Sceptronema orientale Takano, 1983

Gómez, Fernando, Courcot, Lucie & Artigas, Luis Felipe, 2020, Observations of the diatoms Sceptronema orientale Takano and Tabularia parva (Kützing) D. M. Williams & Round on the exoskeleton of copepods in the English Channel and coastal Celtic Seas, Cryptogamie, Algologie 20 (4), pp. 25-30 : 26-27

Sceptronema orientale Takano

DISTRIBUTION. — The consortium of the epizoic diatom Sceptronema orientale was observed in the port of Brest on the copepod Euterpina acutifrons in July 29-30, 2018. In September 2018, the epibioses were found in four stations near the coasts of England and France ( Fig. 1, Table 1). The percentage of copepods that carried the epizoic diatom never exceeded the 30% of the population. The attempt to culture S. orientale was unsuccessful.

DESCRIPTION

The cells were solitary or forming a linear chain of two cells joined with a mucilaginous pad ( Fig. 2 A-L, N, P-Q). Chains of three or more cells or the connection between the foot (narrow) poles of two cells were not observed. The cells were attached to the whole body of the host. The cells attached to the cephalosome showed short stalks ( Fig. 2J, O) whilst the cells attached to the caudal setae showed stalks with more variable length ( Fig. 2K, L).

Cells were heteropolar in valve view and slightly wedgeshaped. Valve was narrowly obovate with a wide rostrate head-pole and a narrow capitate foot-pole, showing a distinctive outline that resembles an Egyptian sarcophagus ( Fig. 2M, N). The frustules were 28-58 µm long and 7-10 µm wide. Sternum was central, straight and spathulate at the ends ( Fig. 2T). The striae were uniseriate, c. 40 in 10 µm, extending almost to the valve mantle edge. The striae were perpendicular to the sternum, except at the poles. Areolae were elliptical, originated in the transapical direction and occluded. There was an apical slit field in the valve mantle composed of longitudinal bars at each valve pole ( Fig. 2 P-U). The head pole contained more longitudinal slits (c. 17) than the basal pole (c. 13 slits) ( Fig. 2P). The mucilage pad

6° 4°

2° 0° 2°E emerged from the slit fields ( Fig. 2P, Q). There was numerous rimoportulae with an elliptical shape on the external valve surface near the poles ( Fig. 2R, S).

REMARKS

The valve outline of Sceptronema orientale is high distinctive when compared to the other epizoic araphid diatoms. In the original description, Takano (1983: fig. 20) illustrated chains of more than two cells, including cells attached by the foot-pole of one cell to a foot-pole of other cells. We have not observed chains of more than two cells or two cells joined by their foot-poles. The length of the stalk of S. orientale differed according to its location on the host. The stalks are short for the cells attached to the frontal part of the copepod ( Fig. 2 H-J), while the stalks are usually longer in the caudal setae ( Fig. 2K, L). There, the stalk may be branched because the stalk is used as attachment point for the stalk of other individuals ( Fig. 2K, L). This suggests an adaption of the diatom to the epizoic life. Sceptronema orientale is only known as epibiont of Euterpina acutifrons . In the Gulf of Biscay, Paulmier (1997: pl. 181, fig. 13) reported the diatom Pseudohimantidium pacificum Hustedt & Krasske in Krasske, on Corycaeus sp. and a line drawing of a diatom identified as Licmosphenia sp. that may correspond to S. orientale .