Hemiaulus hauckii Grunow

Division: Chromophyta
Class: Bacillariophyceae
Order: Biddulphiales
Suborder: Biddulphiineae
Family: Hemiaulaceae Heiberg
Genus: Hemiaulus Ehrenberg
Scientific name: Hemiaulus hauckii Grunow

Cells single or united in chains. Valves elliptical in section, with two narrow, pointed, more or less long processes at the ends of the apical axis, parallel to pervalvar axis. One or more hyaline claws on the end of the processes. Intercalary bands indistinct or absent, without septa. Membrane strongly or weakly siliceous, finer or coarser areolated or punctated. Cells
more or less bent; chromatophores numerous (Cupp, 1943).

In Winter 2006 Cruise, the genus Hemiaulus included three species (H. membranaceus, H.sinensis and H. hauckii). The genus contains medium-sized chain-forming diatom algae. Species of this genus were occasionally observed in the RSA hytoplankton community,however, never reached high concentration (not more 8.4*10^3cells/l; 5.7% of total phytoplankton abundance). Relatively high abundance of Hemiaulus spp. (> 10 3 cells/l) was associated with the northwestern part of the inner RSA; the second maximum was recorded in the Sea of Oman.

References:
Pavillard, 1925: p. 56, Fig. 100; Hustedt, 1930: p. 875, Fig. 518;Cupp, 1943: p. 168, Fig. 118; Crosby and Wood, 1958: p. 509,Pl. 36: 23; Hendey, 1964: p. 106; Simonsen, 1974: p. 33.

Description:
Cells long, straight or twisted, often forming chains more or less turned about the long axis. In broad girdle view oblong, with
long thin processes which are strongly pointed. Apical axis 12-35 µm long. Valve surface flat or slightly concave, grooves absent. Valve mantle high, no groove at junction with girdle band.Apertures between cells large because of long processes. Cell wall weakly siliceous, often without visible sculpturing; puncta when visible, 16-17 in 10 µm on valves (Cupp, 1943).

Dimensions:
Cells length 30-42 µm; width 15-19 µm.

Distribution:
Oceanic or neritic; temperate and tropical species. It has been previously reported from the Indian Ocean (Simonsen, 1974).

Sites of occurrence in RSA:
In Winter 2006 Cruise, this species occurred occasionally at various sites (refer sites map),usually in small numbers; maximum abundance (2.8*10^3cells/l) was associated with Kuwaiti waters in the northwestern part of the inner RSA (St. 1).