Prorocentrum shikokuense

Gómez, Fernando, Zhang, Huan, Roselli, Leonilde & Lin, Senjie, 2021, Detection of Prorocentrum shikokuense in the Mediterranean Sea and evidence that P. dentatum, P. obtusidens and P. shikokuense are three different species (Prorocentrales, Dinophyceae), Acta Protozoologica 60, pp. 47-59 : 53-56

Prorocentrum shikokuense in the North Atlantic

Ocean

The molecular data suggest the presence of P. shikokuense in the European Atlantic ( Figs 2 , 4 ), but studies combining molecular and morphological data remain missing. In addition to the sequence from the subtropical Canary Islands, there is other SSU rRNA gene sequence from a strain isolated in the Baltic Sea that clustered in the clade of P. shikokuense (accession number MH976698 ) ( Fig. 2 ). Monti-Birkenmeier et al. (2019, their fig. 2c) provided a SEM image of this Baltic strain submitted to GenBank as Prorocentrum cordatum . The cell was oval with a length of 10 µm that fit well with P. cordatum ( Monti-Birkenmeier et al. 2019) , while cells of P. shikokuense are more elongated and ~20 µm long. There is no coherence between the morphology of the illustrated cell and the topology of the sequence MH 976698 in the molecular phylogeny ( Fig. 2 ). According Xu et al. (2010), P. shikokuense showed an optimal growth at 27 ºC in the East China Sea. In the Mediterranean Sea, blooms of P. shikokuense were observed at temperatures of 28–32 ºC ( Roselli et al. 2019). The Baltic Sea is characterized by low temperatures, and huge blooms are not expected. Further studies are needed to confirm the occurrence of P. shikokuense in the Baltic Sea.

In the ITS rRNA gene phylogeny, a sequence identified as Prorocentrum rostratum with accession number EU244471 clustered in the clade of P. shikokuense among sequences from the Asian Pacific Ocean ( Fig. 4 ). Prorocentrum rostratum is a distinctive species known from tropical waters ( Stein 1883). The cells are elongated, 5–6 times as long as broad (~60 µm long), valves ending at anterior end in long pointed process, notched in the side view. Prorocentrum rostratum is easily distinguishable from P. shikokuense that is smaller, ~20 µm long, with an inconspicuous shoulder, and a round posterior margin ( Fig. 1 AB, AE–AH). The strain P. rostratum PR 1V ( EU244471 ) was isolated at Vigo, NW Spain. It is not expected to find the tropical species P. rostratum in the cold waters of the Galician Rias. No morphological information of the strain PR1V is available currently. Beyond the problem in the identification associated with the sequences of P. shikokuense in Europe, the presence of P. shikokuense in the European Atlantic Ocean is not unexpected, but a definite conclusion awaits combined morphological and molecular analyses. While the molecular data reveal that P. cordatum is present in the American and European coasts of the North Atlantic Ocean ( Figs 2–4 ), there is no evidence for the presence of P. shikokuense in the American Atlantic coasts.

Figures

Fig. 2. Phylogenetic tree based on SSU rRNA gene sequences, showing the position of the sequence of Prorocentrum shikokuense by Maximum Likelihood (ML). Numbers near branches denote ML bootstrap (BS) and Bayesian posterior probability (PP) values. BS and PP values <70 and 0.8, respectively, are not shown.

Fig. 4. Phylogenetic tree based on ITS rRNA gene sequences, showing the position of the sequence of Prorocentrum shikokuense by Maximum Likelihood (ML). Numbers near branches denote ML bootstrap (BS) and Bayesian posterior probability (PP) values. BS and PP values <70 and 0.8, respectively, are not shown.

Fig. 1. Light micrographs of Lugol’s solution preserved individuals and line drawings of Prorocentrum dentatum (A–I), P. obtusidens (J– AC) and P.shikokuense (AB,AD–AH). (A–B) Prorocentrum dentatum by Stein (1883). (C–I) Individuals from the Pacific Ocean. See Table 1 for location. (J) Prorocentrum obtusidens re-drawn from Schiller (1928). (K–AB) Individuals from a single sample collected offshore Taranto, Ionian Sea. (Y–AB) Valves of the same individual. (AB) One cell of P. obtusidens and two cells of P. shikokuense. (AC) P. obtusidens collected offshore Bari, Adriatic Sea. (AD) Prorocentrum shikokuense redrawn from Hada (1975). (AB, AE–AH) Individuals from a single sample of the harbor of Brindisi, Italy. (AE–AG) Chain-forming individuals. (AH) Note the intraspecific morphological variability. The last individual corresponds to a megacytic cell. Scale bar = 10 µm.