Chaetognaths (Arrow Worms) are an enigmatic group of transparent planktonic invertebrates and play an important role in the marine food web. Their morphological and developmental features have been raised extensive debates since the discovery of the phylum in the 18 th century. Uncertainty in the phylogenetic placement of certain chaetognath species still exists and puzzling many scientists who have tried to clarify this task. Studies using a portion of both SSU and LSU ribosomal RNA (rRNA) genes when integrated with conventional taxonomy were contributed to resolve taxonomical issues in this group. Here we present the first phylogenetic study of Chaetognatha based on a portion of mitochondrial cytochrome oxidase I (COI) gene and compare our results with the earlier morphological and molecular evolutionary hypotheses. This study includes 16 extant species, which represent 8 genera and 6 of the 9 extant families. We recommend the following clade structure for the phylum: Aphragmophora comprising Sagittidae with Pterosagittidae and Krohnittidae included in the Sagittidae and Phragmophora comprising Eukrohniidae, Spadellidae and Heterokrohniidae. Phylogenetic analyses also supported the division of Phragmophora into two monophyletic groups, the Monophragmophora and Biphragmophora. Moreover, Ctenodontina/Flabellodontina and Syngonata/Chorismogonata suborders were not validated. Precise phylogenetic investigations using various molecular markers and specimens from diverse regions are definitely needed to provide an exact evolutionary concept on this phylum.
An attempt was made out to study the distribution and diversity of gelatinous zooplankton in the South Eastern Arabian Sea in the region Kanyakumari to off Kollam. A total of 19 species belonging to 8 groups such as chaetognaths, siphonophores, Hydrozoa and Scyphozoa of the phylum Cnidaria, ctenophores, appendicularians, doliolids and salps were recorded. Chaetognaths were not only predominant group, but also the most numerous. Sagitta enfl ata was the most widely distributed chaetognath species from most of the stations studied. Th e siphonophores were the second most abundant group. Shannon-Weaver species diversity index (H′), Margalef’s species diversity index (D) and Pielou’s species evenness (J′) of gelatinous zooplankton were found to be higher from the off shore, Kollam (Station 10) and minimum from the inshore, the Kanyakumari (Cape) West (Station 1). Diversity of gelatinous zooplankton was found to be positively correlated with atmospheric temperature, sea surface temperature and pH. A continuous seasonal study should be conducted to understand the clear impact of physical and chemical factors on the distribution and diversity of gelatinous zooplankton along this region.
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