Guide to Tursiocola

Species in the genus Tursiocola occur living epizoically on marine mammals and turtles in tropical to polar waters.

Cells are attached to the skin and carapace of animals by one end of the cell or may be motile. According to AlgaeBase (Nov. 10 2024) there are 14 valid species in the genus Tursiocola. Cells are solitary, rectangular in girdle view with rounded corners, and isovalvar.

The valves are isopolar to slightly heteropolar (although at least one species is dorsi-ventral) and narrowly lanceolate, linear-lanceolate or acicular in outline with subacute, obtuse or rostrate apices. The valve face is typically flat or lightly contoured and slopes steeply without any clear transition into a relatively broad mantle. Robust costae or hyaline marginal ridges present in some species. The valve face has uniseriate transapical striae composed of areolae that are transapically elongated, oblong, or rectangular and that extend onto the entire valve mantle. Areolar shape is useful for separating some of the species. The areolae are occluded internally by domed hymenes. The axial area is very narrow and widens at the central area which may be bowtie-shaped, diamond-shaped, rectangular, circular or narrow. Again, variability in the shape of the central area is useful in discriminating between species. The central area is expanded as a stauros or fascia that is highly variable in shape and that extends to the valve margin.

Internal views of the valve reveal a structure connecting the pseudosepta to the central area and stauros or fascia - often referred to as the “butterfly structure” in the literature. The pseudosepta extend from the apices as siliceous plates for approximately 1/3-1/5 of the valve length and then continue as narrow strips that run along the valve margins. The narrow strips of the pseudosepta briefly widen at the valve center where they fuse with the stauros or fascia forming the butterfly structure. The butterfly structure and the pseudosepta enclose two elongated voids on both sides of the central area.

The raphe is filiform and indistinguishable from the straight and strongly silicified axial rib in LM that widens slightly towards the central area. The external proximal raphe ends are simple and are either straight and teardrop-shaped ending in a spathulate groove or deflected towards the secondary side of the valve. The external distal raphe ends are hooked or bifurcated towards the secondary side of the valve and are often obscured by overhanging siliceous flaps that extend from the raphe sternum. In some species there is an extension of the valve mantle at the valve apex that overhangs the apex and extends towards the valve face. This morphological feature is termed a fastigium.. Internally, the raphe slits open along the middle of a strong siliceous rib that widens slightly at the central area. Either 1 or 2 knob-like structures are present on the central nodule between the internal proximal raphe ends. The pseudosepta conceal the internal structure of the distal raphe ends.

The cingulum exhibits perhaps the greatest degree of variability amongst species comprising the genus and is useful for species identifications. It is composed of up to 5 copulae that possess either 1 or 2 longitudinal rows of areolae which are circular, oval, or pervalvarly oriented slits, depending on the species, and are occluded internally by domed hymenes. In addition, the valvocopula may be closed or open and has 3 pairs of advalvar tabs that fit beneath the valve mantle and align with the butterfly structure and the pseudosepta. In whole frustules the advalvar row of pores of the valvocopula is partially obscured by the valve mantle. Additional copulae are open at one end.

All Tursiocola species either live on the skin of manatees and cetaceans or on the skin and carapaces of marine and freshwater turtles. The present hypothesis concerning the distribution and habitat of these species is that they are obligate epizoic taxa. All attempts to grow these species in culture have been unsuccessful to date, suggesting that some compound(s) either produced by the host and/or its associated bacteria are essential for their growth. Observations of living Tursiocola cells have revealed that some species are photosynthetic while others are apochlorotic. The former group possesses a single, highly lobed chloroplast whereas the latter may possess putative leucoplasts and oil droplets and are presumed to be strictly heterotrophic in their nutrition. The only other genera in which photosynthesis has been lost during their evolution are a few species of Nitzschia and Hantzschia in the Order Bacillariales. Molecular phylogenetics show Tursiocola and the Bacillariales to be only distantly related.

Of the 14 Tursiocola species described thus far, 6 are only known from the skin of manatees, 3 only from the skin of cetaceans, and 5 only from the skin and carapace of marine and freshwater turtles. However, it is difficult to make any further statements regarding host specificity, especially with regard to marine turtles and cetaceans, as the sampling effort is minimal and the research effort studying these putatively obligate epizoic diatoms has only been ongoing for the last 10 years.

Tursiocola is morphologically very similar to the genus Epiphalaina, which is also presumed to be an obligate epizoic taxon but its constituent species are known only from cetaceans. The chief difference used to separate these two genera is the butterfly-like structure found in the former which is either highly reduced or absent in the latter. A morphological character analysis by Frankovich et al. (2018) revealed that the butterfly-like structure may be a symplesiomorphy, suggesting that Tursiocola may be paraphyletic. However, a definitive determination to combine the Epiphalaina and Tursiocola species into one genus awaits further phylogenetic analyses as molecular sequence data are lacking for any Epiphalaina species.