Journal of Morphology最新文献

Egg cases in oviparous cartilaginous fishes (sharks, rays, and chimaeras) exhibit diverse morphologies that are closely tied to species-specific reproductive adaptations. However, the diversity and formation mechanisms of these structures remain poorly understood. In this study, we performed a quantitative morphological analysis of egg cases from three species: Okamejei kenojei, Cephaloscyllium sarawakense, and Chiloscyllium plagiosum. The results demonstrated that the egg cases of these species could be distinguished using multiple morphological indices (p < 0.05), supporting species-specificity in egg case morphology. In these species, we observed that egg jelly initially envelops the egg case during early embryonic development and later dissolves, allowing seawater entry—suggesting a conserved reproductive strategy within Elasmobranchii. Furthermore, under artificial breeding conditions, observations of female O. kenojei showed that ovulation occurs before egg case secretion. Specifically, eggs reach the oviduct above the oviducal gland when about half of the egg case has formed. Immunohistochemical staining revealed estrogen and progesterone receptors in the oviductal gland cells. Interestingly, O. kenojei can produce malformed eggs with shark egg case-like features under captive breeding conditions. These findings provide new insights into the species-specificity, timing, and hormonal regulation of egg case formation in cartilaginous fishes, and lay a foundation for future research on their reproductive strategies.

The sperm morphology of Malacobdella japonica, a symbiotic nemertean living in the clam Spisula sachalinensis, has been examined using light and electron microscopy. In this species, the structure of the elongated spermatozoon, consisting of a straight head and a posteriorly oriented flagellum, is similar to that of Malacobdella grossa, as previously studied. In both species, the sperm head has an acrosomal complex, an elongated nucleus, and a neck region. The small acrosomal complex, including a thimble-like acrosomal vesicle, a post-acrosomal ring of electron-dense material, and a subacrosomal space, is located asymmetrically at the anterior end of the nucleus. However, there are some differences between the species in the structure and organization of the mitochondrial compartment. In M. grossa, the mitochondria are elongated along the nucleus and remain separate all along their length. In M. japonica, the mitochondria merge at the base of the neck region, forming a ring around the distal centriole. Above this area, they remain separate, similarly to the pattern observed in M. grossa. This finding suggests M. japonica to be an intermediate stage between M. grossa, distinguished by its unique separate mitochondria, and other hoplonemerteans that have a single, ring-like mitochondrion. We assume that the ultrastructure of spermatozoa in symbiotic nemerteans is shaped rather by the mode of fertilization than by their lifestyle. With external fertilization, without forming clutches, the lifestyle does not lead to the development of highly modified spermatozoa.

Distinguishing between sheep (Ovis aries) and goat (Capra hircus) calcanei is challenging due to their close taxonomic relationship and skeletal similarities, which complicates species identification in veterinary, comparative anatomy, and zooarchaeological contexts. The calcaneus is a robust and functionally significant bone in small ruminants, providing valuable insights into their biomechanical adaptations. This study conducted a comprehensive morphometric analysis of the calcaneus in sheep (n = 163, including Akkaraman, Hamdani, Morkaraman, and Dağlıç breeds) and goats (n = 61, Hair Goats), using a digital caliper to measure 10 standardized linear parameters focused on critical calcaneal tuber and articular surface dimensions, capturing functional weight-bearing and joint articulation aspects. Notably, the maximum width of the sustentaculum tali, which supports the talus and enhances tarsal joint stability, consistently showed larger values in sheep than in goats. Overall, sheep exhibited larger measurements, reflecting adaptations to distinct locomotor and ecological roles. A simplified linear discriminant analysis, based on key functional measurements, achieved a classification accuracy of 87.1%, highlighting the utility of these morphological features in species differentiation. Correlation analysis further revealed strong interrelationships among measurements, indicating coordinated growth patterns in the calcaneus. These findings establish a robust reference framework for future comparative anatomy, veterinary science, and zooarchaeology studies, underscoring the diagnostic and functional importance of the calcaneus in small ruminants.

The sense of touch (mechanoreception) in snakes is not widely appreciated despite emerging evidence of tactile specialisation among sea snakes. This is partly due to the challenges in quantifying small (< 1 mm) and numerous scale mechanoreceptors concentrated on the head. By using a novel application of gel-based 3D profilometry (GelSight scanner) in combination with histology and scanning electron microscopy, we comprehensively quantified the morphology and distribution of scale mechanoreceptors in the olive-headed sea snake, Hydrophis major (Hydrophiinae), for the first time. H. major is one of the few predators to eat eel-tailed catfishes (Plotosidae), which have venomous spines that they lock into erect positions during defence. We discovered that in addition to the radially symmetrical smooth, dome-shaped mechanoreceptors typically found in sea snakes, H. major has asymmetrical, peak-shaped mechanoreceptors that are significantly larger but rarer. Smooth domes are distributed in decreasing density antero-posteriorly on the head with the highest densities on the snout and labial scales. Asymmetrical peaks are rarer; they are detected only on the dorsal and lateral sides of the head, are most dense behind the eye, and their associated dermal papilla (that contains mechanosensitive cells) is spatially offset from the stiff peak. Based on their morphology and distribution, we suggest functional differences in mechanosensory modalities: (1) smooth domes for direct touch used in prey handling to avoid dangerous spines of catfish prey, and (2) asymmetrical peaks that create a lever system capable of amplifying directional water flow. The latter might allow H. major to detect the C-start escape response of free-swimming catfish and/or enhance kinaesthesia for the snake's perception of self-motion during foraging and predatory strikes, but physiological studies are needed to investigate these functional hypotheses further.

The respiratory organs of water-breathing ectotherms (WBEs), typically in the form of gills, extract oxygen from the water. The size and shape of respiratory organs are vital to the efficiency of oxygen diffusion and often unique to each species. Oxygen concentrations in water are < 1%, far lower than in the atmosphere, so WBEs have evolved gill morphologies that maximize their oxygen uptake but are also shaped by their life history and the environment. Gill surface area is related to the body mass of an individual, scaling as a power-law function whose exponent, based on theory and empirical evidence, generally lies between 0.6 and 0.9. However, nearly all estimates of gill surface area are based on 2-dimensional rather than 3-dimensional measurements, assuming that gill thickness is negligible and unimportant to respiration in accord with the physics of oxygen diffusion. This study aimed to develop methods to measure gills in three dimensions and then convert 2-dimensional gill surface area to the 3-dimensional gill surface area. The body mass and gill surface area scaling relationship was then determined for two decapod crustaceans with contrasting life histories: the Caribbean Spiny Lobster (Panulirus argus) and the Caribbean King Crab (Maguimithrax spinosissimus). We found a positive relationship between gill lamellae thickness and body mass for both species, which could be beneficial or detrimental to larger, oxygen-limited WBEs depending on the balance between oxygen diffusion through the additional surface area and the dynamics of oxygen diffusion into a larger volume. The 2-dimensional scaling exponent between gill surface area and body mass was 0.626 for lobsters and 0.779 for crabs, compared to their 3-dimensional scaling exponents (0.809 and 0.702, respectively). These are the first 2D and 3D body mass and gill surface area scaling exponents determined for these species and will set the basis for future physiological research.