Sciency Thoughts: Three new species of Diatoms from the skin of a West Indian Manatee.

Friday, 3 April 2015

Three new species of Diatoms from the skin of a West Indian Manatee.

Most Marine Mammals are thought to host communities of Epizoic Algae living on their skins, though these have only been extensively studied in Cetaceans (Whales and Dolphins). Two closely related genera of Diatoms, Tursiocola and Epiphalaina typically dominate Algal communities on the skins of Cetaceans, though these do not seem to be host specific. To date three species of Epiphalaina have been described, all from Cetaceans, and four species of Tursiocola, three from Cetaceans, and one from a freshwater Turtle. In addition one of the described Cetacean-dwelling species of Tursiocola has also been found on a Barnacle, and an undescribed species has been reported from Marine Turtles.

In a paper published in the journal Phytotaxa on 27 February 2015, Thomas Frankovich of the Florida Bay Interagency Science Center of the Florida International University, Michael Sullivan of Madison, Mississippi and Nicole Stacy of Large Animal Clinical Sciences at the University of Florida, describe three new species of Tursiocola from the skin of a dead adult female West Indian Manatee Trichechus manatus, found near Coon Key in Florida Bay within the Everglades National Park.

Diatoms are single celled algae related to Kelp and Water Moulds. They are encased in silica shells with two valves. During reproduction the cells divide in two, each of which retains one valve of the shell, growing a new opposing valve, which is slightly smaller and fits flush within the older valve. This means that the Diatoms grow smaller with each new generation, until they reach a minimum size, when they undergo a phase of sexual reproduction, giving rise to a new generation of full-sized cells.

Tursiocola and Epiphalaina are linear or lanceolate (straight or spear-shaped) Diatoms thought to be very closely related. They are separated by two features, members of Tursiocola have a ‘Butterfly-structure’ which joins the sides of each valve at the centre, which Epiphalaina lacks, and Epiphalaina has a single row of pores along its copulae (the connecting band, or area where the two valves connect), while Tursiocola has a double row.

The first new species described is named Tursiocola ziemanii, in honour of Joseph Zieman for his generous support of Diatom research in Florida Bay. These are lanceolate Diatoms 20–61 μm in length and 2.4–5.2 μm in width. They have Butterfly-structures with broad ‘wings’ and a double row of elongate pores on their copulae.

Tursiocola ziemanii, (14–15) external views of whole valves showing axial and central areas, (16) internal view of whole valve showing pseudosepta and butterfly structure, (17) detail of butterfly structure and internal central area with stauros (arrow) and two knobs on the raphe rib. Scale bars: (14–16) = 5 μm; 17 = 2 μm. Frankovitch et al. (2015).

The second new species is named Tursiocola varicopulifera, in meaning ‘bearing a different copulae’; this species has a Butterfly-structure and a double row of copulae pores on one valve, and is assigned to the genus Tursiocola because of it, but the second valve has only a single row of copulae pores. These are lanceolate Diatoms 31–57 μm in length and 2.9–4.7 μm in width.

Tursiocola varicopulifera, SEM images; (42) internal view of whole valve showing pseudosepta and Butterfly-structure; (43) detail of butterfly structure and internal oval central area with two knobs on the raphe rib; (44) entire frustule in girdle view showing differentiated copulae and wide mantle areas. Long arrow indicates valvocopula; short arrow indicates abvalvar copula. Scale bars: (42, 44) = 10 μm; (43) = 1 μm. Frankovitch et al. (2015).

The third new species is named Tursiocola costata, meaning ‘ribs’ in reference to the clearly visible internal ribs of the species. These are lanceolate Diatoms 17–29 μm in length and 2.5–3.9 μm in width. This species possesses only a single round copulae pores.

Tursiocola costata, SEM images; (65) internal view of whole valve showing pseudosepta and Butterfly-structure, (66) detail of butterfly structure and internal hexagonal central area with two knobs on the raphe rib, (67) isolated copula showing internal structure and opening at one end (arrow), (68) isolated copula in girdle view showing opening at one end (short arrow), a single row of pores, and a central tab (long arrow) on the advalvar side. Scale bars: (65, 67–68) = 5 μm; (66) = 1 μm. Frankovitch et al. (2015).

At the moment the two closely related Diatom genera Tursiocola and Epiphalaina are separated by tow features; the presence or absence of a Butterfly-structure, and the number of pore-rows on the copulae. These newly discovered species all possess Butterfly-structures and are placed within the genus Tursiocola. However they also possess variable numbers of copulae pore rows, and Frankovich et al. suggest that it is possible that future discoveries of more species within these genera may reveal other intermediate states, calling into question the separation of the genera.