![Sample our Environment & Agriculture journals, sign in here to start your access, latest two full volumes FREE to you for 14 days]({"type":"image" , "src" : "/sda/5934/TOC-Subject-Sample-2021-Environment-Agriculture.jpg"})
Abstract
The frustule of Cocconeis costata var. costata, its varieties hexagona and pacifica, and C. pinnata (Bacillariophyceae), mainly collected in SE Pacific Ocean coastal and antarctic waters, have been studied. Ultrastructural information is provided for the first time for most of them. The taxa show several common features in relation with the low density and areolation of the striae (except for C. costata var. pacifica); broad and highly internally silicified interstriae; areola opening and arrangement, and location of the areola occlusion; type of valvocopula and absence of fimbriae in both valves, and presence of a transverse hyaline central area in the sternum valve (or araphid valve). Only the valvocopula of the sternum valve of C. costata var. pacifica displays fimbriae which are similar to the crenate edge of a leaf. Though only partially studied, the areola occlusion shows great uniformity on comparing both valves. The taxa differ with respect to the overall morphology and the areolar density of both valves.
Several ultrastructural features are shown for the first time for Cocconeis taxa. The sternum valve of C. costata var. pacifica displays an axial plate with variable width in different specimens. The raphe-sternum valve (or raphid valve) of C. costata var. hexagona possesses a marginal ridge or crista marginalis, similar to that of some centric diatoms but never as well developed. C. pinnata shows overall similarity with C. costata varieties but differs mainly in the usually irregular areolar pattern in both valves, presence of a closed valvocopula in the sternum valve and the recognition of a fascia in the raphe-sternum valve. The recognition of Pleuroneis Cleve as a separate genus and comparison with other Cocconeis groups is considered.
