Seasonal Changes in the Tetrodotoxin Content of the Flatworm Planocera multitentaculata.

Tetrodotoxin (TTX) is a potent neurotoxin that acts specifically on voltage-gated sodium channels on excitable membranes of muscle and nerve tissues. The biosynthetic process for TTX is unclear, although marine bacteria are generally thought to be the primary producers. The marine flatworm Planocera multitentaculata is a known TTX-bearing organism, and is suspected to be a TTX supplier to pufferfish. In this study, flatworm specimens were collected from an intertidal zone in Hayama, Kanagawa, Japan, the TTX content of the flatworm was measured using liquid chromatography with tandem mass spectrometry (LC-MS/MS), and seasonal changes in TTX content were investigated. No significant difference in TTX concentration of the flatworm body was found between the spawning period and other periods. However, the TTX content in individual flatworms was significantly higher in the spawning period than at other times. The TTX content rose in association with an increase in the body weight of the flatworm.

The planocerid flatworm is a main supplier of toxin to tetrodotoxin-bearing fish juveniles.

Tetrodotoxin (TTX), a potent neurotoxin, is found in various phylogenetically diverse taxa. In marine environments, the pufferfish is at the top of the food chain among TTX-bearing organisms. The accumulation of TTX in the body of pufferfish appears to be of the food web that begins with bacteria. It is known that toxic pufferfishes possess TTX from the larval/juvenile stage. However, the source of the TTX is unknown because the maternally sourced TTX is extremely small in quantity. Therefore, the TTX has to be obtained from other organisms or directly from the environment. Here, we report evidence that the source of TTX for toxic fish juveniles including the pufferfish (Chelonodon patoca) and the goby (Yongeichthys criniger) is in the food organisms, as seen in their gut contents. Next generation sequencing analysis for the mitochondrial COI gene showed that the majority of the sequence recovered from intestinal contents of these toxic fishes belonged to the flatworm Planocera multitentaculata, a polyclad flatworm containing highly concentrated TTX from the larval stage. PCR specific to P. multitentaculata also showed that DNA encoding the planocerid COI gene was strongly detected in the intestinal contents of the goby and pufferfish juveniles. Additionally, the planocerid specific COI sequence was detected in the environmental seawater collected from the water around the sampling locations for TTX-bearing fish. These results suggest that planocerid larvae are the major TTX supplier for juveniles of TTX-bearing fish species.

Species composition of the genus Scombrops (Teleostei, Scombropidae) in the waters around the Japanese Archipelago: detection of a cryptic species.

Current literature states that family Scombropidae consists of a single genus Scombrops comprising three species worldwide, with two of them, Scombrops boops and Scombrops gilberti, distributed in the waters around the Japanese Archipelago. Although these two scombropids are commercially important species, little is known about the ecology of these fishes. It is difficult to discriminate between these two species based on external characteristics because of their morphological similarity. Here, we report two different morphotypes characterized by the relative growth between the otolith size and the standard length (SL) of the scombropid specimens caught in southern waters off Kyushu Island, Japan, and show the genetic relationship between the morphotypes by means of phylogenetic analyses using complete DNA sequences of the cytochrome b gene. The relationship between otolith weight and SL was significantly different between specimens < 505 mm SL and those > 550 mm SL. Phylogenetic analysis demonstrated that the sequences from these scombropid specimens formed three clades: two corresponded to S. boops and S. gilberti, while the third did not correspond to any sequence recorded in databases, suggesting that these specimens are undescribed scombropid species. Almost all the specimens with SL < 505 mm (n = 76) were identified as S. boops, and only nine as S. gilberti. On the other hand, almost all the specimens with SL > 550 mm (n = 41) fell in the unidentified group except for four specimens, whose sequences were identical to that of S. boops.

Including planocerid flatworms in the diet effectively toxifies the pufferfish, Takifugu niphobles.

Beginning with the larval stages, marine pufferfish such as Takifugu niphobles contain tetrodotoxin (TTX), an extremely potent neurotoxin. Although highly concentrated TTX has been detected in adults and juveniles of these fish, the source of the toxin has remained unclear. Here we show that TTX in the flatworm Planocera multitentaculata contributes to the toxification of the pufferfish throughout the life cycle of the flatworm. A species-specific PCR method was developed for the flatworm, and the specific DNA fragment was detected in the digesta of wild pufferfish adults. Predation experiments showed that flatworm larvae were eaten by the pufferfish juveniles, and that the two-day postprandial TTX content in these pufferfish was 20-50 µg/g. Predation experiments additionally showed flatworm adults were also eaten by pufferfish young, and after two days of feeding, TTX accumulated in the skin, liver and intestine of the pufferfish.

Complete mitochondrial genome of an undescribed gnomefish of the genus Scombrops (Teleostei, Scombropidae) from southern waters off Kyushu Island, Japan.

The complete mitochondrial genome of an undescribed gnomefish species of the genus Scombrops was determined using a PCR-based method. The total length of mitochondrial DNA (mtDNA) was 16,521 bp, and included 13 protein-coding genes, two ribosomal RNA (rRNA) genes, 22 transfer RNA (tRNA) genes and one control region. The mitochondrial gene arrangement of this gnomefish species was identical to that of two previously described scombropid species, Scombrops boops and Scombrops gilberti, and also to those of other teleosts. Maximum likelihood analysis showed that the undescribed scombropid species is most closely related to S. boops.

Phylogenetic position of Scombropidae within teleostei: the complete mitochondrial genome of the gnomefish, Scombrops gilberti.

The complete mitochondrial genome of the Japanese gnomefish, Scombrops gilberti, was determined using a PCR-based method. The total length of mitochondrial DNA (mtDNA) is 16 518 bp, which includes 13 protein-coding genes, two ribosomal RNA (rRNA) genes, 22 transfer RNA (tRNA) genes, and one control region. The mitochondrial gene arrangement of S. gilberti was found to be identical to that of other scombropid and indeed, other teleosts as well. Maximum likelihood analysis revealed that Scombropidae forms a sister group to Pempheriformes.