161 Bacteriastrum 輻桿藻 Phylum: Heterokontophyta 门：不等鞭毛門 Class: Mediophyceae 纲：中型硅藻綱 Order: Chaetocerotales 目：角毛藻目 Family: Chaetocerotaceae 科：角毛藻科 Morphology Bacteriastrum and Chaetoceros are similar in morphology, and both were previously classified in Chaetoceraea (subfamily Biddulphioideae) (Huistedt, 1930). However, Bacteriastrum possess radially symmetrical round valves while Chaetoceros valves are bilaterally symmetrical on the apical and transapical axes. Chaetoceros possess two setae per valve, one at each end of the apical axis while Bacteriastrum has numerous setae regularly radiating from valve margin. Due to this, Lebour (1930) established Bacteriastraceae and moved Bacteriastrum (Centricae, Diacoideae) to this family. There are approximately 20-30 species in the genus Bacteriastrum, but only 15 species have been taxonomically accepted (based on AlgaeBase). Bacteriastrum hyalinum and Bacteriastrum mediterraneum are frequently observed in Hong Kong coastal waters. Cells of Bacteriastrum are generally short and cylindrical. Gaps between adjacent cells varies depending on species. Valves are circular with numerous setae radiating from the margins. Setae of two adjacent cells are fused for a certain distance beyond the base, and divide again, displaying Y shape. The length of the fused and bifurcating part of sibling setae between intercalary cells is an important species identification criterion in this genus. Terminal setae are morphologically different from intercalary cells, usually thicker, not fused, and therefore not branched. Morphology and curved direction of terminal setae are different between species. 形態
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