Two new taxa of A chnanthidium and E ncyonema ( B acillariophyceae) from the Y om R iver, T hailand, with special reference to the areolae occlusions implying ontogenetic relationship

Summary We discovered two new taxa of A chnanthidium and E ncyonema occurring with high abundance in the upstream of the Y om R iver, which is one of four major rivers in N orthern T hailand. A chnanthidium pseudoconspicuum var. yomensis var. nov. closely resembles A chnanthidium pseudoconspicuum var. pseudoconspicuum in scanning electron microscopy ( SEM ), but has clearly different valve morphometry under the light microscopy ( LM ). A chnanthidium pseudoconspicuum var. yomensis is also similar to A. japonicum in valve morphology under LM , but differs to this species based on SEM observation. E ncyonema yuwadeeanum sp. nov. shows similarities to E ncyonema leei and E ncyonema subkukenanum , but differs in valve shape and/or polar fissure shape. We discuss the possible valve ontogenetic relationship between A chnanthidium and C ymbellales based on the detailed areolae structures of these two new taxa.