New observations on the Antarctic Asteromphalus darwinii/ hookeri diatom species-complex (Asterolampraceae)

Abstract Antarctic diatom populations of Asteromphalus hookeri and related species such as A. hyalinus and A. parvulus exhibit a highly variable number of hyaline rays ranging from 3 broad + 1 narrow (3 + 1) in the smallest valves, with 4 + 1 (27%) and 5 + 1 rays (35%) most common, and 6 + 1, 7 + 1, and rarely 8 + 1 rays only in larger cells. During December 1959 to April 1960 in the southern sector of the Atlantic Ocean, 6% of valves occurred as “double forms” with epitheca and hypotheca of the same cell exhibiting 4 + 1/3 + 1, 5 + 1/4 + 1, 6 + 1/5 + 1 and 7 + 1/6 + 1 ray combinations. Smaller cells (3 + 1, 4 + 1) always exhibited jagged separation lines in the central area, but larger cells (7 + 1, 8 + 1) had mostly smooth lines, and either jagged or smooth separation lines occurred in intermediate 5 + 1 and 6 + 1 forms, respectively. Epitheca and hypotheca of one and the same cell always exhibited jagged or smooth separation lines, but never mixtures. Observations of silica deposition during October to November 2011 around the Kerguelen Island plateau using the PDMPO fluorescent marker suggest that Asteromphalus separation lines play a key role in silica cell wall development. We discuss implications for taxonomy and our understanding of ecophysiology of what we designate as two highly variable and often confused and overlapping diatom taxa, A.darwiniii (jagged separation lines; synonyms A. beaumontii, A. hyalinus, A. leboimei, A. parvulus, A. rossii) and A. hookeri (smooth separation lines; synonym A. antarcticus, A.buchii, ?cuvierii, ?humboldtii).