Algae World: Sellaphora

The valve poles

At the poles, the raphes are bent towards the secondary side of the valve (as in many other raphid diatoms). By contrast, in another genus with a similar stria and pore structure – Cavinula – the raphe slits point in opposite directions at the two poles. In this terminal region of the raphe, the raphe slits do not penetrate through to the interior of the valve: they are blind external grooves beyond the position of the helictoglossa (see previous page) and here comprise the 'terminal fissures'. 

Close examination suggests that the terminal fissures vary subtly but apparently consistently between different Sellaphora species, as shown above. In some species, the terminal fissures bend abruptly above the helictoglossa, whereas in others they curve smoothly. Elsewhere, double inflections are found. Another characteristic possessed by some but not all species is that the raphe slits are sinuous between pole and centre (above, third from left).

The polar bars present in some Sellaphora species are difficult to detect externally and appear as slight interruptions of the striae.