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Stelladinium robustum

Zonneveld, K.A.F. and Pospelova V. (2015). A determination key for modern dinoflagellate cysts. Palynology 39 (3), 387- 407.

 
dorsal view
photographs Karin Zonneveld
cross section
ventral view

Field characteristics

Stelladinium robustum Zonneveld 1997

Field characteristics:
Pale brown, dorsoventrally compressed, cyst with pentagonal outline. Cyst wall is formed by a smooth pedium. Epicyst triangular with a prominent apical horn. Hypocyst trapezoidal with two distinct antapical horns. Two lateral horns are present at the lateral sides of the cingulum. No additional horns are present in cingular region. Horns show no thickening of the cyst wall at their distal ends. Tabulation reflected in cingulum and archeopyle only. Archeopyle is formed by the loss of the 2a plate and is pentagonal, with enlarged sutures between plates 4' and 2a and between the intercalary and precingular plates. Operculum is often attached.

Dimensions: length (spines included): 92-125 μm; body width (spines included): 82-126 μm

Comparison with other species:
The species most closely relates to Stelladinium stellatum but differs from it by largeer size and the lack of any spines/processes in its sulcal region. This makes this species very easy to identify.

Geographic distribution

Geographic distribution based on :
Zonneveld et al., 2013. Atlas of modern dinoflagellate cyst distribution based on 2405 datapoints. Review of Palaeobotany and Palynology, v. 191, 1-197
Although recently the species has been recorded from the Omura Bay (Japan) Stelladinium robustum can be considered as endemic to the Indian Ocean where it is exclusively observed in tropical to equatorial, mesotrophic to eutrophic settings where full-marine conditions prevail and bottom waters are well ventilated. It occurs in upwelling regions and cysts are produced during active upwelling.
Distribution:
Stelladinium robustum is restricted to full marine tropical and equatorial regions of the Indian Ocean and adjacent seas. High abundances (up to 5%) occur in the eastern part of the Arabian Sea and off the Island Sumatra (Bay of Bengal).

Environmental parameter range:
SST: 23.6 - 29.8°C (summer - spring), SSS: 33.0 - 36.7 (autumn - summer), [P]: 0.13 - 0.73 μmol/l, [N]: 0.3 - 5.1 μmol/l, chlorophyll-a: 0.1 - 2.6 ml/l, bottom water [O2]: between 1.0 - 4.4 ml/l.
Although Stelladinium robustum occurs in mesotrophic regions, highest relative abundances are found in areas, which are (seasonally) eutrophic. These regions include upwelling areas, where large inter-annual variability in the trophic state of the upper waters can occur.

Comparison with other records:
So far, Stelladinium robustum has only been observed outside the Indian Ocean regions in sediment trap samples of the Omura Bay and in sediments of the Southeast Asian coasts (Japan, Fujii and Matsuoka, 2006, southeast Asia: Furio et al., 2012 as Stelladinium abei). It is not clear if this is the result of a recent introduction of the species in this region as before 2006, this species had not been reported from this area.
In a sediment trap of Somalia (Arabian Sea) these cysts are produced during active upwelling (Zonneveld and Brummer, 2000). No seasonal production pattern has been observed in the Omura Bay (Fujii and Matsuoka, 2006).