Genus Microheliella Cavalier-Smith and Chao, 2012
Diagnosis: As for the order Microhelida.
Microheliella maris Cavalier-Smith and Chao, 2012
Diagnosis: Characters of the genus; cell body about 4 µm, axopodia about twice that but so slender that easily overlooked.
Ecology: brackish habitat. River Ebro Delta, Spain
Remarks: Microheliella maris is so small that it is hard to see morphological characteristics under the light microscope, but it can be recognized by four:
1) radiating axopodia,
2) central centrosome,
3) the nucleus situated aside from the cell centre,
4) absence of cilia.
This morphological combination is shared only with centrohelid heliozoa and gymnosphaerids; in the light microscope alone one cannot see their tiny extrusomes and cannot tell that the axopodia actually have microtubules, as shown here for the first time. Gymnosphaerids are far larger, more complicated and adaptively quite distinct; none of those known could be confused with Microheliella. It would be hard to separate Microheliella from the smallest scale-free centrohelids (Oxnerella, Chlamydaster); the smallest of these so far described is typically three times as large as Microheliella, but there is a new species of Oxnerella which is nearly as small (7 µm as opposed to Microheliella’s 4 µm (Chao and Cavalier-Smith, unpubl. observ.). It can be distinguished from Microheliella by careful light microscopy that reveals its much larger and more obvious extrusomes (Cavalier-Smith and Chao, 2012).
Similar looking amoeboids are specimens of the genus Limnofila, which are also very small, but lack microtubules.