INTRODUCTION
Dinoflagellates are ubiquitous and major components of marine ecosystems and play key roles as primary producers, prey, predators, parasites, and symbionts (Coats 1999, Stat et al. 2008, Jeong et al. 2010, 2021, Lim and Jeong 2021, Kang et al. 2023a, 2023b, Ok et al. 2023a, Park et al. 2024a). Recently, dinoflagellates have been identified as approximately half of the microalgae that cause global red tides (Jeong et al. 2021). Moreover, dinoflagellates account for 75% of all harmful algal bloom species that can cause human illness and large-scale mortality in fish, shellfish, and marine mammals (Smayda 1997). Given their ecological and economic impacts, understanding the ecophysiological characteristics of dinoflagellates is essential for understanding the structure and function of marine ecosystems. This knowledge is also critical for mitigating economic losses in industries such as aquaculture and tourism.
During 2000–2003, some dinoflagellate species that had previously belonged to the genus Gymnodinium in the family Gymnodiniaceae were relocated to new genera into Akashiwo, Karenia, Karlodinium, and Takayama because they did not possess the morpho-anatomical features of Gymnodinium, such as a horseshoe-shaped apical groove, a nuclear envelope chamber, and a nuclear fibrous connective (Daugbjerg et al. 2000, de Salas et al. 2003). Later, based on phylogeny and pigment composition analyses, Karenia, Karlodinium, and Takayama, with fucoxanthin or its derivatives as the major carotenoid pigment, were relocated to the Kareniaceae family (Bergholtz et al. 2005). In 2019, the genus Asterodinium was added to the Kareniaceae based on analyses on phylogeny and pigment composition (Benico et al. 2019). In the same year, the genus Gertia was also newly added to Kareniaceae, although Gertia does not possess fucoxanthin but peridinin (Takahashi et al. 2019). In 2021, the genus Shimiella, which did not possess its own permanent pigments but temporarily survived using plastids from phototrophic prey, the so-called kleptoplastids, was assigned to Kareniaceae (Ok et al. 2021b). Thus, six genera have been officially described: Asterodinium, Gertia, Karenia, Karlodinium, Shimiella, and Takayama. The dinoflagellate family Kareniaceae is now regarded as having diverse types of pigments such as fucoxanthin or its derivatives (Asterodinium, Karenia, Karlodinium, and Takayama), peridinin (Gertia), and kleptoplastids (Shimiella), highlighting its evolutionary significance (Novák Vanclová et al. 2024).
Several species of Kareniaceae are known to cause noxious blooms associated with mortality in both vertebrates and invertebrates (e.g., Heil et al. 2001, Kempton et al. 2002, Magaña et al. 2003, Lim et al. 2014, Iwataki et al. 2022, Yamaguchi and Tomaru 2022). Because of their ecological significance, many plankton ecologists have investigated the ecophysiological characteristics of kareniacean dinoflagellates. In general, biological interactions, including predator-prey relationships and inhibition by physical contact or chemical effects, play crucial roles in the survival and bloom outbreaks of dinoflagellates (Jeong et al. 2015), and numerous studies have explored the biological interactions of each kareniacean dinoflagellate species (e.g., Adolf et al. 2007, 2008, Berge et al. 2008a, 2008b, Glibert et al. 2009, Place et al. 2012, Jeong et al. 2016, Ok et al. 2017, 2019, 2021a, 2022, 2023b, 2024, Lim et al. 2018, Song et al. 2020, Yang et al. 2020, Park et al. 2021, 2024b). To understand the eco-evolution of the genera in the family Kareniaceae, comparing biological interactions of different genera in the family is needed.
In this study, we review the biological interactions of the genera in the family Kareniaceae, focusing on trophic modes, prey spectra, feeding mechanisms, growth and ingestion rates, and protistan predators. Additionally, to enhance our understanding of biological interactions of Kareniaceae, we investigated the feeding occurrence in Gertia stigmatica, the prey spectrum of Karenia brevis, and the predation of Takayama tasmanica by heterotrophic protists. Furthermore, to better understand the eco-evolutionary strategies of kareniacean dinoflagellates, we estimated their divergence time based on large subunit ribosomal DNA sequences (LSU rDNA). This review provides insights into the eco-evolution of genera in the family Kareniaceae.
TROPHIC MODES
Among the 38 officially described species belonging to the family Kareniaceae, the trophic modes of 13 species (one Gertia, five Karenia, four Karlodinium, one Shimiella, and two Takayama species) have been explored up to date (Li et al. 1999, de Salas et al. 2005, Jeong et al. 2005, Berge et al. 2008a, Zhang et al. 2011, Lim et al. 2018, Benico et al. 2020, Ok et al. 2021a, 2023b, this study). They have diverse trophic modes, including autotrophy (lack of mixotrophy), mixotrophy (performing both photosynthesis and predation to obtain energy and carbon), and kleptoplastidy (performing photosynthesis using plastids obtained from ingested prey) (Table 1).
Gertia
In the present study, we examined the feeding occurrence of Gertia stigmatica by providing the cyanobacterium Synechococcus sp., cryptophytes Rhodomonas salina and Storeatula major, chlorophyte Dunaliella salina, and dinoflagellates Heterocapsa rotundata, Amphidinium carterae, and Akashiwo sanguinea (Supplementary Table S1). None of these potential prey items provided were observed to be fed by G. stigmatica. Therefore, G. stigmatica lacks mixotrophic ability. To date, G. stigmatica is the only species in this genus. If a new species is described in this genus, its mixotrophic ability should be explored.
Karenia
Previously, only two Karenia species, K. brevis and K. mikimotoi, were known to possess feeding abilities, leading to the assumption that the genus Karenia was mixotrophic (Jeong et al. 2005, Zhang et al. 2011). However, a recent study revealed that K. bicuneiformis, K. papilionacea, and K. selliformis lack feeding ability (Ok et al. 2023b). Thus, 60% of Karenia species tested for mixotrophy did not exhibit mixotrophic abilities, indicating that the genus is not uniformly mixotrophic. Due to this diversity, generalizing the trophic modes across the entire genus requires careful consideration. Moreover, findings from the previous study (i.e., Ok et al. 2023b) suggest that reporting whether a dinoflagellate species in a genus lacks mixotrophic ability is as important as reporting whether it possesses mixotrophic ability (e.g., You et al. 2023).
Among the five Karenia species explored, K. brevis and K. mikimotoi are mixotrophic, whereas K. bicuneiformis, K. papilionacea, and K. selliformis are primarily autotrophic (Jeong et al. 2005, Zhang et al. 2011, Ok et al. 2023b). Furthermore, Ok et al. (2023b) found that in a phylogenetic tree based on LSU rDNA sequences, the mixotrophic species K. brevis and K. mikimotoi belong to one clade, whereas the autotrophic Karenia species belong to the other clade (Fig. 1). Therefore, the presence or absence of mixotrophic ability in Karenia species may be partially influenced by their shared evolutionary history, suggesting potential evolutionary divergence between mixotrophic and autotrophic species within the genus.
Karlodinium
Among total of 16 officially described Karlodinium species, the trophic mode of only four species (25%) has been investigated (Li et al. 1999, de Salas et al. 2005, Berge et al. 2008a, Benico et al. 2020). All examined Karlodinium species (i.e., K. armiger, K. australe, K. azanzae, and K. veneficum) were mixotrophic. In the phylogenetic tree based on LSU rDNA sequences, the mixotrophic species K. armiger, K. australe, and K. azanzae formed a closely related group within the Karlodinium genus (Fig. 1). Therefore, the mixotrophic abilities of these three species, which cluster closely together in the Karlodinium genus, may be partially influenced by their shared evolutionary history. However, a mixotrophic ability of one species in the clade, K. digitatum, should be explored to test this hypothesis. Additionally, the trophic modes of other species in Karlodinium should be investigated.
Shimiella
The genus Shimiella includes only one officially described species, S. gracilenta, which lacks its own plastid but survives using kleptoplastidy, obtaining plastids from plastid-containing prey (Skovgaard 1998, Ok et al. 2021b). The undescribed Ross Sea dinoflagellate, which phylogenetically belongs to the genus Shimiella, has also been reported to be kleptoplastidic (Gast et al. 2006). Therefore, kleptoplastidy may be a unique characteristic defining the genus Shimiella.
Takayama
Among the eight species in the genus Takayama, the mixotrophic ability of only two species (T. helix and T. tasmanica) has been explored (Jeong et al. 2016, Lim et al. 2018); and both T. helix and T. tasmanica have been identified as mixotrophic. To test whether the mixotrophic ability is a unique characteristic of this genus, the trophic modes of other Takayama species should be explored.
The trophic mode of the genus Asterodinium remains unknown.
PREY SPECTRA
Species in the genera Karenia, Karlodinium, Shimiella, and Takayama feed on diverse prey (Li et al. 1999, Jakobsen et al. 2000, de Salas et al. 2005, Jeong et al. 2005, Adolf et al. 2008, Berge et al. 2008a, Zhang et al. 2011, Place et al. 2012, Jeong et al. 2016, Ok et al. 2017, 2021a, 2023b, Lim et al. 2018, Benico et al. 2020, Song et al. 2020, Yang et al. 2020). However, the prey spectrum (prey taxa and sizes) of one genus in Kareniaceae differed from that of the other genera (Tables 2–6, Supplementary Table S2).
Prey taxa
Previously, K. brevis was known to feed on the cyanobacterium Synechococcus sp. (Jeong et al. 2005, Glibert et al. 2009). In the present study, we examined the prey spectrum of K. brevis by providing cryptophytes Rhodomonas salina, Storeatula major, and Teleaulax amphioxeia, diatom Chaetoceros socialis, and dinoflagellates Heterocapsa rotundata and Akashiwo sanguinea (Supplementary Table S3). None of these potential prey items provided were observed to be fed by K. brevis.
The prey taxon spectrum of the genus Karenia is relatively moderate; Karenia species can feed on the heterotrophic bacterium, cyanobacterium, cryptophyte, or prymnesiophyte (Table 2) (Jeong et al. 2005, Zhang et al. 2011, Ok et al. 2023b). Karenia mikimotoi was able to feed on the heterotrophic bacterium Marinobacter sp., cryptophyte Teleaulax amphioxeia, and prymnesiophyte Isochrysis galbana (Table 3) (Zhang et al. 2011, Ok et al. 2023b). However, K. mikimotoi was unable to feed on Synechococcus sp. that Karenia brevis is able to feed on (Table 3) (Jeong et al. 2005, Ok et al. 2023b). Moreover, the result of the present study showed that K. brevis did not feed on T. amphioxeia that K. mikimotoi can feed on. The feeding occurrence of K. brevis on Marinobacter sp. and I. galbana that K. mikimotoi can feed on has not yet been explored. Further research is required to determine whether these two Karenia species that belong to the same clade have largely different prey spectra.
Species in the genus Karlodinium feed on the most diverse prey taxa among the genera in this family (Table 2) (Li et al. 1999, de Salas et al. 2005, Adolf et al. 2008, Berge et al. 2008a, 2012, Place et al. 2012, Benico et al. 2020, Song et al. 2020, Yang et al. 2020). The number of the reported prey items that K. armiger is able to feed on is highest (Supplementary Table S2) (Berge et al. 2008a, 2012); this species is able to feed on a variety of prey, including cryptophytes, prasinophytes, prymnesiophytes, a diatom, raphidophytes, dinoflagellates, ciliates, and metazoans (Table 2). Karlodinium veneficum feeds on various prey taxa including cryptophytes, a prymnesiophyte, a diatom, dinoflagellates, and metazoans (Table 2). Moreover, K. armiger and K. veneficum are the sole species feeding on diatoms among kareniacean dinoflagellates, although diatoms are suggested not to be the preferred prey for K. armiger (Berge et al. 2008a, Place et al. 2012). Karlodinium australe feeds on a cryptophyte, a prymnesiophyte, dinoflagellates, and metazoans (Table 2) (de Salas et al. 2005, Song et al. 2020). The prey taxon spectrum of Karlodinium azanzae has not been fully explored; however, this dinoflagellate has been reported to feed on diverse types of metazoans (Tables 2 & 6) (Benico et al. 2020). Owing to the wide range of prey taxa, Karlodinium species may have survival advantages in diverse environments.
The prey taxon spectrum of Shimiella gracilenta is relatively moderate (Table 2), and it is able to feed on cryptophytes, a dictyochophyte, a prasinophyte, and prymnesiophytes, whereas it does not feed on a chlorophyte, a diatom, a raphidophyte, dinoflagellates, and a ciliate (Tables 2–5) (Jakobsen et al. 2000, Ok et al. 2021a). All three Karlodinium species tested for their prey taxa—K. armiger, K. australe, and K. veneficum—feed on cryptophytes (Li et al. 1999, de Salas et al. 2005, Berge et al. 2008a, Song et al. 2020, Yang et al. 2020). Therefore, S. gracilenta might compete for cryptophyte prey with K. armiger, K. australe, and K. veneficum.
The prey taxon spectra of T. helix and T. tasmanica were narrower than those of Karenia, Karlodinium, and Shimiella. Two Takayama species, T. helix and T. tasmanica, have been observed to feed selectively on dinoflagellates, including phototrophic and heterotrophic species (Tables 2, 4 & 5) (Jeong et al. 2016, Ok et al. 2017, Lim et al. 2018). The dinoflagellate prey that these two Takayama species were able to feed on had diverse ecophysiological characteristics, such as single and chain-forming, planktonic and benthic, thecate and naked forms, and phototrophic and heterotrophic. Dinoflagellates are ubiquitous and constitute a major component of marine ecosystems. Therefore, T. helix and T. tasmanica may readily encounter dinoflagellate prey in marine ecosystems.
Because of the different prey taxon spectra, one genus in the family Kareniaceae may have an ecological niche different from that of other genera.
Prey size
The prey size spectrum of one genus in Kareniaceae differed from that of the other genera (Tables 3–6, Supplementary Table S2).
The Karenia species is able to feed on prey species having sizes of 1.0–5.6 μm in equivalent spherical diameters (ESD) (Supplementary Table S2) (Jeong et al. 2005, Zhang et al. 2011, Ok et al. 2023b). K. mikimotoi has been reported to feed on 0.5 and 2.0 μm microspheres, suggesting its potential ability to consume 0.5 μm prey (Zhang et al. 2011). The range of prey sizes that Karenia species can feed on is the narrowest among the four genera that have mixotrophic or kleptoplastidic species. To contrast, Karlodinium species showed the widest range of prey size (Supplementary Table S2): K. armiger is able to feed on algal prey having sizes of 4.2–35.0 μm, heterotrophic ciliate prey having a size of 150 μm, and metazoan prey having a size of ca. 400 μm (Berge et al. 2008a, 2012); K. veneficum is able to feed on algal prey having sizes of 3.9–42.5 μm and metazoans having lengths of 56–130 μm (Brachionus plicatilis), 1,220 μm (Artemia salina), and 3,500 μm (Oryzias melastigma) (Li et al. 1999, Adolf et al. 2008, Place et al. 2012, Yang et al. 2020); K. australe is able to feed on algal prey having sizes of 4.8–33.9 μm and metazoan prey with lengths of 56–130 μm (Brachionus plicatilis) and 1,220 μm (Artemia salina) (Song et al. 2020). Thus, mixotrophic Karlodinium species can feed on prey of diverse sizes, particularly on prey larger than themselves, because they feed on prey using a peduncle (feeding tube).
The Shimiella species is able to feed on prey items having sizes of 3.1–10.1 μm (Supplementary Table S2). The Takayama species are able to feed on prey items having sizes of 13.5–67.4 μm, the second widest range of prey size among the four genera (Supplementary Table S2); the size range of prey that T. helix is able to feed on is 15.0–67.4 μm, while that for T. tasmanica is 13.5–67.4 μm.
Among kareniacean dinoflagellates, species belonging to Karlodinium and Takayama species are capable of feeding on prey cells that are much larger than themselves. In the phylogenetic tree based on the LSU rDNA sequences, Karlodinium and Takayama are sister genera within the family Kareniaceae (Fig. 1). This suggests that the clade containing Karlodinium and Takayama may exhibit broad prey size spectra.
FEEDING MECHANISMS
In general, protist predators capture prey through raptorial, filter / interception, and diffusion feeding (Fenchel 1987, Jeong et al. 2010). After capturing the prey, raptorial feeders ingest the prey through engulfment feeding, peduncle feeding (feeding tube), and pallium feeding (feeding veil) (Jeong et al. 1997, 2010, 2012). Some raptorial feeders ingest prey by sucking on prey materials (e.g., Takayama helix and Takayama tasmanica) (Jeong et al. 2016, Lim et al. 2018); however, determining their exact feeding mechanism is challenging because it is unclear whether they use a peduncle. No peduncle or feeding tube was observed during the prey-sucking process in these dinoflagellate species. Therefore, the present study distinguished sucking prey materials from engulfment and peduncle feeding. All kareniacean species that could feed on prey were raptorial feeders that ingested prey by engulfment, peduncle feeding, or sucking (Table 7).
Four mixotrophic Karlodinium species (K. armiger, K. australe, K. azanzae, and K. veneficum) were observed to ingest prey by peduncle or engulfment feeding (Li et al. 1999, de Salas et al. 2005, Berge et al. 2008a, Benico et al. 2020, Song et al. 2020, Yang et al. 2020, 2021); K. armiger, K. australe, and K. veneficum ingested prey by both peduncle and engulfment feeding (Song et al. 2020, Yang et al. 2020, 2021); and Benico et al. (2020) observed feeding by K. azanzae on larger zooplankton using a peduncle.
Shimiella gracilenta possessing a peduncle in its protoplasm, was observed feeding on prey using a peduncle (Ok et al. 2021a). T. helix was observed to ingest prey cells using two different feeding mechanisms; it ingested a whole prey cell through direct engulfment for small prey cells (i.e., 15 μm in mean ESD), whereas it sucked prey materials for larger prey cells (>15 μm) (Jeong et al. 2016, Ok et al. 2017). T. tasmanica has been observed to feed on all prey by sucking the prey materials (Lim et al. 2018).
The feeding mechanisms of two mixotrophic Karenia species (K. brevis and K. mikimotoi) have not yet been reported. No peduncle or tube-like structure in K. brevis and K. mikimotoi has been observed under scanning and transmission electron microscopy (Steidinger et al. 1978, Fukuyo et al. 1990, Hansen et al. 2000). Thus, the two mixotrophic Karenia species, whose prey species are smaller than the two Karenia species, are likely to feed on prey materials by engulfment. However, further analyses are needed to confirm the feeding mechanisms of K. brevis and K. mikimotoi.
Overall, K. armiger, K. australe, K. veneficum, and T. helix have been observed to have two different feeding mechanisms. These four species showed the wide prey size spectra among kareniacean dinoflagellates (Supplementary Table S2). Thus, having dual feeding mechanisms may be partially responsible for having various prey species of different sizes.
DIVERGENCE TIME
The divergence time of the kareniacean dinoflagellates was estimated by analyzing LSU rDNA sequences (Fig. 1). The branching of the family Kareniaceae was estimated to have occurred at 139.6 million years ago (Mya) (95% highest posterior density [HPD] 104.8–175.6 Mya). The genus Karenia was estimated to have diverged from its common ancestor at 95.1 Mya (95% HPD 68.9–122.9 Mya). Moreover, the genus Gertia separated from its common ancestor at 75.9 Mya (95% HPD 53.9–99.7 Mya), whereas the genus Shimiella diverged at 59.5 Mya (95% HPD 42.4–77.4 Mya). The genera Karlodinium and Takayama were estimated to have diverged at 48.7 Mya (95% HPD 34.9–63.7 Mya). Thus, in this divergence time estimate, the genera Karlodinium and Takayama, which have broader prey size spectra, emerged later than Gertia, Karenia, and Shimiella. This suggests that the family Kareniaceae may have evolved toward genera with broader prey size spectra. To verify this hypothesis, the trophic modes and prey spectra of other kareniacean species that have not been investigated yet should be further explored.
GROWTH AND INGESTION RATES
Among the kareniacean dinoflagellates, the reported maximum autotrophic growth rate of Karenia mikimotoi, 0.82 d−1, is the highest, whereas that of Shimiella gracilenta, −0.15 d−1, is the lowest (Table 7). However, the reported maximum mixotrophic growth rate of S. gracilenta, 1.51 d−1, is the highest, whereas that of Takayama helix, 0.42 d−1, is the lowest. Therefore, mixotrophy elevated the ranking of S. gracilenta with the maximum growth rate among kareniacean dinoflagellates. The maximum mixotrophic growth rates of kareniacean dinoflagellates were significantly negatively correlated with their ESDs (p < 0.05), although the maximum autotrophic growth and ingestion rates of kareniacean dinoflagellates were not significantly correlated with their ESDs (p > 0.05) (Fig. 2A–C). The size of S. gracilenta is smallest, 9.3 μm, among the mixotrophic kareniacean dinoflagellates. Therefore, the smallest size of S. gracilenta was partially responsible for its highest maximum mixotrophic growth rate.
Interestingly, the three species with the highest maximum mixotrophic growth rates, S. gracilenta, K. veneficum, and K. armiger, utilized peduncles for feeding (Table 7) (Berge et al. 2008a, Yang et al. 2020, Ok et al. 2021a). This suggests that peduncle feeding may be an efficient feeding strategy that contributes to the high growth rates of kareniacean dinoflagellates. In contrast, the species with the lowest maximum mixotrophic growth rates, T. tasmanica and T. helix, feed on prey by sucking prey materials (Table 7) (Jeong et al. 2016, Lim et al. 2018), which appears to be less effective in supporting high growth rates than peduncle feeding.
EFFECTS OF ENVIRONMENTAL FACTORS ON FEEDING AND GROWTH
Several studies have demonstrated that environmental factors, including light intensity, nutrient availability, and temperature, can influence the ingestion and growth rates of certain kareniacean dinoflagellates (Jakobsen et al. 2000, Li et al. 2000, Ok et al. 2019, 2022). The ingestion rates of K. veneficum, T. helix, and S. gracilenta were significantly affected by light intensity (Jakobsen et al. 2000, Li et al. 2000, Ok et al. 2019). T. helix and S. gracilenta exhibited increasing ingestion rates with higher light intensity. However, the ingestion rates of K. veneficum on prey showed a saturation at light intensities above 60 μmol photons m−2 s−1 (Li et al. 2000). This suggests that light intensity effects may vary among kareniacean species. The growth rates of K. veneficum, T. helix, and S. gracilenta with added prey were also significantly affected by light intensity (Li et al. 1999, Jakobsen et al. 2000, Ok et al. 2019). At high light intensities (≥ 247 μmol photons m−2 s−1), the autotrophic growth rates of T. helix were negative, indicating photoinhibition. However, feeding reversed these negative growth rates to positive values, suggesting the importance of feeding for T. helix survival under high light conditions (Ok et al. 2019). Photoinhibition was not observed in K. veneficum and S. gracilenta (Jakobsen et al. 2000, Li et al. 2000). These findings indicate that while light intensity plays a crucial role in regulating ingestion and growth rates of kareniacean species, the extent of its effects varies among species.
Ingestion rates of K. veneficum was also significantly affected by nitrate and phosphate concentrations (Li et al. 2000). Moreover, nitrate and phosphate depletion led to high ingestion rates of K. veneficum (Li et al. 2000). There is currently a lack of research on the effects of nutrient availability on the ingestion and growth rates of kareniacean dinoflagellates. Further studies are needed to better understand how nutrients influence the ingestion and growth rates of kareniacean dinoflagellates under varying nutrient conditions.
Ingestion and growth rates of T. helix and S. gracilenta with added prey were significantly affected by water temperature (Ok et al. 2019, 2022). T. helix exhibited peak ingestion at 15°C, whereas S. gracilenta exhibited peak ingestion at 25°C, suggesting species-specific thermal optima for ingestion. Feeding enhanced the growth rates of both T. helix and S. gracilenta at <30°C, but not at ≥30°C. Moreover, the optimal temperatures for growth in T. helix and S. gracilenta with prey (28 and 25°C, respectively) were higher than those without prey (26 and 20°C, respectively). These results suggest that feeding enables these species to thrive at elevated temperatures below 30°C. However, almost all T. helix and S. gracilenta cells died at ≥30°C whether incubated with or without prey, indicating that the presence of prey does not support the survival of T. helix and S. gracilenta at these temperatures.
Based on the previous studies, effects of various environmental factors on the ingestion and growth rates of kareniacean dinoflagellates are species-specific. However, there is still limited research on the effects of environmental factors on the ingestion and growth rates of kareniacean dinoflagellates. Further studies on other kareniacean dinoflagellate species are needed under varying conditions.
PREDATION BY HETEROTROPHIC PROTISTS
Predators
To date, feeding occurrence by heterotrophic protists on four Karenia, one Karlodinium, one Shimiella, and two Takayama species have been explored (Table 8). Feeding occurrence by heterotrophic protists on kareniacean dinoflagellates was species-specific rather than genus-specific.
The biological interactions between the four Karenia species and heterotrophic protists were different. The percentage of the number of heterotrophic protists feeding on the four Karenia species to the number of tested heterotrophic protists range from 0 to 100% (Table 8); none of 8 tested potential heterotrophic protists fed on K. selliformis (0%); 5 of 8 tested potential heterotrophic protists fed on K. bicuneiformis (63%); 5 of 6 tested potential heterotrophic protists fed on K. mikimotoi (83%), and 2 of 2 tested potential heterotrophic protists fed on K. brevis (100%). Gyrodinium moestrupii can feed on K. bicuneiformis, K. mikimotoi, and K. brevis (Yoo et al. 2013, Kang et al. 2020, Park et al. 2024b). G. moestrupii is known to feed on toxic dinoflagellates such as Alexandrium minutum CCMP113 and Margalefidinium polykrikoides (Yoo et al. 2013, Kang et al. 2020). Therefore, the toxic materials produced by K. mikimotoi and K. brevis are unlikely to affect feeding by G. moestrupii on K. mikimotoi and K. brevis. However, K. selliformis lysed all tested heterotrophic protists, including G. moestrupii (Table 8) (Park et al. 2024b). Therefore, toxicity of K. selliformis may be greater than K. mikimotoi and K. brevis. Moreover, K. mikimotoi is known to lyse the heterotrophic dinoflagellate Gyrodinium jinhaense cells, although it is fed on by G. moestrupii (Table 8) (Kang et al. 2020). Therefore, different species of the genus Gyrodinium respond differently to K. mikimotoi.
K. brevis and K. mikimotoi were fed on by engulfment feeders among heterotrophic dinoflagellates, and K. bicuneiformis cells were preyed upon by both engulfment-feeding and pallium-feeding heterotrophic dinoflagellates (Table 8). However, feeding by peduncle-feeding heterotrophic dinoflagellates on Karenia species has not yet been reported. Park et al. (2024b) reported that K. bicuneiformis cells were attacked by the small-sized peduncle feeder Gyrodiniellum shiwhaense but immediately escaped before being captured. Thus, Karenia species may escape predation by peduncle-feeding heterotrophic dinoflagellates.
The percentage of the number of heterotrophic protists feeding on Shimiella gracilenta to the number of the tested heterotrophic protists was 50% (Table 8); 4 of 8 tested potential heterotrophic protists fed on S. gracilenta. Shimiella gracilenta is fed on by the peduncle-feeding dinoflagellate Pfiesteria piscicida, engulfment-feeding dinoflagellates Oxyrrhis marina and Gyrodinium dominans, and the filter-feeding ciliate Rimostrombidium sp. (Table 8) (Park et al. 2021). The predators O. marina, P. piscicida, and G. dominans are known to feed on cryptophyte prey, which are also the preferred prey taxa of S. gracilenta (Jakobsen et al. 2000, Jeong et al. 2006, Adolf et al. 2008, Kang et al. 2020, Ok et al. 2021a). Thus, predation by O. marina, P. piscicida, and G. dominans on S. gracilenta may serve as a strategy to eliminate competitors feeding on the same prey, such as cryptophytes.
The percentages of the number of heterotrophic protists feeding on Takayama helix and T. tasmanica to the number of the tested heterotrophic protists were 62 and 23%, respectively (Table 8); 8 of 13 tested potential heterotrophic protists fed on T. helix (Ok et al. 2017) and 3 of 13 tested potential heterotrophic protists fed on T. tasmanica (this study). The results of the present study showed that among 13 tested heterotrophic protists, the heterotrophic dinoflagellates Luciella masanensis, Aduncodinium glandula, and Oxyrrhis marina fed on T. tasmanica, but the heterotrophic dinoflagellates Gyrodiniellum shiwhaense, Gyrodinium dominans, Gyrodinium moestrupii, Noctiluca scintillans, Oblea rotunda, Pfiesteria piscicida, Polykrikos kofoidii, and Protoperidinium pellucidum, and the naked ciliates Pelagostrobilidium sp. and Strombidinopsis sp. did not feed on T. tasmanica (Table 8, Fig. 3, Supplementary Table S4). Therefore, G. dominans, G. moestrupii, Pfiesteria piscicida, Polykrikos kofoidii, and Strombidinopsis sp. were able to feed on T. helix but not on T. tasmanica, indicating that the interactions of T. tasmanica with heterotrophic protists are different from those of T. helix.
Strategies minimizing mortality due to predation
Kareniacean dinoflagellates employ four strategies to minimize mortality from predation (Deeds and Place 2006, Adolf et al. 2007, Ok et al. 2017, Kang et al. 2020, Park et al. 2024b): lysis, toxin production, immobilization, and reciprocal predation.
K. selliformis, K. mikimotoi, and T. helix lyse heterotrophic dinoflagellates (Table 8); K. selliformis lyses all eight heterotrophic dinoflagellates tested, T. helix lyses Gyrodiniellum shiwhaense and P. kofoidii, but K. mikimotoi lyses Gyrodinium jinhaense (Ok et al. 2017, Kang et al. 2020, Park et al. 2024b). Interestingly, T. helix can reduce its mortality due to predation by lysing P. kofoidii cells, although P. kofoidii has been reported to feed on T. helix (Ok et al. 2017). Ok et al. (2017) demonstrated that the growth rate of P. kofoidii on T. helix decreased with increasing T. helix concentrations, and P. kofoidii cells were rarely observed at the end of the experiments. Thus, lysing potential predator cells is an important strategy for maintaining the populations of K. mikimotoi, K. selliformis, and T. helix.
High concentrations of karlotoxin purified from K. veneficum have been shown to lyse O. marina cells, which contain cholesterol as the major membrane sterol (Deeds and Place 2006). Therefore, kareniacean dinoflagellate toxins may cause lysis. Furthermore, the toxic strain Karlodinium veneficum CCMP 2064, which produces karlotoxins, inhibits grazing by O. marina on K. veneficum CCMP 2064 (Adolf et al. 2007). This finding suggests that toxin production in kareniacean dinoflagellates is an effective strategy for minimizing mortality due to predation.
Immobilization is another strategy used by kareniacean dinoflagellates to minimize mortality due to predation. T. helix immobilizes the heterotrophic dinoflagellate O. rotunda, leading to preventing O. rotunda from feeding on T. helix (Table 8) (Ok et al. 2017). Similarly, the results of the present study showed that T. tasmanica immobilized the naked ciliate Pelagostrobilidium sp. (Table 8, Supplementary Table S4).
Reciprocal predation is defined as a phenomenon in which two species feed on each other (Jeong et al. 1997), and it can be a strategy for minimizing the mortality of one species due to predation by the other species. Among the kareniacean dinoflagellates, T. helix is known to engage in reciprocal predation with O. marina (Ok et al. 2017). When T. helix and O. marina were co-incubated, T. helix became predominant, outcompeting O. marina with increasing mean T. helix concentrations (Ok et al. 2017). Moreover, reciprocal predation between T. tasmanica and the heterotrophic dinoflagellates L. masanensis, O. marina, and A. glandula was observed (Table 8, Fig. 4, Supplementary Table S4). Furthermore, at a high mean T. tasmanica concentration (4,510 cells mL−1), the specific growth rate of O. marina was negative (−3.0 d−1; unpublished data), whereas that of T. tasmanica was positive (0.1 d−1; unpublished data), indicating that T. tasmanica predominated over O. marina under reciprocal predation.
To date, among the genera within Kareniaceae, only Takayama species have been observed to engage in reciprocal predation with heterotrophic protists. The maximum mixotrophic growth rates of T. helix and T. tasmanica were largely lower than those of S. gracilenta, K. veneficum, K. armiger, and K. brevis (Table 7). Thus, reciprocal predation between Takayama species and heterotrophic protists may lower mortality from predation, which could compensate for the relatively low growth rates of Takayama species in marine environments.
Collectively, these findings suggest that kareniacean dinoflagellates have evolved diverse and complex mechanisms to mitigate predation pressure, allowing them to persist and maintain population stability in marine ecosystems.
KARENIACEAN SPECIES HUBS
Jeong et al. (2010) described the predator-prey relationships of the dinoflagellates Prorocentrum cordatum (= previously P. minimum) and Prorocentrum micans as “the Prorocentrum minimum hub” and “the Prorocentrum micans hub,” respectively, likening them to airport hubs. These dinoflagellate hubs provide insights into the diverse ecological roles of Prorocentrum species, which function as both prey and predators, and interact with a wide range of marine organisms. Based on the results of the present study and previous literature, each kareniacean species “hub,” encompassing species from the genera Karenia, Karlodinium, Takayama, and Shimiella, was established (Figs 5–8). These hubs show that kareniacean dinoflagellates interact with diverse marine organisms with various characteristics such as taxa, sizes, and trophic modes.
Furthermore, Karenia species hubs clearly showed that each species in the genus Karenia had distinct predator-prey relationships (Fig. 5). Similarly, the species in Karlodinium and Takayama exhibited variations in their biological interactions within each genus (Figs 6 & 7). Thus, species in the same genus have distinct ecological niches owing to differences in their biological interactions with prey and predators. Moreover, S. gracilenta hub showed that the biological interaction of S. gracilenta with prey and protistan predators was different from that of the species in Karenia, Karlodinium, and Takayama (Fig. 8).
Kareniacean species hubs can enhance our understanding of the causes of harmful algal blooms because they provide information on nutrient accumulation in kareniacean species and the subsequent transfer of these nutrients to predators (Glibert and Burkholder 2018). Thus, these hubs can be used to develop models for more accurate prediction of harmful algal bloom outbreaks for each kareniacean species (Yoo et al. 2017).
As shown in Figs 5–8, predator-prey relationships between several kareniacean dinoflagellates and other protists have not yet been reported. Therefore, to better predict the outbreaks of harmful algal blooms caused by kareniacean dinoflagellates, their predator-prey relationships should be explored and hubs should be further established based on the results.
CONCLUSION
The present study provides insight into the ecological diversity and adaptive strategies of kareniacean dinoflagellates based on the data on their distinct trophic modes, prey spectra, feeding mechanisms, predators, and survival strategies. The genera Karenia, Karlodinium, Shimiella, and Takayama have different prey taxa and size spectra. Moreover, each species within the same genus exhibits different biological interactions with prey and protistan predators. By demonstrating that different genera and species within Kareniaceae occupy different ecological niches, this study enhances our understanding of ecological roles and evolutionary adaptations of kareniacean dinoflagellates in marine ecosystems.
To date, feeding by only 13 species out of 38 officially described species in the family Kareniaceae has been explored. Moreover, feeding by heterotrophic protists on only eight kareniacean species has been investigated. Further studies on other kareniacean species, whose biological interactions with prey and predators and the effects of environmental factors on feeding have not yet been explored, are needed to better understand the eco-evolutionary characteristics of Kareniaceae. Moreover, given these knowledge gaps, careful consideration is required when interpreting the present study.
