Pseudo nitzschia guide
  1. Valves lanceolate, fusiform, or linear
  2. Frustules are joined in "stepped" colonies
  3. Raphe strongly eccentric
  4. Plastids two; arranged about transapical plane

Species of Pseudo-nitzschia are strictly marine and widely distributed in the phytoplankton of the world’s oceans.

The genus is best known for its production of a potent neurotoxin. The genus currently includes over 60 species, with nearly half of the species capable of producing domoic acid. Domoic acid accumulates in the filter feeding organisms that consume Pseudo-nitzschia such as shellfish, zooplankton, and finfish. Marine birds, marine mammals, and humans that eat organisms contaminated with domoic acid may experience the syndrome termed amnesic shellfish poisoning.

Frustules are elongated. Valves are lanceolate, fusiform, or linear in outline, with acute or rounded apices. In girdle view, valves are rectangular or fusiform. In most species, striae are indistinct or unresolvable in LM. Virgae may be distinct or unresolvable in LM. The valve face is flat or nearly flat, with an acute valve/mantle interface and a shallow mantle. The raphe is strongly eccentric and subtended by fibulae. Central raphe ends may be present or absent. The raphe not elevated above the plane of the valve (i.e., a keel is absent). The raphe canal lacks poroids in the wall. Striae are uniseriate to multiseriate, consisting of small hymenate areolae commonly referred to as “poroids” in the Pseudo-nitzschia literature.

Each areola is occluded by a single hymenate velum perforated by nano-pores in a regular distribution or by a more complex velum divided into hymenate sectors separated by unperforated areas. The morphology of the pore occlusion (TEM) is useful in delineating species. The rows of poroids continue uninterrupted onto the mantle distal to the raphe and are separated from the valve face poroids by a solid strip of silica at the valve face/mantle junction. The poroids on the mantle proximal to the raphe may or may not match those on the valve face in size and/or number. The virgae are not raised above the external valve surface, but internally, the striae lie within depressions between adjacent virgae. Asymmetry or heteropolarity may be present in some species either by the two hemivalve margins differing slightly in outline or the two apices of the same valve having different poroid densities.

The cingulum consists of three open, narrow, distinctly pointed, perforated bands. An additional unperforated band may sometimes be present. Each band is widest at mid-valve and tapers sharply toward both apices. The closed end of the band is often referred to as a "loop" and typically breaks in slide preparations such that half bands may dominate in the sample. One to several linear rows of perforations characterize most bands, which are occluded by hymenate vela as in the valve poroids.

Living cells of Pseudo-nitzschia contain two plate-like chloroplasts arranged symmetrically about the transapical plane of the cell (i.e. fore-and-aft arrangement). Except for the lone exception of P. americana, Pseudo-nitzschia cells form stepped colonies where adjacent cells overlap one another at their ends. Peragallo 1900 (in Peragallo and Peragallo 1897–1908) indicate the filamentous, stepped colonies as a defining character of the genus.

The fibulate raphe, hymenate pore occlusions, and fore-and-aft plastid arrangement of the genus Pseudo-nitzschia also characterize the vast majority of species within the Bacillariaceae (Mann et al. 2021). The absence of poroids in the raphe canals of Fragilariopsis and Pseudo-nitzschia was noted by Lundholm et al. (2002) and by Mann et al. (2021) and correlates with the planktonic habit of these genera, in which the raphe has lost much of its usual functional significance.

Pseudo-nitzschia is commonly divided into morphological complexes to assist in species determinations (Meave del Castillo and Zamudio Resendiz 2018). The americana complex consists of species with broadly rounded apices and occur as solitary cells, or are colonial in stepped chains as in other Pseudo-nitzschia species (Lundholm et al. 2002). The seriata complex consists of species with wider (typically > 3.5 µm) valves and more heavily silicified frustules. The delicatissima complex consists of species with narrow (typically < 3.5 µm) valves and less silicified frustules. The delicatissima complex is further divided into delicatissima and pseudodelicatissima groups. The delicatissima and pseudodelicatissima groups are differentiated by poroid ultrastructure (SEM and TEM). The delicatissima group (Lundholm et al. 2006) consists of species with two rows of simple poroids within a stria, while the pseudodelicatissima group (Lundholm et al. 2003) consists of species with a single row of rectangular areolae with complexly divided pore occlusions within a stria. The morphological groups broadly conform to molecular phylogeny inferred from ITS2 gene sequences, although the Pseudo-nitzschia genus is paraphyletic as Fragilariopsis is embedded within the larger clade (Lim et al., 2018). Fragilariopsis is easily distinguished from Pseudo-nitzschia in live or preserved material by its habit of forming valve face attached ribbon-like colonies as opposed to the stepped chains of Pseudo-nitzschia.

The distributions of the various taxa are imprecisely known because of the necessity of employing SEM and TEM to confidently identify most of the species.