Zoological Letters, an open access online journal launched in 2015 is entering its third year of publication, and now seeks to drive new insights in evolutionary and comparative zoology by the inclusion of paleontological studies into its scope.

The tardigrade class Mesotardigrada was erected on the basis of the description of Thermozodium esakii by Gilbert Rahm in 1937. In some characteristics, T. esakii is intermediate between members of the classes Eutardigrada and Heterotardigrada. The class Mesotardigrada is known only from Rahm's published drawings of T. esakii; no voucher specimens are known, and subsequent attempts to collect it at the locus typicus have been unsuccessful. Among the possible explanations for this situation are that Rahm may have collected specimens of a more typical tardigrade, but misinterpreted what he saw. Alternatively, changes in habitat in the area may have led to the tardigrade's extirpation. Perhaps T. esakii is a rare species, such that recent sampling efforts have been insufficient to rediscover it. Finally, Rahm's 1937 description may be an attempt at deception. Until physical evidence of T. esakii is found, the species, and by extension the class Mesotardigrada, should be considered nomen dubium.

Extreme environments sometimes support surprisingly high meiofaunal diversity. We sampled runoff from the acidic hot springs of Unzen, Japan. This is the type locality of Thermozodium esakii Rahm, 1937, the only tardigrade in the class Mesotardigrada, which remains contentious in the absence of corroboration or supporting specimens. Our sampling revealed at least three species of arthropods, four rotifers, and five nematodes living in the hot (ca. 40°C) and acidic (ca. pH 2.5) water, but no tardigrades.

Chinese populations of the endangered Siberian salamander Ranodon sibiricus are reported to have diverged only about 120 years ago, and to have the lowest genetic diversity of any amphibian. However, these conclusions require verification, as the main range of the species is in Kazakhstan. Moreover, the generation time used for estimating divergence time has a weak ground. In order to clarify these problems, we investigated the molecular phylogenetic relationship and historical demography of the species covering its whole distribution range using the mitochondrial DNA region reported for Chinese population (1072 bp sequences of the control region), while conducting skeletochronological analysis to estimate accurate generation time. As a result, the range expansion was estimated at 88,000–50,000 YA, based on the generation time of 6–10 years. Degree of intraspecific genetic differentiation is actually very small, but, as a single species, is not so small as had been reported for Chinese population alone.

Geographic variation of the sizes of lower molar (M₁ size) and relative lower molar sizes (size proportions among M₁, M₂, and M3) were examined in two species of closely related Japanese field mice (Apodemus speciosus and Apodemus argenteus). To determine the cause of the geographic variations observed, phylogeographic structure, interspecific competition, climate, and location (mainland or island) were compared. With regard to the phylogeographic structure, the sizes of the molar and the relative molar sizes in A. speciosus did not differ between two major clades (mainland vs. Hokkaido and peripheral islands), whereas the phylogeographic structure was not examined in A. argenteus, as no clear phylogeographic structure was evident. The sizes of M₁ and relative molar size (M₃/M₁ score) in A. speciosus differed significantly between the mainland and islands; however, there was no significant difference between islands within and outside the distribution of A. argenteus. Interspecific competition between the two species may thus not be considerable. Climatic factors (temperature) and relative molar sizes (M₂/M₁ and M₃/M₁ scores) were significantly correlated in the mainland populations of A. speciosus, indicating that geographic variations in relative molar sizes may be affected by climate. In addition, M₃/M₁ scores varied more in the islands than on the mainland, suggesting effects of genetic drift. However, M₁ size increases in the island populations of the two species are not attributed to the climate, but are explained by the so-called Island Rule. Geographic variation in A. speciosus is thus likely attributable to various effects.

We examined geographic variations of absolute and relative lower molar sizes (size proportions among M₁, M₂, and M₃) in the Japanese macaque (Macaca fuscata) using skull specimens obtained from 13 locations. We compared the geographic patterns and climatic factors. The size of M₁ significantly and negatively correlated to the annual and coldest month mean temperatures and precipitation for both males and females. The M₂/M₁ and M₃/M₁ scores significantly and positively correlated to the annual and coldest month mean temperatures. The geographic pattern in the size of M₁ was consistent with Bergmann's rule; however, the sizes of M₂ and M₃ did not correlate with temperature, and were not consistent with the rule. The geographic pattern in relative molar sizes (M₂/M₁ and M₃/M₁ scores) indicated that populations living in colder climates possess a larger M₁ in relation to M₂ and M₃. Therefore, the correlations of M₁, M₂/M₁, and M₃/M₁ scores to temperature involve an increase in the size of M₁ in a colder climate. In macaques, the functions of the different molars (M₁, M₂, and M₃) do not differ (they all exhibit grinding function, unlike differentiation between carnassial and other molars in Carnivora), whereas the timing of molar eruption does. In other words, at young ages (1.5–3.5 years), M₁ erupts and is in occlusion, whereas M₂ and M₃ do not erupt and are not used for mastication. Therefore, the geographic pattern in the relative molar sizes may be attributed to increasing survivability in harsh winter climates by increasing occlusal surface in younger animals.

Pseudokeronopsis erythrina produces three new secondary metabolites, erythrolactones A2, B2 and C2, and their respective sulfate esters (A1, B1, C1), the structures of which have been recently elucidated on the basis of NMR spectroscopic data coupled to high resolution mass measurements (HR-MALDI-TOF). An analysis of the discharge of the protozoan pigment granules revealed that the non-sulfonated erythrolactones are exclusively stored in these cortical organelles, which are commonly used by a number of ciliates as chemical weapons in offense/defense interactions with prey and predators. We evaluated the toxic activity of pigment granule discharge on a panel of free-living ciliates and micro-invertebrates, and the activity of each single purified erythrolactone on three ciliate species. We also observed predator—prey interactions of P. erythrina with unicellular and multicellular predators. Experimental results confirm that only P. erythrina cells with discharged pigment granules were preferentially or exclusively hunted and eaten by at least some of its predators, whereas almost all intact (fully pigmented) cells remained alive. Our results indicate that erythrolactones are very effective as a chemical defense in P. erythrina.

Juvenile hormone (JH) plays a crucial role in the determination of developmental timing in insects. In Drosophila melanogaster, reports indicate that JH titers are the highest immediately following eclosion and that the mating experience increases the titers in females. However, the titers have not been successively measured for an extended period of time after eclosion. This study reveals that JH titers are increased after eclosion in virgin females when supplied with food that is occupied by eggs and larvae as well as in mated females. With the presence of eggs and larvae, food induced the virgin females to lay unfertilized eggs. When combined with previous work indicating that females are attracted to such food where they prefer to lay eggs, these results suggest that flies can prepare themselves to lay eggs by changing the titers of JH under the presence of growing larvae, ensuring that the food is an appropriate place to oviposit.

The distribution and local movement patterns of humpback whales in waters off the west coast of Okinawa Island, southwest Japan, were investigated using line transect and photo-identification methodologies. Line transect surveys were conducted from 2011 to 2014 and photo-identification survey from 2006 to 2012. During the surveys, humpback whales aggregated in the areas around Ie and Kerama Islands, and tended to travel along the inshore coast of Okinawa Island when they move locally between those two sites. A total of 496 humpback whales of the known sex were photo-identified (322 males, 75 females and 99 females with a calf). Of these, 24.8% were confirmed moving locally between the sites of Ie and Kerama Islands within the same season. Frequency rates of the local movement for males, females and females with a calf were 41.9, 25.0, and 15.1%, respectively; the frequency of local movement for males was significantly higher than that for females and females with a calf. These results indicate that male humpback whales tend to move more actively between the local breeding sites as compared to females and females with a calf. We speculate that the males search for more opportunities to mate, whereas females with a calf tend to remain in the same areas to nurse their calves. These findings extend our knowledge of the habitat use and reproductive ecology of humpback whales in Okinawan waters, which remain poorly understood.

Previous studies in Xenopus have shown that forced expression of Nodal signaling can change ectodermal cells to a mesodermal fate by the early gastrula stage, suggesting mesodermal competence in early ectoderm cells. This mesodermal competence in ectodermal cells has been shown to be regulated at the level of nucleocytoplasmic localization of Smad2 in Xenopus. However, the regulation of mesodermal competence through epigenetic mechanisms has not been fully elucidated. Here, we used a constitutively active form of zebrafish Smad2 (Smad2ca) to overcome the inhibition of Nodal signaling via the nuclear exclusion of Smad2. While heat-shock-dependent expression of Smad2ca at 5 h post fertilization (hpf) induced ectopic expression of mesendodermal genes in zebrafish ectodermal cells, responsiveness to Smad2ca was lost by 7 hpf. Chromatin immunoprecipitation-quantitative PCR analyses revealed that in ectodermal cells, levels of H3K27me3, but not H3K9me3, at both transcriptional start site (TSS) and 3′-flanking regions of mesendodermal genes at 9 hpf were markedly higher than those at 5 hpf. In contrast to mesendodermal genes, the levels of H3K27me3 at the TSS, but not 3′-flanking regions, of ectodermal genes remained low in ectodermal cells even at 9 hpf. We also found that chemical inhibition of H3K27me3 modification was able to recover the mesendodermal competence in ectodermal cells at 7 hpf, but not at 10 hpf. Taken together, our results suggest that the mesendodermal competence in zebrafish ectodermal cells is restricted by multiple mechanisms, including upregulation of H3K27me3 levels at the TSS of mesendodermal genes during early gastrulation.

The pond snail Lymnaea stagnalis can acquire conditioned taste aversion (CTA) as a long-term memory. CTA is caused by the temporal pairing of a stimulus, such as sucrose (the conditioned stimulus; CS), with another stimulus, such as electric shock (the unconditioned stimulus; US). Previous studies have demonstrated changes in both cellular and molecular properties in a pair of neurons known as the cerebral giant cells (CGCs), suggesting that these neurons play a key role in CTA. Here we examined the necessity of the pair of CGC somata for the learning, memory formation and memory recall of CTA by using the soma ablation technique. There was no difference in the feeding response elicited by the CS before and after ablation of the CGC somata. Ablation of the CGC somata before taste-aversion training resulted in the learning acquisition, but the memory formation was not observed 24 h later. We next asked whether memory was present when the CGC somata were ablated 24 h after taste-aversion training. The memory was present before performing the somata ablation. However, when we tested snails five days after somata ablation, the memory recall was not present. Together the data show that: 1) the somata of the CGCs are not necessary for learning acquisition; 2) the somata are necessary for memory formation; and 3) the somata are necessary for memory recall. That is, these results demonstrate that the CGCs function in the long-term memory of CTA in Lymnaea.