marine drugs Article TTX-Bearing Planocerid Flatworm (Platyhelminthes: Acotylea) in the Ryukyu Islands, Japan Hiroyuki Ueda, Shiro Itoi * and Haruo Sugita Department of Marine Science and Resources, Nihon University, Fujisawa, Kanagawa 252-0880, Japan; [email protected] (H.U.); [email protected] (H.S.) * Correspondence: [email protected]; Tel./Fax: +81-466-84-3679 Received: 10 December 2017; Accepted: 17 January 2018; Published: 19 January 2018 Abstract: Polyclad flatworms comprise a highly diverse and cosmopolitan group of marine turbellarians. Although some species of the genera Planocera and Stylochoplana are known to be tetrodotoxin (TTX)-bearing, there are few new reports. In this study, planocerid-like flatworm specimens were found in the sea bottom off the waters around the Ryukyu Islands, Japan. The bodies were translucent with brown reticulate mottle, contained two conical tentacles with eye spots clustered at the base, and had a slightly frilled-body margin. Each specimen was subjected to TTX extraction followed by liquid chromatography with tandem mass spectrometry analysis. Mass chromatograms were found to be identical to those of the TTX standards. The TTX amounts in the two flatworm specimens were calculated to be 468 and 3634 µg. Their external morphology was found to be identical to that of Planocera heda. Phylogenetic analysis based on the sequences of the 28S rRNA gene and cytochrome-c oxidase subunit I gene also showed that both specimens clustered with the flatworms of the genus Planocera (Planocera multitentaculata and Planocera reticulata). This fact suggests that there might be other Planocera species that also possess highly concentrated TTX, contributing to the toxification of TTX-bearing organisms, including fish. Keywords: flatworm; Planocera; planocerid; Polycladida; tetrodotoxin (TTX) 1. Introduction Tetrodotoxin (TTX, C11H17N3O8), also known as pufferfish toxin, is one of the most potent neurotoxins. It specifically blocks voltage-gated sodium channels on excitable cell membranes of muscle and nerve tissues [1,2]. TTX was long believed to occur exclusively in pufferfish. However, after its molecular structure was found to be consistent with that of tarichatoxin from the California newt Taricha torosa [3], TTX has been detected in various taxonomic organisms including toad Atelopus spp. [4]; toxic goby Yongeichthys criniger [5]; blue-ringed octopus Hapalochlaena maculosa [6]; xanthid crabs [7]; marine bivalves [8]; gastropods [9]; flatworms [10]; and ribbonworms [11]. Furthermore, TTX-producing bacteria have been isolated from TTX-bearing organisms, as well as the environment [12,13]. In addition, non-toxic pufferfish were produced when grown from hatching with a non-toxic diet, and these cultured non-toxic pufferfish became toxic when TTX was administered orally [14–20]. Therefore, it is generally thought that TTX is produced primarily by bacteria, and accumulates in the pufferfish body via the food web [21,22]. Nevertheless, the source of TTX in pufferfish is actually unknown since the amount of TTX produced by the bacteria is too little to account for that present in pufferfish. Recently, our lab showed that the pufferfish Takifugu niphobles ingested the toxic eggs of another pufferfish Takifugu pardalis, thereby efficiently increasing their own toxicity [23], suggesting that there may be other such unknown avenues of TTX accumulation. TTX accumulation in pufferfish via the ingestion of TTX-bearing organisms is also likely. Among the many TTX-bearing organisms listed above, Planocera multitentaculata, widely distributed in the waters around the Japanese Archipelago, Mar. Drugs 2018, 16, 37; doi:10.3390/md16010037 www.mdpi.com/journal/marinedrugs Mar. Drugs 2018, 16, 37 2 of 11 was the first to be reported as a TTX-bearing flatworm [10]. Subsequently, TTX was detected in a closely related species, Planocera reticulata [24], and a conspecific [25]. Our lab recently found that the pufferfish of the genus Takifugu (Takifugu rubripes and T. niphobles) became toxic after feeding on the polycladMar. flatworm, Drugs 2018, 16P., 37 multitentaculata , and a DNA fragment of the P. multitentaculata cytochrome-2 of 10 c oxidase subunit I (COI) gene was detected in the intestinal contents of wild specimens of the pufferfish above, Planocera multitentaculata, widely distributed in the waters around the Japanese Archipelago, T. niphobles (unpubl data). These results suggest that planocerid flatworms could contribute to the was the first to be reported as a TTX-bearing flatworm [10]. Subsequently, TTX was detected in a toxificationclosely of related pufferfish. species, A Planocera different reticulata polyclad [24], flatworm, and a conspecificStylochoplana [25]. Oursp., lab recently has been found implicated that the in the toxificationpufferfish of sea of slugs, the genus resulting Takifugu in (Takifugu dog neurotoxicosis rubripes and T. inniphobles New) Zealand became toxic [26– after28]. feeding After a on hiatus the since the 1940spolyclad [29], these flatworm, findings P. multitentaculata have spurred, and renewed a DNA interestfragment in of understandingthe P. multitentaculata these cytochrome- flatworms,c with a spate ofoxidase reports subunit in the pastI (COI) decade gene [was30– 34detected]. in the intestinal contents of wild specimens of the Recently,pufferfish our T. lab niphobles showed (unpubl [35] that data). the classificationThese results ofsugges polycladst that basedplanocerid onthe flatworms 28S rRNA could gene was contribute to the toxification of pufferfish. A different polyclad flatworm, Stylochoplana sp., has been approximately consistent with the morphological classification reported thus far [29,36,37]. In another implicated in the toxification of sea slugs, resulting in dog neurotoxicosis in New Zealand [26–28]. recent study,After a we hiatus reported since the that 1940s the [29], TTX these content findings of the have flatworm spurred renewedP. multitentaculata interest in understandingrose in association with anthese increase flatworms, in body with weight a spate [of38 re].ports Almost in the nothing past decade is known [30–34]. about how this increase takes place as there is littleRecently, information our lab showed on the [35] diet that or the even classifica the spatialtion of (verticalpolyclads orbased horizontal) on the 28S distribution rRNA gene of the genus Planocerawas approximatelyin the waters consistent around with the the Japanesemorphological Archipelago. classification Only reported five speciesthus far of[29,36,37]. this genus In have been recordedanother fromrecent thestudy, area we [reported29,36]. Ofthat these, the TTX only content two of species, the flatwormP. multitentaculata P. multitentaculataand roseP. reticulatain , association with an increase in body weight [38]. Almost nothing is known about how this increase have beentakes frequently place as there observed is little at information the intertidal on zonethe di ofet theor even main the islands spatial [29(vertical,38]. or horizontal) In thisdistribution paper, toof the clarify genus the Planocera processes in the for waters the toxification around the Japanese of TTX-bearing Archipelago. organisms, Only five including species fish, we reportof thethis presencegenus have of been planocerid-like recorded from flatworm the area [29,36]. specimens Of these, in theonly waters two species, around P. multitentaculata the RyukyuIslands, Japan, classifyand P. reticulata them, with have been the helpfrequently of phylogenetic observed at the analysis intertidal usingzone of 28S the main rRNA islands and [29,38]. COI nucleotide sequences, andIn this measure paper, to the clarify toxicity the ofprocesses the flatworm for the toxification by means of of TTX-bearing liquid chromatography organisms, including with tandem fish, we report the presence of planocerid-like flatworm specimens in the waters around the Ryukyu mass spectrometry (LC-MS/MS) analysis. Finally, we discuss the distribution of planocerid flatworms Islands, Japan, classify them with the help of phylogenetic analysis using 28S rRNA and COI in Japan,nucleotide and their sequences, contribution and measure to the toxificationthe toxicity of of the pufferfish. flatworm by means of liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis. Finally, we discuss the distribution of 2. Resultsplanocerid flatworms in Japan, and their contribution to the toxification of pufferfish. 2.1. External2. Results Morphology Two2.1. polyclad External Morphology flatworms were collected from waters off the city of Nago, Okinawa main island, Japan (Figure1, Table1), in August 2017. Specimen-1 (hereafter referred to as S1) weighed 1.88 g Two polyclad flatworms were collected from waters off the city of Nago, Okinawa main island, and Specimen-2 (S2) weighed 2.69 g. Both appeared to be planocerids based on external morphology, Japan (Figure 1, Table 1), in August 2017. Specimen-1 (hereafter referred to as S1) weighed 1.88 g and althoughSpecimen-2 some features (S2) weighed were different2.69 g. Both from appeared those ofto beP. multitentaculataplanocerids based andon externalP. reticulata morphology,, based on the only reportalthough so far some [29 features]. Their were external different morphology from those of P. was multitentaculata characterized and P. by reticulata a translucent, based on body the with brown reticulateonly report mottle so far [29]. (light Their in colorexternal for morpholo S1 andgy dark was for characterized S2; Figure by2A,B) a translucent