after tube formation, with concentric rings of pores within the cribral surface and siliceous caps when mature [2.144].
Coscinodiscus has radial hexagonal loculate areolae, proximal circular rimmed foramen, and complex domed cribra [2.116, 2.126]. Areolae become higher at the margin with increased silica deposition in contrast to the center [2.116, 2.126]. The cribra are perforated and formed via differentiation of the outer velum covering each areola [2.116], and cribella fill the cribra as a small sieve plate structure [2.126]. Coscinodiscus has solid silica ribs outlining areolae radially with variably spotted and striated hyaline rays in the central area [2.126]. A central rosette from which silica strings diverge and branch form a general overall valve pattern [2.116]. There is a rimoportula at the marginal end of every two to three areolae, rimoportulae below the rimmed mantle edge, and other rimoportulae are scattered on the Coscinodiscus valve face, including the terminus of hyaline rays [2.126]. The timing of each daughter cell forming within the mother cell may be different for different species [2.116], which may have implications for symmetry during development.
Cyclotella is characterized by its mantle fultoportulae, clear central area with fultoportulae, and distinctive striations as ribs regularly placed at the valve margin covering about half of the valve face [2.56, 2.126]. At the valve margin, rimoportulae are present in varying numbers. The central area may have tubular fultoportulae and associated pores unlike mantle fultoportulae that may have a collar. Cyclotella initial cells have an unstructured central area and many valve fultoportulae but are hemispherically shaped, unlike vegetative cells which have an undulated shape. Salinity affects Cyclotella and may produce abnormal cells in terms of their internal structure [2.56].
Asterolampra valves form from a raised annulus from which siliceous rays as “spokes on a wheel” are formed, then bifurcated twice so that the spaces between the siliceous “tines” are the sites of areolae formation. Vertically, round pores form prior to the hexagonal honeycomb, and a central area fuses over the rays as a “roof” from the center to the periphery, producing a convex shape. The rays vary in number but are all of similar size and shape [2.146]. The regularity in the valve formation of Asterolampra is an indicator of valve symmetry.
Asteromphalus valves form so that a singular ray is slimmer and shorter than the rest of the rays which are uniform in shape and size but vary in number. Hexagonal areolae form between the rays prior to mantle formation, and large columnar structures form on the edges of areolae for all but the singular ray. Cribra are very complex. The central area fuses over the singular ray prior to the roof forming over the rest of the rays, and the overall surface produces an undulating shape. Internal formation of the rimoportulae is larger for the singular ray than the rest of the rays. Valve features point to asymmetry as does the eccentric annulus of Asteromphalus in contrast to Asterolampra which has a central annulus. Although Spatangidium arachne is similar to Asteromphalus, this taxon is dissimilar in having a central rimoportulae, a different cribral pattern, and a singular ray that is longer than the remaining four rays. Both genera have asymmetric valve faces unlike Asterolampra [2.146].
For species in Actinoptychus, there is a hyaline central area on external and internal valves. A narrow hyaline zone is present in A. splendens and A. senarius that extends from the central area to around halfway to the valve margin. Areolae are irregular in most species, and taxa have a variable number of sectors on the valve face [2.79]. Actinoptychus has a sectored valve face and curled rimoportulae. Areolae form a netted or reticulated pattern on the valve surface with a marginal ridge in species such as A. senarius, and other species have a loculus with a smooth valve face with no marginal ridge [2.57].
Arachnoidiscus has auxospore attachment to the hypovalve of the mother cell, while auxospore attachment to the mother cell of Amphitetras occurs on the epivalve [2.125]. The implication is that valve patterning forms differently depending on the attachment site.
Eupodiscus radiatus has marginal equally spaced ocelli with intercalated rimoportulae extending to the ocelli, a scalloped mantle edge [2.30], and loculate hexagonal areolae and cribral pores arranged in parallel rows [2.30, 2.31]. There are siliceous strips with flanges on the spines and other structures defining the valve mantle as well [2.30]. In contrast, Amphitetras has pseudoloculi with siliceous strips on the mantle without other structures present [2.30]. E. radiatus and Amphitetras symmetry may be based on the number and position of equally spaced ocelli or pseudocelli structures.
Triceratium species have elongated ocelli and poroidal areolae with domed cribra [2.31]. Unlike Triceratium, Lampriscus shadboltianum has pseudocelli [2.30], and Glyphodiscus stellatus has four-part symmetry in which the valve is concentrically undulate. Triceratium pentacrinus fo. quadrata and T. bicorne are uncommon four-part symmetrical species, as most are three-part symmetrical valves [2.31].
Pseudoloculi or loculi are variably present in Triceratium, e.g., as in pseudoloculi surrounding areolar clusters on the valve surface of Triceratium dubium [2.31]. Rimoportulae openings differentiate species, such that Triceratium dubium having an elongated tube opening with apical spinules is different from T. favus having a spatulate opening that is hemispherical internally [2.31].
Trigonium have pseudocelli, centrally located rimoportulae with oval openings, and honeycombed loculate areolae [2.143] in a radial arrangement [2.38, 2.143]. In Trigonium arcticum, areolae are covered by rota-type vela, and a straight cingular suture is present [2.38]. Toward the end of cribra formation, this structure finishes at valve surface level in Trigonium arcticum, and rotae within porelli of the pseudocelli are also being formed [2.143]. For Trigonium formosum, rimoportulae are centrally located and cribra are highly domed [2.143].
2.2.2 Centric Diatoms, Morphology, and Valve Formation
Centric diatom valve formation occurs from a central area to the valve margin in three general stages [2.126] and is illustrated in Figure 2.5. First, horizontal silica deposition occurs to form a basal layer. From an annulus in the central area at the site of valve formation initiation, radial rows of silica are deposited as areolae with cross extensions and connections so that a branching pattern of silica strands emerges. Gaps in this overall pattern are filled in with silica, and internal rimoportulae tubes commence [2.126]. Second, vertical silica deposition occurs so that areolae walls increase in height. Round pores are transformed to hexagonal or other shapes, and external rimoportulae are evident [2.126]. Third, areolar associated structures of cribra and cribella are completed during horizontal silica deposition so that the size and spacing of these structures occur as a response to the constraints imposed by the areolae [2.126]. Successive layers of silica are deposited, and the completion of the valve and its structures extends to the margin [2.116].
Figure 2.5 Three stages of valve formation (as defined in [2.125]) in Biddulphia. Row 1: First stage shows formation of the elongated annulus, branching virgae and vimines producing open holed areolae, and the internal tubes of rimoportulae just beginning. Row 2: Second stage shows the external emergence of rimoportulae, the filling in of areolae, and ridges are more hyaline with increasing silica deposition. Row 3: Third stage shows the beginning of pseudocelli formation, fusing of the valve margin, and the mature cribra. All photos by Mary Ann Tiffany.
Prior to valve formation, mitotic and cytokinetic processes occur [2.122]. Silica aggregation occurs within the silicalemma/SDV where new wall formation occurs [2.59, 2.116, 2.119, 2.126]. The general outline of the cell wall is molded via the plasmalemma [2.122].
As silica deposition occurs from center to the periphery, it is
