The order Polycladida comprises a highly diverse and cosmopolitan group of marine turbellarian flatworms. Owing to the great morphological diversity and the absence of a molecular phylogeny, the classification of this group has always been controversial. Here we seek to add resolution by reporting the results of molecular phylogenetic analysis based on sequences of the 28S ribosomal RNA gene, thus providing a framework for understanding relationships and the evolution of characters within the group. The phylogeny provides strong support for 10 and 7 distinct families within the suborders Acotylea and Cotylea, respectively. In addition, an analysis based on the mitochondrial gene sequences (cytochrome c-oxidase subunit I) reveals further details of the relationships within Acotylea, which were classified by morphological analysis, but not by 28S rRNA sequence-based analyses. These analyses also showed that several species corresponded to previously described genera based on morphological features and character combinations. We conclude that a classification of genera in Acotylea and Cotylea based on molecular phylogeny reflects the morphological diversity of these polyclad flatworms.
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.
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.
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.
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