Familiy Oxnerellidae Cavalier-smith and Chao, 2012

Diagnosis: Naked centrohelids, lacking scales, spicules or mucus coat; axopodia either radiating from the centrosome in all directions or appressed to substratum. Extrusomes conspicuous.
Type genus: Oxnerella Dobell 1917.

 

 

Oxnerella micra Cavalier-Smith and Chao, 2012

 

Diagnosis: tiny naked centrohelid heliozoan with extremely slender axopodia with prominent extrusomes; axopodia are normally stretched across the substratum.
Differs from the only previously named type species O. maritima by smaller size (~7 µm not 10-22 µm) and by the many fewer axopodia (6 or 7) lying along the substratum during feeding and by feeding on bacteria not algae.
Type locality: muddy coastal marine sand, Walney Island, Cumbria, UK.

 

Remarks: Oxnerella micra cells are so small that the nucleus and centrosome are both hard to identify in the presence of ingested bacteria. The only clearly distinguishing features are the long slender axopodia, which do not radiate in all directions as in the much larger Oxnerella maritima (Dobell 1917) but extend along the substratum in much the same way as in many of the smaller granofilosean Cercozoa (Bass et al. 2009). The large prominent extrusomes are the easiest way of differentiating them from the similarly sized marine granofilosean Minimassisteria (Howe et al. 2011), which has much smaller extrusomes that are far less obvious. Minimassisteria also differs by having a flagellate phase (as does the related Massisteria) and an ability to extrude relatively thick pseudopodia, but neither of these features is always expressed. Chlamydaster fimbriatus also spreads its axopodia on the substratum like O. micra when feeding, but is readily distinguished by its substantially larger size and conspicuous fimbriated mucus coat (Dürrschmidt and Patterson 1987). No signs of a mucus coat, scales, cilia, contractile vacuoles, cysts or pseudopodia were visible in O. micra. Young cells of the granofilosean Limnofila with unbranched pseudopods could easily be confused with O. micra (though all known Limnofila species are from freshwater) as they are about the same size, their filopodia have the same thinness, and extrusomes are essentially the same size as in O. micra, but most limnofilids have obviously branching filopodia. The marine Nanofila is smaller with tinier extrusomes and should not be confusable with O. micra (Bass et al. 2009; Bhattacharya and Oliveira 2000). The noncentrohelid heliozoan Microheliella is also smaller and its extrusomes are too small to be clearly detected by light microscopy (Yabuki et al. 2012), so is obviously distinct. O. micra is thus distinguishable in the light microscope from all previously named protists. (Remarks by Cavalier-Smith and Chao, 2012)