Fine structure, taxonomy, and systematics of the centric diatom Fryxelliella gen. nov. (Eupodiscaceae, Bacillariophyta) having a new valve structure, the circumferential marginal tube, with descriptions of F. floridana sp. nov. and F. inconspicua (Rattray) comb. nov.

Abstract

A new genus of eupodiscoid diatoms, Fryxelliella, collected from the Atlantic coast of Florida is described and placed in the centric diatom family Eupodiscaceae. Fryxelliella is characterized by the following features: (1) a circumferential marginal tube (L. ‘sipho marginalis’), hitherto undescribed in diatoms, having small areolae on the part away from the margin; (2) loculate (with an external cribrum) or pseudoloculate valve; (3) a variable number of ocelli, each with closely packed porelli surrounded by a hyaline rim; and (4) at least one rimoportula between a pair of ocelli, the external part of which is present as a short tube. Fryxelliella floridana Prasad, a living species from the Atlantic Ocean, has been selected as the generitype. This taxon is characterized by loculate areolae with external cribra and internal foramina; a circumferential marginal tube whose internal part is areolated and whose external part has several rows of siliceous granules near the edge and a ring of marginal subtriangular holes but no areolae or pores; three ocelli, each with packed porelli and a narrow hyaline rim; and three rimoportulae, one between a pair of ocelli, of which the external part is a reduced tube. Observations of the type material of Eupodiscus inconspicuus Rattray from the Greville Collection, and of Miocene material from the Hanna Collection, identified as Rattrayella inconspicuua (Rattray) Hanna, suggested that this species is closely related to the new genus. It has been transferred to Fryxelliella as the second species of the genus. The fossil representative differs from the generitype by the presence of pseudoloculate valves; in areolation of both the internal and external parts of the marginal tube and absence of both siliceous granules and a ring of subtriangular apertures; in having seven or eight small ocelli of variable diameter, each with packed porelli and a wide hyaline rim; and in having one or two rimoportulae between a pair of ocelli. The relationships of the two species within the family Eupodiscaceae are discussed. Although the two species are accommodated in a single genus, certain morphological heterogeneity is apparent within Fryxelliella. The presence or absence of a marginal tube is considered an important character even at the familial level, and a separate family may be justifiable.