Didinium gargantua
Meunier, 1910

Key features
Cell ellipsoidal, anterior end truncate with distinct conical protrusion (proboscis); two ciliary bands (anterior - ACB and posterior - PCB), two dorsal brosses, one underneath each ciliary band; one macronucleus, kidney-shaped to C-shaped

Measurements
Length: 70 (40-200) µm
Width: 50 (30-120) µm
No of K: 45 (50-75)
Ma size: 30 x 15 µm
Biovolume: 85,000 µm3

Movement
Rotates about main body axis.

Food
Mainly heterotrophic flagellates, dinoflagellates, cryptophytes, chlorophytes (cell diameter 5-10 µm); can ingest cells of equal size or larger

Ecological data
Temperature: 8-18 ºC (-2 ºC from sea ice)
Salinity: 12-34 ‰ (41 ‰ from sea ice)

Species description
Cell broadly ellipsoidal, posterior widely rounded; Lugol’s fixed cells are occasionally globular, 70 (40-200) µm long and 50 (30-120) µm wide; anterior end truncated, with a distinct conical protrusion (= proboscis), occupying 15-25% of the body length (Fig 1,2,5); extrusomes scattered throughout cell, toxicysts are located between the nematodesmata (Fig 1).

Cytostome on the apical end of the proboscis; nematodesmata reach deep into the cell (Fig 1); cytostome flexible, ciliate can ingest prey larger than its own size.

45 (50-75) equally spaced somatic kineties, only partly ciliated, forming anterior and posterior ciliary bands (ACB, PCB, wreath of pectinelles: about 15 kinetosomes per row; Fig 1,2,5) inclined to the body axis (Fig 1,5); cilia ~16 µm long; two dorsal brosses, one underneath each ciliary band (Fig 1), consist of 2-4 rows of dikinetids (characteristic for the different populations studied, however, not seen in Lugol’s fixed material).

One macronucleus, kidney-shaped to C-shaped, near the center of the body located (Fig 1,2).

Similar species
Didinium nasutum (freshwater or brackish water, dorsal brosse with 5-9 rows, macronucleus considerably longer); D. chlorelligerum (freshwater, dorsal brush with more rows, with symbiontic green algae); Monodinium balbianii (only one ciliary band - NOTE: two ciliary bands might be present during division).

List of synonyms
-

Taxonomical remarks
Didinium gargantua was originally described from the Arctic Sea (Meunier 1910) and recently redescribed from Antarctic sea ice (Petz, Song & Wilbert 1995). Most records from marine and brackish waters refer to this species. However, differences in the infraciliature between polar and North Sea populations suggest that at least three marine species of Didinium exist. Since protargol staining is neccessary to reveal the species specific features, we only refer to D. gargantua. A detail taxonomic study on Didinium from the Arctic type location is required, before further taxonomical steps are taken. These taxonomical problems should be recognised, when interpreting ecological data.

References
Agatha S (unpubl) Morphology, infraciliature, and ecology of some planktonic and tychoplanktonic ciliates (Protozoa, Ciliophora) from coastal waters of the North Sea.

Leegaard C (1920) Microzooplankton from the Finnish waters during the month of May 1912. Acta Soc Sci Fenn 48:1-44

Meunier A (1910) Microplankton des mers de Barents et de Kara. In: Bulens C (ed) Duc d’Orléans. Campagne arctique de 1907. Bulen, Bruxelles:1-355

Petz W, Song W, Wilbert N (1995) Taxonomy and ecology of the ciliate fauna (Protozoa, Ciliophora) in the endopelagial and pelagial of the Weddell Sea, Antarctica. Stapfia 40:1-223

Protist Information Server 1995-2001 (http://protist.i.hosei.ac.jp/PDB/Images/Ciliophora/Didinium/index.html)

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