Zoological Science

Aposematic animals advertise their unprofitability to potential predators via morphological and behavioral signals. Strong signals are detectable and memorable for the predators, and such signals would therefore be expected to be most effective. However, many apparently well-defended animals do not have very conspicuous signals. To better understand this paradoxical phenomenon, I compared intra-individual variation in an immobile aposematic behavior of the newt Cynops pyrrhogaster under different thermal gradients, using 40 animals from four populations. I found a negative relationship between ambient temperature and the frequency of performing this behavior, independent of body size. Newts kept in low temperature conditions showed a stronger tendency to display the immobile aposematic behavior than those in high temperature conditions, indicating that this behavior in the newt is temperature-dependent. However, interpopulational comparison of thermal thresholds governing the decision to flee or remain immobile as an antipredator strategy suggested that the effect of temperature might not be responsible for interpopulational variation in the immobile aposematic behavior of the newt.

The grass puffer (Takifugu niphobles) exhibits unique spawning behavior. Large numbers of fish aggregate to coastal spawning beds several hours before high tide during the spring tide. In order to examine the environmental and physiological regulation of this semilunar-synchronized spawning rhythm, the aggregating and spawning behaviors of the grass puffer were observed in the field, and in an aquarium without tidal changes. The fish aggregated to a spawning bed in the rising tidal phases both in the morning and evening during the spring tide, and several days after the spring tide. Spawning occurred on several days when large numbers of fish (200–1000) aggregated to the spawning bed. The timing of aggregation and spawning was tightly connected to the tidal changes; aggregation occurred 2–3 h before high tide, and spawning occurred 2 h before high tide. In the aquarium, in which a slope was constructed with pebbles, small groups of mature fish aggregated on the slope only in the rising tidal phases during and after the spring tide, when the fish aggregated in the field. However, there was no spawning in the aquarium. The aggregating behavior observed in the aquarium without tidal changes suggests that the semilunar reproductive rhythm is endogenously maintained with surprising precision during the spawning period in grass puffer.

The biliary bile salts of the medaka, the Japanese rice fish (Oryzias latipes) were isolated and identified. Only bile acids were present, and all were N-acylamidated with taurine. Three bile acids, constituting 98% of total bile acids, were isolated by chromatography and their structure inferred from their properties compared to those of synthetic standards when analyzed by liquid chromatographytandem mass spectrometry. The dominant bile acid was the 25R-epimer (82%) of 3α,7α,12α-trihydroxy-5β-cholestan-27-oic acid. The 25S-epimer was also present (11%), as was cholic acid (5%). Complete ¹H and ¹³C NMR signal assignments of the C-25 epimers were made by using a combination of several 1D- and 2D-NMR techniques. The ¹H and ¹³C NMR chemical shifts and spectral patterns of the hydrogen and carbon atoms, being close to the asymmetric centered at C-25, provided confirmatory evidence in that they distinguished the two epimeric diastereomers. The medaka is the first fish species identified as having C₂₇ biliary bile acids as dominant among its major bile salts.

Flounder metamorphosis, marked by eye migration, lateralized pigmentation, and tissue differentiation in the stomach and skeletal muscle, is stimulated by thyroid hormone (TH). It is known that tri-iodothyronine (T3) produced by iodothyronine deiodinase type-1 (Dio1) from thyroxine (T4) enters the blood, whereas T3 produced by Dio2 penetrates into the nucleus of the Dio2-expressing cells, and then Dio3 inactivates both T4 and T3. To better understand the distinct functions of these three deiodinases in T3 regulation during flounder metamorphosis, we examined the tissue expression patterns of dio1, dio2, and dio3 in larvae of the Japanese flounder, Paralichthys olivaceus, by section in situ hybridization (SISH). We found that each deiodinase is expressed in a spatially and temporally specific pattern. dio1 is expressed in liver parenchymal cells from pro-metamorphosis to early climax, while dio2 is expressed in limited regions of the eyes, tectum, and skeletal muscles from pro-metamorphosis to post-climax. Considering these findings together with reports on other vertebrates, we predict that the liver cells expressing dio1 supply T3 to the blood, and that this systemic T3 synchronizes metamorphosis of differentiating tissues throughout the larval body, whereas the eyes, tectum, and skeletal muscles autonomously produce additional T3 for local tissue differentiation. Finally, dio3 expression is detected in skeletal muscle and gastric gland blastemas, which both undergo marked tissue differentiation at metamorphic climax. We hypothesize that dio3 expression protects these tissues from basal T3 levels early in metamorphosis, ensuring, together with the T3 surge from the liver, the synchronization of tissue differentiation at metamorphic climax.

The adult ascidian neural complex forms from a thin tube called the neurohypophyseal duct and from the primordium of the cerebral ganglion from the sensory vesicle in metamorphosing larvae. Neurohypophyseal duct cells, located in the anterior left side of the sensory vesicle of swimming larvae, are derived from the anterior embryonic neural plate, which expresses common transcription factors in vertebrates and urochordates. The cerebral ganglion primordium is probably derived from the posterior sensory vesicle during metamorphosis. After metamorphosis begins, the duct elongates anteriorly and fuses with the stomodeal ectoderm, where the dorsal tubercle, a large ciliated structure that opens into the upper part of the pharynx, later develops. The rudiment of the cerebral ganglion and the duct elongate posteriorly. The duct also differentiates into the neural gland. The dorsal wall of the neural gland in adult ascidians has a thick epithelium (placode), the central part of which forms the dorsal strand by repeated invaginations along the visceral nerve. Both gonadotropin-releasing hormone (GnRH) neurons and prolactin-like (non-GnRH) neurons are generated in the dorsal strand and migrate to the cerebral ganglion along the visceral nerve throughout adulthood. Thus, the epithelium derived from the neurohypophyseal duct possesses neurogenic potential to generate neural stem cells of the central (cerebral ganglion) and peripheral (dorsal strand) nervous systems. The generation of prolactin-like neurons and their migration into the brain with GnRH neurons suggest that the ascidian dorsal strand is homologous to the craniate olfactory placode, and provide unequivocal support for the existence of the clade Olfactores.

The nutcracker Nucifraga caryocatactes belongs to a group of bird species that use their beak and tongue as tools for obtaining food, such as seeds from hard-to-reach cones or nuts from shells. The aim of the present study, carried out with a scanning electron microscope, was to define the morphological features of the tongue of the nutcracker, which seems to be adapted to its environment through specific methods of obtaining food. One of the characteristic features of the nutcracker's tongue is the unique structure of the anterior part of the tongue, which has two long and highly keratinized processes — a product of the renewable keratinized layer of the epithelium covering the ventral surface of the tongue. These dagger-like processes, which are a modified “lingual nail,” take a major role in levering up and shelling seeds, which are transported over the short sulcus-shaped body of the tongue. A unique feature of the nutcracker's tongue is the groove separating the body from the root. Two rows of highly keratinized, mechanical, conical papillae are located at the junction of the body and the root. These papillae are mechanically protective elements for passing food particles in the form of seeds. Among lingual glands, only the posterior lingual glands on the root of the tongue have been observed. Their secretion agglutinates dry food before it is swallowed. Results of the present study indicate that the nutcracker's tongue is an efficient tool resembling a lever that is helpful in shelling seeds.

We investigated the mechanical properties of the flexor carpi radialis muscle (FCR), a forelimb muscle used mainly for amplexus in the breeding season (February to March), of the male Japanese brown frog, Rana japonica. In the present experiment, the changes in force and stiffness of the FCR before, during, and after contraction were measured at 4°C. The total time from the end of stimulation to the end of relaxation was about 30 min. The time course of this prolonged relaxation was fitted by two exponential decay processes. Stiffness decreased during prolonged relaxation, but stayed higher than force, when normalized to peak values. These mechanical properties of the FCR were different from those of the glutaeus magnus muscle (GM) in the hindlimb, used for jumping. When a quick release was applied to the FCR during relaxation, the force recovered gradually after a sudden decrease. The time course of this force recovery was fitted by a single exponential term, and the rate constant decreased as the prolonged relaxation proceeded. The possible involvement of active process(es) in the prolonged relaxation is discussed.

In this study, we asked whether neurons of the enteric nervous system (ENS) isolated from the central nervous system (CNS) are competent to mediate ordered autonomous gut motility (i.e. descending peristalsis, which may be functional for food propulsion down the gut) in the snail Lymnaea stagnalis. Firstly, we explored the origin of autonomous gut contraction in the esophagus and crop. Using extracellular recording, we were able to detect excitatory junctional potentials (EJPs) elicited by motor neuron activity in autonomous rhythmic bursts, showing that neurogenic autonomous contractions existed. We also determined the motor neuron as cholinergic from the antagonistic effects of d-tubocurarine (d-TC) on the EJPs. Interestingly, the “pacemaker region”, which drives the rhythm of the automaticity, was found in the crop, which is located distally to the esophagus. Thus, the burst first occurs in the crop and propagates in an ascending direction (i.e. in the opposite direction of the peristalsis) along the esophagus. From the observation of the relationship between the motor neuron activity and the driven motility, proximally decreasing time lag between the neuronal burst and the peak contraction (T-p; Time to peak) was found to be crucial in producing peristalsis. Regional T-p difference was also observed in the electrically evoked contractions in the isolated esophagus. Blocking the motor neuron activity by d-TC attenuated the regional difference. The above suggest that the processes leading up to the elicitation of EJPs by ENS activity contribute to producing the regional T-p difference.

Doubly uniparental inheritance (DUI) of mitochondrial DNA transmission to progeny has been reported in the mussel, Mytilus. In DUI, males have both paternally (M type) and maternally (F type) transmitted mitochondrial DNA (mtDNA), but females have only the F type. To estimate how much M type mtDNA enters the egg with sperm in the DUI system, ratios of M type to F type mtDNA were measured before and after fertilization. M type mtDNA content in eggs increased markedly after fertilization. Similar patterns in M type content changes after fertilization were observed in crosses using the same males. To compare mtDNA quantities, we subsequently measured the ratios of mtDNA to the 28S ribosomal RNA gene (an endogenous control sequence) in sperm or unfertilized eggs using a real-time polymerase chain reaction (PCR) assay. F type content in unfertilized eggs was greater than the M type in sperm by about 1000-fold on average. M type content in spermatozoa was greater than in unfertilized egg, but their distribution overlapped. These results may explain the post-fertilization changes in zygotic M type content. We previously demonstrated that paternal and maternal M type mtDNAs are transmitted to offspring, and hypothesized that the paternal M type contributed to M type transmission to the next generation more than the maternal type did. These quantitative data on M and F type mtDNA in sperm and eggs provide further support for that hypothesis.

A new melitoid Amphipoda, Psammogammarus mawatarii, is described from Kuchinoerabu Island, Kagoshima Prefecture, Japan. This is the first record of the genus from Asia. The new species is morphologically similar to P. garthi, but differs from the latter in the following features: 1) lateral cephalic lobe of head not strongly produced; 2) head lacking antennal sinus; and 3) posteroventral corner of epimeral plate 3 rounded. Morphology of maxillae 1 and 2, and mandible, and gut contents (harpacticoid Copepoda) of P. mawatarii indicate that the feeding type of the species seems to be, at least facultatively, carnivorous.