New classification key in the genus Chaetoceros (Bacillariophyceae) based on setae structures

Abstract

In order to develop new classification key of the genus Chaetoceros, we identified 33 Chaetoceros species using LM and SEM focused on the fine structures of setae. We found remarkable characteristics which were forms of cross section and arrangements of spines on setae. The genus Chaetoceros were divided into two groups based on these characteristics. In the first group, the key characters were used a circular form in cross section of setae and spines radials arrangement on that. In this group, Chaetoceros affinis, C. brevis, C. contortus, C. costatus, C. curvisetus, C. debilis, C. diadema, C. diversus, C. messanensis, C. pseudocurvisetus, C. radicans, C. salsuguineus, C. socialis, C. subtilis, C. subtilis var. abnormis were involved. In the second group, the key characters were used a polygonal of setae and straight arrangement of spies. C. atlanticus, C. atlanticus var. skeleton, C. coarctatus, C. concavicornis, C. danicus, C. decipiens, C. densus, C. dichaeta, C. didymus, C. didymus var. anglica, C. eibenii, C. laciniosus. C. lorenzianus, C. peruvianus, C. protuberans, C. pseudodichaeta, C. rostratus, C. vanheurckii were comprised of this group. Futhermore, each group was subdivided into six and eleven subgroups according to the others except for the cross section forms and spines arrangement, respectively; shape of spines, existence of special intercalary setae, shape, numbers of poroids. Chaetoceros has been traditionally divided into two subgenus based on cell shape, number and position of the chloroplasts in LM. In spite of the previous classification system, the fine structures of the setae are considered as useful taxonomic characters.