Possible aplanochytrid (Labyrinthulea) prey detected using 18S metagenetic diet analysis in the key copepod species <i>Calanus sinicus</i> in the coastal waters of the subtropical western North Pacific

Abstract

Metagenetic diet analyses of the 18S V9 region were conducted in 40 adult female Calanus sinicus during winter in Tosa Bay (Japan). The majority of prey items were small crustaceans (of Copepoda and Cirripedia) and diatoms, taxa that are dominant in the environment and have been previously reported as important prey items of Calanus. The abundance of sequences attributable to Dinophyta and Chlorophyta was significantly lower in C. sinicus gut contents than in environmental plankton communities, suggesting that C. sinicus avoids prey from these groups. Hydrozoans were also observed, and aplanochytrids (Labyrinthulea) were detected for the first time as a major prey of C. sinicus. Additionally, high proportions of unclassified eukaryote material were observed, suggesting undetected predator-prey relationships in key copepod species in marine ecosystems. The dietary importance of aplanochytrids, heterotrophic protists that accumulate unsaturated fatty acids such as docosahexaenoic acid, has been overlooked in previous research. Calanus sinicus is a key copepod species in the subtropical coastal regions of the western North Pacific, and a major food source for the larvae of commercially important fish; therefore, further investigation into novel prey items such as aplanochytrids is recommended to understand the complex food web structures in marine ecosystems.

Nutritional intake of Aplanochytrium (Labyrinthulea, Stramenopiles) from living diatoms revealed by culture experiments suggesting the new prey–predator interactions in the grazing food web of the marine ecosystem

“…Aplanochytrids have been detected in the gut and fecal pellets of copepods and salps in the tropical Indian and North Pacific Oceans [34, 37, 39, 65]. Hirai et al (2018) [39] reported that aplanochytrids, which were recognized by 18S rDNA metagenomic analysis, were possibly the major food source of Calanus sinicus , a large copepod common in the coastal waters of the subtropical western North Pacific [66]. The proportion of Aplanochytrium was higher in gut contents than in environmental samples, suggesting that C .…”

“…The proportion of Aplanochytrium was higher in gut contents than in environmental samples, suggesting that C . sinicus positively selected aplanochytrids as prey [39]. This copepod was shown to actively capture particles with a diameter of 20–50 μm, but few particles smaller than 10 μm [67].…”

“…Aplanochytrids have also been isolated from the gut and fecal pellets of zooplankton [34, 37–38]. Metagenomic analyses revealed a higher proportion of aplanochytrids in the gut contents of copepods than in environmental samples [39], suggesting that aplanochytrids are associated with zooplankton in predator–prey or commensal relationships. In addition, it was reported that Aplanochytrium cells secrete cellulase, so it suggests aplanochytrids play a role as a decomposer for plant substances [22].…”

Carbon and nitrogen dynamics in the coastal Japan Sea inferred from 15 years of measurements of stable isotope ratios of Calanus sinicus

“…Calanus sinicus is one of the important secondary producers that connect primary producers with higher trophic levels in the coastal area of the Japan Sea, although C. sinicus is known to prey on heterotrophic plankton in addition to phytoplankton (Hirai et al, 2018;Yi et al, 2017;Uye and Yamamoto, 1995). The δ 15 N values of C. sinicus (6.9 ± 1.2‰) are consistent with this scenario: their δ 15 N values are intermediate between those of particulate organic matter (POM) in coastal areas of the Japan Sea (around 2-6‰) (Kogure, 2004;Antonio et al, 2012) and predatory fishes such as Japanese anchovy (8.9-11.7‰) (Tanaka et al, 2008) and Japanese sardines (9.4 ± 0.7‰) (Ohshimo et al, 2019).…”

Application of Next-Generation Sequencing for the Determination of the Bacterial Community in the Gut Contents of Brackish Copepod Species (Acartia hudsonica, Sinocalanus tenellus, and Pseudodiaptomus inopinus)

“…For copepod gut bacterial community analyses, a method of dissecting the gut from individual specimens has been commonly used [ 19 ]. This method has been used for large species such as Calanus (2–3 mm of prosomal length); however, it is difficult to apply it to small-sized species [ 20 , 21 ]. Although the DNA analysis methods of the gut contents of small zooplankton (rotifers, under 400 μm) have been proposed [ 22 ], zooplankton species-specific differences, such as exoskeleton hardness, must be considered, and methods that can be applied to small-sized copepod species (700 μm–1 mm of prosomal length) are still required [ 23 ].…”