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Gymnodinium mikimotoi Miyake and Kominami ex Oda, 1935

Species Overview:

Gymnodinium mikimotoi is an unarmoured, marine, planktonic dinoflagellate species. It is a common red tide former in Japan and Korea associated with massive fish kills.

Taxonomic Description:

Gymnodinium mikimotoi is an athecate species; i.e. without thecal plates. Cells are small, broadly oval to almost round (Figs. 1-7) and compressed dorso-ventrally (Fig. 3). Cells are slightly longer than wide (Fig. 3) with a characteristic long and straight apical groove to the right of the sulcal axis (Figs. 1,7). The apical groove extends from the ventral side to the dorsal side of the epitheca creating a slight indentation at the apex of the cell (Figs. 1,2). Cells range in size from 18-40 µm in length to 14-35 µm in width (Takayama and Adachi, 1984, Fukuyo et al., 1990, Hallegraeff, 1991, Taylor et al., 1995, Steidinger and Tangen, 1996).

The epitheca is broadly rounded and smaller than the hypotheca (Figs. 1-3). The hypotheca is notched by the widening sulcus at the antapex resulting in a lobed posterior (Figs. 1-3). The wide and deeply excavated cingulum is pre-median, and is displaced in a descending spiral about 2 times its width (Figs. 1,3,4,7). The sulcus slightly invades the epitheca extending from above the cingulum to the antapex (Figs. 1,3,7) (Takayama and Adachi, 1984, Fukuyo et al., 1990, Hallegraeff, 1991, Taylor et al., 1995, Steidinger and Tangen, 1996).

Morphology and Structure:

G. mikimotoi is a photosynthetic species with several oval to round yellow-brown chloroplasts, each with a pyrenoid (Fig. 7). The large ellipsoidal nucleus is located in the left hypothecal lobe (Figs. 5,7) (Takayama and Adachi, 1984, Fukuyo et al., 1990, Hallegraeff, 1991, Taylor et al., 1995, Steidinger and Tangen, 1996).

Reproduction:

G. mikimotoi reproduces asexually by binary fission; cells divide obliquely during mitosis (Fig. 6) (Yamaguchi and Honjo, 1990).

Species Comparison:

G. mikimotoi resembles G. breve: both species are dorso-ventrally flattened and their nucleus is located in the left half of the hypotheca. However, these species differ in several features: G. mikimotoi does not have an apical process; G. breve cells are flatter (dorso-ventral compression is greater); and the sulcal invasion of the epitheca is deeper in G. breve (Takayama and Adachi, 1984).

The Pacific Gymnodinium mikimotoi and the European Gyrodinium aureolum are morphologically similar and have been in a state of taxonomic turmoil for over 20 years (Takayama et al., 1998). They are generally regarded as con-specific, although genetic differences between the two populations do exist (Partensky et al., 1988). Controversy, therefore, still remains over the taxonomic status of the Pacific and European populations.

Recently, Takayama et al., 1998 conducted an extensive taxonomic study on the morphological differences between the Pacific Gymnodinium mikimotoi and the European Gyrodinium aureolum. There were several morphological differences reported, namely swimming behavior, cell thickness, and shape and position of nucleus: cells of G. aureolum are thicker; the nucleus of G. aureolum is spherical and central, while that of G. mikimotoi is longitudinally elliptical and located in the left lobe of the hypotheca.

Ecology:

G. mikimotoi is a planktonic species first described from western Japan (Oda, 1935). This species is a recurring bloom former in coastal waters of Japan and Korea; red tides commonly occur in warmer months and are associated with massive fish and shellfish kills (Takayama and Adachi, 1984). Reported to be eurythermal and euryhaline, populations of G. mikimotoi could presumably over-winter as motile cells, which could then serve as seed populations for a summer red tide (Yamaguchi and Honjo, 1989). Moreover, studies conducted in Omura Bay, Japan, revealed that this species can tolerate anoxic or near anoxic conditions utilizing sulfide from the sediment (Iizuka, 1972).

Cells have a distinct swimming pattern: turning over through water like a falling leaf (Takayama and Adachi, 1984).

Toxicity:

G. mikimoitoi is a toxic species associated with massive kills of benthic invertebrates and of both wild and farmed fishes in coastal waters off Japan and Korea; e.g. in 1933 pearl oyster mortalities near Nagasaki, Japan, resulted in an economic loss of $7 million (Oda, 1935). For decades red tides of G. mikimoitoi have resulted in devastating marine life mortalities, yet the toxin mechanism and principles are poorly understood. Research indicates that this species produces hemolytic and ichthyotoxic substances (Hallegraeff, 1991, Taylor et al., 1995). Recently, Seki et al., 1996 extracted a lipid-soluble toxin, gymnodimine, from shellfish in Southland, NZ (dubbed 'Southland toxin') after a Gymnodinium cf. mikimotoi red tide event. This toxin produced a quick kill in both mice and fish, but was less toxic than brevetoxins. No reported human illnesses have resulted from consumption of fish or shellfish from bloom affected areas (Hallegraeff, 1991).

Habitat and Locality:

G. mikimotoi is a cosmopolitan species commonly found in temperate to tropical neritic waters. Blooms have been reported from Australia, Denmark, Ireland, Japan, Korea, Norway and Scotland (Taylor et al., 1995, Steidinger and Tangen, 1996).

Gymnodinium mikimotoi