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

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 separating lines in the central area, but larger cells (7 + 1, 8 + 1) had mostly smooth lines, and either jagged or smooth separating 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 separating 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 separating lines play a key role in silica cell wall development. We discuss implications for taxonomy of what we designate as two highly variable and often confused and overlapping cold-water diatom taxa, A. darwinii (jagged separating lines; synonyms A. beaumontii, A. hyalinus, A. leboimei, A. parvulus, A. rossii) and A. hookeri (smooth separating lines; synonyms A. antarcticus, A. buchii).

Automated summary

Antarctic diatom populations, such as Asteromphalus hookeri, A. hyalinus and A. parvulus, exhibit a wide range of hyaline rays. Different combinations of hyaline rays are observed in these diatoms, with 4 + 1 and 5 + 1 rays being the most common. Observations suggest that the separating lines in Asteromphalus play a crucial role in silica cell wall development. The taxonomy of two closely related diatom taxa, A. darwinii and A. hookeri, is discussed and their synonyms are listed.