stations across the latitude (5°N-5°S) and longitude (80°-95°E) sampled.
Table 2—Pearson’s correlation relationships between larvaceans and copepods, and environmental variables in the eastern equatorial Indian Ocean.n.s., not significant. P<0.05* Group Variables Surface Surface Surface Surface Integrated temperature salinity Chl a < 3 μmChl a Chl a Larvaceans Richness 0.388* n.s. n.s. n.s. n.s. Diversity index n.s. n.s. n.s. n.s. n.s. Evenness index n.s. n.s. n.s. n.s. -0.443* Abundance n.s. -0.469* 0.412* n.s. n.s. Copepods Richness 0.412* n.s. 0.453 * 0.414* n.s. Diversity index n.s. n.s. n.s. n.s. n.s. Evenness index n.s. n.s. n.s. n.s. -0.422* Abundance n.s. n.s. n.s. n.s. n.s.
Fig.4—Distribution of larvacean and calanoid copepod abundance
LI et al.: LARVACEANS AND CALANOIDS IN THE EASTERN INDIAN OCEAN 1377
Abundance distribution The distribution of five larvacean species was Average larvacean abundance was 40.6±14.9 ind consistent with the pattern of total larvaceans (Fig. m-3, with a range of 11.6–69.3 ind m-3. Average 5a). There was a significant difference in O. calanoid abundance was lower with 28.4±9.1 ind m-3, longicauda (P=0.020) and F. pellucida (P=0.003) accounting for 65% of total copepods (this statement abundance with respect to latitude and longitude, with is already repeated above) (43.6±12.8 ind m-3). High high values in the equatorial section. The abundance larvacean abundances occurred south of Sri Lanka of C. furcatus was significantly higher at the (I25-I26, I21) and off Sumatra Island (I1, I6-I7) (Fig. equatorial section than at the more northern and 4). In contrast, high abundances of calanoids mainly southern latitudinal stations (P=0.006) (Fig. 5b). The occurred along the eastern equatorial section and same was also observed for A. negligens (P=0.010) north to 3°N (Fig. 4). The difference in calanoid and A. longicornis (P=0.014). Results from CCA abundance with respect to latitude and longitude was showed that temperature at the 100 m layer and Chl a significant (P<0.05), but not for larvaceans. concentration were the main factors influencing Larvacean abundance was positively correlated with species distribution (Figs. 6a-b), and stations were surface Chl a concentration and negatively correlated classified by latitude and longitude. with surface salinity (Table 2). No significant correlations were found for calanoids.
Fig.6—CCA triplots of species (a-larvaceans, b-calanoid copepods) abundance (empty triangles) in relation to environmental factors (arrows) and stations in the eastern equatorial Indian Ocean. Note: species is abbreviated as the first letter of genus name followed by the first three letters of the species name; Environmental parameters refer to temperature (T), salinity (S), chlorophyll a (c) at depths of 0, 50, 100, 150 and 200 m as shown in Fig.1 and the location of stations (Log- longitude, Lat-latitude); The number of stations was in accordance with I1- I26, with solid triangles representing latitudinal stations and circles representing longitudinal stations. The three scales of empty triangles correspond to dominant species (large), common Fig.5—Distribution of dominant species abundance of larvaceans species (median) and rare species (small). (a) and calanoid copepods (b) across the latitudinal (5°N-5°S) and longitudinal (80°-95°E) stations sampled. Discussion Species composition and distribution The five dominant larvacean and calanoid species The present study provides fundamental were identified based on the ranking of their information on the species composition and abundances. Oikopleura fusiformis, Oikopleura abundance distributions of larvaceans and calanoid longicauda, Oikopleura cophocerca, Fritillaria copepods in the eastern equatorial Indian Ocean. formica and Fritillaria pellucida were dominant in Twenty-five species of larvaceans were identified in larvaceans, accounting for 83% of total larvacean the eastern equatorial Indian Ocean, which account abundance. Calanoids were numerically dominated by for 63% of appendicularians observed in the Indian Clausocalanus furcatus, Clausocalanus farrani, Ocean27, and 38% of those described in the world Acartia negligens, Euchaeta spp. and Acrocalanus oceans as a whole2. The diversity of epipelagic longicornis, with these species collectively larvaceans in the study area was higher than that in representing 61% of total calanoid copepod tropical neritic waters5, 28, and temperate29 or subarctic abundance (Table1). waters30, 31.
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Observations of species diversity may depend on the Rare occurrences of deeper living copepods was also method and frequency of sampling,. For example, the made by recent studies in the Bay of Bengal32, the Bay of Villefranche has been the best-known reasons could be vertical migration or due to transport temperate area for - larvaceans, due to having been by water masses. studied in detail for over 50 years27. Temora discaudata, a cosmopolitan species, very Although a high diversity of larvaceans was found common in coastal waters33, only occurred at I1, I5, in the eastern equatorial Indian Ocean, five species I25 and I26, which would have been influenced by were observed to be highly numerically dominant low-salinity coastal waters from the Bay of Bengal15. (Table 1, Fig.5a). This striking observation is similar The species distributions of Centropages furcatus, to that found in the Bay of Villefranche, where Centropages gracilis and Centropages orsinii were Fritillaria borealis sargassi, Oikopleura longicauda, also confined to low-salinity areas. The genus O. dioica, O. fusiformis, and F. pellucida made up Clausocalanus is one of the most dominant and more than 80% of total individuals27. Larvaceans are widespread taxa of the small-sized calanoids33, and in often irregularly distributed, and are influenced by this study, C. furcatus and C. farrani were observed temperature and water mass27, 29, 31. In our study, some in high abundances in the eastern equatorial Indian species occurred only occasionally at few stations. Ocean (Table 1, Fig.5b). A. negligens was one of the Such species included Oikopleura albicans, dominant species in the study area. The most Fritillaria abjornseni and Fritillaria megchile, which abundant families were the Paracalanidae and were found mainly at the equator, and Oikopleura Clausocalanidae, which have also been reported to be gracilis, Fritillaria fraudax and Fritillaria gracilis rich in the Spermonde Archipelago33, but not in the which were found at stations I21-I26 in very low northern Indian Ocean32 due to the larger mesh size of abundances (Fig.6a). Fenaux et al.27 reported that the the nets used. It has also previously been reported that predominant species in tropical coastal waters include small copepods dominate the total mesozooplankton O. longicauda, O. fusiformis and Fritillaria community in the tropical Atlantic34. The diversity of haplostoma, whereas shelf waters are often dominated calanoid copepods was higher than that in the tropical by O. longicauda, O. fusiformis and F. pellucida. neritic environment dominated by Parvocalanus Based on this study, O. fusiformis, O. longicauda and crassirostris, Temora turbinata, Paracalanus F. formica may be dominant in tropical or subtropical aculeatus and Acartia spp.11,35. Generally, the oceanic waters27-28 . composition and distribution of calanoid copepods Recent studies of calanoid copepods in the northern collected in the eastern equatorial Indian Ocean may Indian Ocean show that about 183 species occur be influenced by species distribution, ecological frequently in the epipelagic realm 32. In the Bay of habitat, water mass and the size of sampling mesh Bengal, Copepoda form the dominant group, used. Because the sampling was concentrated in the represented by 132 species17. In our study, a total of euphotic zone that was~200 m deep, truly 123 copepod species were identified and calanoid mesopelagic and bathypelagic species of larvaceans copepod represented 69 of those species (Table 1). and calanoids may have been missed in this study. Some of these observed calanoid species are typical inhabitants of coastal or deep waters. Most of the Abundance distribution family Aetideidae species are meso-or bathypelagic In most cases, copepods are the most abundant forms. However, Aetideusgies brechti and Aetideus zooplankters, followed by ostracods or chaetognaths armatus occasionally occurred in surface collections in the Indian Ocean and its adjacent waters18, 27, 33, 36. with low numbers in the study area. Euchirella In this study, however, the abundance of larvaceans messinensis was present only at I12, and Gaetanus was almost equal to that of copepods, and higher than pileatus at I24 (Fig.6b). Species of the family that of calanoid copepods (Table 1, Fig.4). Larvacean Heterorhabdidae are generally deep-dwelling forms, production may exceed that of copepods due to their occurring from the surface to a depth of 3000m32, but rapid growth4,5,7,8. Species abundance seems to be Heterorhabdus papilliger was frequently detected at affected by food availability29, thermohaline fronts37, our sampling stations. Another deep-living species, or large-scale oceanographic boundaries8, 30. Phaenna spinifera, occurred at I7, I15, I21 and I23. During the spring inter-monsoon period, the eastern The abundance of these species was generally <0.10 equatorial Indian Ocean is influenced by low-salinity ind m-3 (Table 1). water from the Bay of Bengal and high-salinity water
LI et al.: LARVACEANS AND CALANOIDS IN THE EASTERN INDIAN OCEAN 1379
from the Arabian Sea, which would increase the Chl a excellent support. This work was supported by the concentration and provide a potential food source for Strategic Priority Research Program of the Chinese larvaceans and copepods25,28. The Chl a concentration Academy of Sciences (XDA11020305); the National above the 150 m layer fluctuates due to the intrusion Project of basic Sciences and Technology of two water masses25. Appendicularians can respond (2017FY201400); and the National Natural Science more quickly to primary production with their unique Foundation of China (41576125). filtration system in the equatorial Pacific than in sub- tropical and temperate waters10, 38. Jaspers et al.8 References reported that larvaceans became more abundant 1. Turner, J. T., The importance of small planktonic toward subtropical/tropical areas, whereas copepods copepods and their roles in pelagic marine food webs. Zool. 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