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Figure 1

Figure 2.

Species Name: Minidiscus trioculatus (Taylor) Hasle
Common Name: None
Synonymy: None

    Kingdom Phylum/Division Class: Order: Family: Genus:
    Protista Bacillariophyta Mediophyceae - - Minidiscus

    Species Description

    As with M. chilensis, this species can be identified with certainty only with the use of electron microscopy because of its small size. In common with other Minidiscus species, the structural processes of the valve are clustered in or near the valve center. The main distinguishing characteristic of M. trioculatus is the comparatively wide hyaline flange at the margin (Fig. 1, TEM). The valve surface is covered with areolae that have sieve-like vela (Fig. 2, TEM; detail of Fig. 1) arranged in a tangential or linear pattern. There is a single sub-central rimoportula and two (rarely three) fultoportulae that have mostly two (but occasionally three) supporting struts (Fig. 2). The fultoportulae and rimoportula are each separated by 2-3 areolae. Separating the rimoportula from one of the fultoportulae is an occluded pore (Fig. 1, arrow). The pervalvar axis is longer than the diameter; the diameter range is reported as 2.0-5.0µm (Throndsen et al. 2007). In the IRL, cells are 2.8-3.9µm.

    The geographic distribution is cosmopolitan in both coastal and oceanic environments. Cell concentrations exceeding 106 per liter are widely known (Buck et al. 2008; Zingone et al. 2011). It is not rare in the IRL but seasonality is unknown. Kaczmarska et al. (2009) have established a separate variety, M. trioculatus var. monoculatus, which differs morphologically in having only one fultoportula. Minidiscus comicus (previously reported in the IRL) and M. chilensis are similar in size to M. trioculatus, but the hyaline margin is lacking in M. comicus, and M. chilensis lacks areolae in the central part of the valve.


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    Throndsen J, Hasle GR, Tangen K. 2007. Phytoplankton of Norwegian Coastal Waters. Almater Vorlag As, Oslo. 343p.

    Kaczmarska I, Lovejoy C, Potvin M, MacGillivary M. 2009. Morphological and molecular characteristics of selected species of Minidiscus (Bacillariophyta, Thalassiosiraceae). Eur J Phycol 44:461-475.

    Buck KR, Chavez FP, Davis AS. 2008. Minidiscus trioculatus, a small diatom with a large presence in the upwelling systems of central California. Nova Hedw Beiheft 133:1-6.

    Zingone A, Sarno D, Siano R, Marino D. 2011. The importance and distinctiveness of small-sized phytoplankton in the Magellan Straits. Polar Biol 34:1269-1284.

Unless otherwise noted, all images and text by PE Hargraves
Editing and page maintenance by LH Sweat
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Page last updated: 28 April 2015

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