Morphology and taxonomy of marine planktonic diatoms of the genus Asteromphalus (Bacillariophyceae) in Antarctic waters and the Southwestern Atlantic Ocean

Abstract

Diatoms of the genus Asteromphalus are common planktonic species in marine waters all over the world, widespread in warm, temperate and cold water regions. The most conspicuous morphological feature is the presence of distinctive hyaline rays on the valve surface, one of them narrower than the others, alternating with areolate sectors. Some morphological characters traditionally used for differentiating species have limited or no taxonomic value when used in isolation, due to a high degree of variation (e.g. number of rays, proportion of the central portion). However, when ambiguous morphological characteristics are considered together they may be diagnostic, e.g. the shape and width of the rays, position of central portion, shape of separating line and the areolae pattern (type of cribrum). Though previous investigations on this genus have thoroughly described the frustule, only a few have studied inter-specific morphological variations. In order to study the morphology and distribution of Asteromphalus species, a large number of water samples collected during different cruises carried out between 1986 and 2009 in the Southwestern Atlantic and Southern Oceans were examined with light and electron microscopes. Eight species of Asteromphalus have been found in the area, A. arachne, A. emergens, A. flabellatus, A. heptactis, A. hookeri, A. hyalinus, A. parvulus and A. sarcophagus. All the documented species were described and illustrated, and have been supplemented with environmental data (temperature and salinity) and a review from the literature. Additional considerations on their regional occurrence and a morphological comparison with other Asteromphalus species are provided.