286 T
. Uchida et al. J. Exp. Mar. Biol. Ecol. 241 1999 285 –299 Keywords
: Cell contact; Gymnodinium mikimotoi; Heterocapsa circularisquama; Red tide; Species inter- action; Temporary cyst
1. Introduction
Heterocapsa circularisquama Horiguchi is a novel red tide causing species of which occurrences have been increasing in embayments of west Japan. This species has caused
significant damage to shellfish aquaculture by killing the bivalves Matsuyama et al., 1995; Nagai et al., 1996. Several physiological and ecological studies have been
conducted in order to clarify the mechanism of red tide outbreaks of this species Uchida et al., 1995; Yamaguchi et al., 1997; Matsuyama et al., 1995, 1996. Matsuyama et al.
1996 showed that the disturbance of water stratification is a likely mechanism triggering the red tide occurrences of H
. circularisquama. Yamaguchi et al. 1997 reported that this species prefers higher temperatures compared to other representative
red tide species of Japan. In addition to physical and chemical factors, species interactions are also considered to be important for the development of phytoplankton
blooms. H . circularisquama sometimes appears after the bloom of Gymnodinium
mikimotoi Miyake et Kominami ex Oda which is one of the most representative red tide flagellate in Japan. For example, red tide by H
. circularisquama replaced G. mikimotoi red tide at Gokasho Bay in August 1994 Uchida, unpublished data. The same sequence
pattern from G . mikimotoi to H. circularisquama was observed at Hiroshima Bay in
August 1998 Matsuyama, unpublished data. It is possible that H . circularisquama and
G . mikimotoi interact with each other in nature.
Growth interactions between phytoplankton species mediated by extracellular organic substances released allelopathy by one or both interacting species have been consid-
ered an important factor affecting phytoplankton sequence Maestrini and Bonin, 1981; Rice, 1984; Honjo, 1994. For example, Pratt 1966 observed the alternate appearances
of Olisthodiscus luteus and Skeletonema costatum, and verified that O . luteus produces a
substance inhibiting the growth of S . costatum. Similarly, some flagellate species have
been found to secrete substance inhibiting coexisting phytoplankton species Uchida, 1977; Honjo et al., 1978.
Differing from ‘allelopathy’, Uchida et al. 1995 found that H . circularisquama is
able to kill the dinoflagellate Gyrodinium instriatum by direct cell contact. Furthermore, several species of flagellates have been observed to die when cultured with H
. circularisquama Uchida et al., 1996, although it was not clear if cell contact caused
this phenomenon. On the other hand, H . circularisquama has been shown to be
suppressed in bialgal cultures by several diatom species Uchida et al., 1996. In this case, H
. circularisquama is transformed into immotile cells with a round or elliptical form. Thus, unique interactions have been observed between H
. circularisquama and other phytoplankton species.
Subsequently, we have extended these investigations to examine the relationships between H
. circularisquama and Gymnodinium mikimotoi. This study deals with the effect of initial cell density on the competitive success between these two dinoflagellates.
T . Uchida et al. J. Exp. Mar. Biol. Ecol. 241 1999 285 –299
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Furthermore, the growth of each species in bialgal cultures were simulated using a mathematical model to quantify the relationships between H
. circularisquama and G. mikimotoi.
2. Materials and methods
