Animal coloration serves various signaling and non-signaling functions. In damselflies and dragonflies (Odonata), such colors may not only play photoprotective and/or thermoregulatory roles but also serve as visual signals during courtship and/or agonistic interactions. Here, we analyzed the coloration of Perithemis tenera wings, a potential secondary sexual ornament, applying spectrophotometry and visual modeling to gain a deeper understanding of their color mechanisms and functions. The amber coloration of the P. tenera wings results from the interaction of light with both the melanized chitin matrix and possibly ommochrome pigments. Additionally, by fitting the absorbance curve of P. tenera wings to the extinction coefficient of different melanins, we deduced that pheomelanin is likely the pigment embedded in the wing's chitinous matrix. The amber coloration of P. tenera wings stands out against their natural habitat, making it detectable by conspecifics. Finding multiple pigments in the P. tenera wings not only enhances our understanding of the functional roles of pigmentation in Odonata but also offer broader insights into how structural and pigment-based colorations evolve as multifunctional traits.

Ovarian development in r-selected species is a highly dynamic process widely studied in various insect groups. An array of biotic and abiotic factors may influence it. So, the present investigation was to evaluate the impact of a female's age and mating status on the ovarian development, ootheca formation, body colour polymorphism, and fat content of the tortoise beetle, Aspidomorpha miliaris (Fabricius). Females of a certain age and mating status were dissected to demonstrate changes in their ovarioles, lateral oviduct, accessory gland, elytral colour, fat body content, and body size. It was predicted that age or mating status would not affect the ovarian parameters like length and width of ovarioles, lateral oviducts, accessory glands, body size, fat body content, and elytral colour. However, the ootheca-forming modifications would be initiated by ageing and mating. The current study demonstrated the substantial effect of age and mating status on the growth of the accessory glands and ovarioles. On the other hand, fat body contents declined comparatively in multiply-mated females. Besides this, the beetle exhibits elytral colour polymorphism till sexual maturity. An accessory gland was exclusively detected in multiply-mated females. This discovery opens opportunities for further investigation into the precise function of the gland, which is likely involved in the formation of ootheca.

The liver, a crucial organ for metabolic processes, has a generally uniform histological structure across amphibian taxa. However, ontogenetic changes, particularly those related to biphasic life cycle and metamorphosis, are less documented. Here, we explored and described the liver histology of an emerging model organism, the Balkan crested newt (Triturus ivanbureschi) at three ontogenetic stages: larval, juvenile (just after metamorphosis) and adult. At the larval stage, the liver is characterized by hepatocytes containing large lipid droplets, poorly developed basement membranes in the blood vessels, and a lack of melanin-rich macrophage centers. Juveniles show transitional characteristics between larvae and adults. Lipid droplets in hepatocytes are abundant, but also, at the juvenile stage the well-developed basement membrane of blood vessels and melanomacrophages are present, as in adults. The presence of lipid droplets in hepatocytes during larval and juvenile stages suggests the liver's role in fat storage and energy provision during development and growth. Melanomacrophages, which synthesize melanin, perform phagocytosis, and neutralize free radicals, have been found in juveniles (after metamorphosis) and increase with age. The biphasic life cycle and liver histology transition in Triturus newts provide an insight in changes in liver histology and make them a suitable model for studying fat deposition regulation and the evolution of the immune system in terrestrial vertebrates.

Phoronida is a small phylum of benthic marine invertebrates that can occur in large numbers globally. The study of phoronid morphology and anatomy is important for understanding phoronid biology and the function of benthic communities dominated by phoronids. Because all phoronids are tube-living animals, the study of the morphology and ultrastructure of the body wall is an important step toward understanding the processes of the tube formation, growth, and renovation. This study used epoxy histology, scanning and transmission electron microscopy to describe the body regionalization and ultrastructure of the body wall epithelium of the unusual Phoronis embryolabi, which lives as a commensal in burrows of digging shrimps. The trunk of P. embryolabi consists of 8 zones, which are clearly distinguishable in living individuals. These zones are as follows: long head region, median sphincter with its three different parts (waist, upper and lower), muscular region, reproductive region, zone 7, and ampulla. Such body division can correlate with specificity of life style of P. embryolabi. The ultrastructure of the epithelium of all zones differ from each other in thickness, set and abundance of gland cells, structure of the extracellular matrix that underlies the epithelium, and abundance of neurites. The capacity and distribution of glandular cells correlate with tube formation and remodelling. Bacteria of two different types are described along body wall of all parts of the trunk; reciprocally advantageous phoronid-bacteria interaction is suggested. Our data suggest that P. embryolabi is able to build the tube at the anterior end rather than at the posterior end, as previously suggested for other phoronid species. At the same time, the certain mechanism of phoronid tube growth and remodelling is still unknown for phoronids as well as for many other tube-living invertebrates.

Chelicerae, the mouthparts of chelicerates, are essential for food processing. Particularly within harvestmen (Opiliones), some species have greatly elongated their tripartite chelicerae and utilize them for mating behavior, defense, and primarily for predation. We investigated two European species, Ischyropsalis muellneri and Ischyropsalis hellwigii, which occupy different niches (caves, forests), exhibit different feeding ecologies (opportunist, specialist), and first and foremost possess different chelicerae morphologies (long and thin, short and robust). We scanned the specimens using state-of-the-art micro-CT, generated surface reconstructions, and equipped one chelicera of each specimen with artificial joints to explore their Range of Motion in a 3D kinematic approach. For a size-corrected comparison of the two species, we analyzed the Range of Motion in addition to three different settings (original body size, body scaled to 5 mm, chelicerae scaled to 5 mm). Ischyropsalis muellneri reached a higher maximum excursion angle (= single Range of Motion) in all three joints, also exhibiting a greater total Range of Motion in the original body length setting, as well as the scaled body length setting. Only in the third setting, the total Range of Motion of Ischyropsalis hellwigii was slightly higher, although Ischyropsalis muellneri still extended further ventrally. Our results suggest that the sturdier, more massive chelicerae of Ischyropsalis hellwigii, attributable to strong specialization on snails as prey, are associated with reduced Range of Motion. The less food-specialized species Ischyropsalis muellneri apparently requires higher flexibility of its chelicerae for prey capture, likely due to its restriction to cave ecosystems, where food availability is relatively scarce. We could show that virtual Range of Motion analyses in harvestmen chelicerae can play a pivotal role in understanding the theoretical feeding ecology and functional morphology of this group. This approach can be verified by in-vivo observations and measurements or extended to other arachnid taxa and other body parts.

Octodontids are South American caviomorph rodents endemic to mesic and arid biomes displaying a wide range of substrate preferences, from terrestrial to subterranean habits. However, the hind limb morphology of these rodents remain relatively poorly understudied, particularly from an ecomorphological perspective. To investigate the association between hind limb morphology and substrate preference-epigean, semifossorial, fossorial, and subterranean- this study analyzed six skeletal measurements of femur and tibia, along with five morphological indices. We employed phylogenetic mapping, allometry, and multivariate analyses (Phylogentic Flexible Discriminant and Principal Component analyses) on log-transformed variables and indices. The results suggest that the epigeans and subterraneans possess hind limb skeletal features that enhance their mechanical capabilities, which are advantageous for their respective lifestyles. However, in the absence of clear behavioral adaptations or associations, the functional habits of Octodontidae do not requires significant structural modifications of the proximal bones of the hind limbs. These results indicate that understanding the form-function relationship in octodontids requires direct field or laboratory observations of behavior and environmental interactions, highlighting the limitations of current research without such data.

Alternative hypotheses suggest that the reptiles at the origin of snakes were primarily either burrowing, terrestrial or marine. It is possible that the ability to swim varies between the major snake lineages and lifestyles; for example, the highly fossorial blind snakes (Scolecophidia), a lineage that emerged early in snake evolution over 100 My ago, may not be able to swim. However, it is sometimes stated that all snakes can swim suggesting that swimming ability may not be discriminatory. To find out whether this is true, we used a systematic search (PRISMA), including personal communications and information on websites. Of the 3951 species considered, no information was found for 89 % of all snakes. Of the 454 species for which information was found, 382 species were aquatic, only 62 were terrestrial, 6 were arboreal, and only 4 were burrowing. Moreover, almost all belonged to the speciose Colubroides (e.g. 58 % Colubridae, 20 % Elapidae). No reliable information was available for important early diverging lineages (e.g. Scolocophidia, Aniliidae). Faced with this lack of information, we filled in important phylogenetic gaps by testing the swimming capacity of 103 diverse snake species and 13 species of diverse limbed and limbless ectothermic tetrapod vertebrates (Amphisbaenia, Lacertilia, Gymnophiona). All tests were positive. The results show that, 1) all snakes for which information is available (525 species) appear to be able to swim, 2) this is a trait shared by many land vertebrates that undulate laterally. As swimming ability is non-discriminatory, we need to collect detailed measurements on the performance, kinematics and energetic efficiency of swimming snakes. It is also necessary to finely describe the ecology and morphology of the species studied to better understand form∼function relationships and the occupation of ecological niches in snakes.

Seasonal dimorphism in the body shape of marine invertebrates has been poorly explored compared to vertebrates. We aim to investigate through traditional (body length/width ratio, dorsal elevation ratio and angle of elevation) and geometric (centroid size and shape geometric configurations) morphometrics the effect of gonad maturity (via the gonadosomatic index [GSI] and gonad development stages [GDS]) on changes in body shape in males and females of two latitudinally different populations of the broadcast-spawning intertidal mollusc Chiton articulatus. We confirmed that C. articulatus does not present external sexual dimorphism since sex does not have a significant effect on body shape (1 %); instead, dimorphism was seasonal and related to the reproductive season, and varied across populations, probably because in the subtropical zone additional energy is invested in shell (scleritome) bending at the same time as the gonad matures, which is opposite of what occurs in the tropical zone. C. articulatus shows a narrower body shape (i.e., diminished body width) during its gonad maturity compared to the rest of the GDS and is corroborated by a body length/width ratio that contributes the greatest variation in the geometric shape descriptors (18 %), just below the centroid size (24 %). The expression of centroid size differences shows a correspondence with the change in body dimensions expressed by the three morphometric ratios during gonad maturity. The use of traditional body ratios over time may be useful in polyplacophoran molluscs as a proxy tool to estimate gonad maturity and to provide a reliable indication of the reproductive season.

Sexual activity (mating) negatively affects immune function in various insect species across both sexes. In Drosophila melanogaster females, mating increases susceptibility to pathogenic challenges and encourages within-host pathogen proliferation. This effect is pathogen and host genotype dependent. We tested if mating-induced increased susceptibility to infections is more, or less, severe in hosts experimentally adapted to pathogenic infection. We selected replicate D. melanogaster populations for increased post-infection survival following infection with a bacterial pathogen, Enterococcus faecalis. We found that females from the selected populations were better at surviving a pathogenic infection compared to the females from the control populations. This was true in the case of both the pathogen used for selection and other novel pathogens (i.e., pathogens the hosts have not encountered in recent history). Additionally, the negative effect of mating on post-infection survival was limited to only the females from control populations. Therefore, we have demonstrated that experimental selection for increased post-infection survival ameliorates negative effects of mating on host susceptibility to infections.

Living at high altitudes impose physiological and ecological challenges to which species may respond altering their body size, body proportions, and the shape of their body parts. Despite the importance of this topic for understanding the origin of species diversity, little attention has been invested in this phenomenon at the populational level. This paper study the relationship between altitude and body size, body proportions, and forewing shape venation of two populations of the parasitoid wasp Cotesia flavipes. Wasps were collected from Diatraea spp. larvae from sugarcane crops in two Colombian mountain ranges that cover between 600 m and 2143 m of altitude. Linear measurements of different body regions and geometric morphometrics of the forewing were subject to multivariate comparisons and allometric analyses to assess variation and to compare trends between ranges. Central (600 m to 1704 m) and Eastern Cordillera (877 m to 2143 m) populations showed different trends between body size and altitude. Allometric trends were not uniform within or between populations nor between structures. The allometric slopes of five body measurements from a single altitude differed from these from its own mountain range suggesting that body size trends along the cordilleras are a consequence of altitude and not of intrinsic body resource allocation processes. Wing shape between populations differed; however, these changes were poorly related to altitude. In agreement with recent studies in other groups, the observed allometric and wing shape differences between the two C. flavipes populations could be a plasticity response to altitude with interesting implications for posterior genetic differentiation.

Spotted hyaenas are generally considered to have resilience against injuries and pathogens, and the incidence of pathologies is scarcely reported. However, it is important for ecological studies and conservation efforts to have a clear overview of the physical threats that can occur in the life of a species. The present paper describes and interprets several osteopathologies in the crania of the spotted hyaena, Crocuta crocuta, in the collections of the Royal Museum for Central Africa (RMCA) in Tervuren, Belgium. Of the fifty-two specimens from central Africa examined, twenty-three (i.e., 44 %) of them showed evidence of pathologies, including injuries, congenital pathologies, and disease. Selected specimens were scanned using Computed Tomography and their internal cranial structures were studied. Here we describe and discuss the different types of pathologies evident in the crania of these hyaenas: craniosynostosis, microtia, osteomyelitis and periodontitis, as well as a trauma indicated by cranial fractures, dental fractures, and bite puncture marks. Some pathologies reported herein are novel for hyaenas, and there are some instances of multiple pathologies in the same individual. Implications of these pathologies, including the social interactions of hyenas, conservational threats, as well as biased data sampling, are highlighted. This study improves our knowledge of disease and trauma experienced by hyaenids, and provides a better understanding of their biology, and the potential threats faced by these iconic animals. This case study offers a comparative base for similar studies in other carnivorans, to clarify the pathological background of apex predators.

Islands provide excellent settings for studying the evolutionary history of species, since their geographic isolation and relatively small size limit gene flow between populations, and promote divergence and speciation. The endemic Bolle's Laurel Pigeon Columba bollii is an arboreal frugivorous bird species distributed on laurel forests in four islands of the Canary archipelago. To elucidate the population genetics, we genotyped ten microsatellite loci using DNA obtained from non-invasive samples collected across practically all laurel forest remnants, and subsequently grouped into eight sampling sites. Analyses including F-statistics, Bayesian clustering approaches, isolation by distance tests and population graph topologies, were used to infer the genetic diversity and the population differentiation within and among insular populations. Additionally, we evaluated the effect of null alleles on data analysis. Low genetic diversity was found in all populations of Bolle's Laurel Pigeon, with no significant differences in diversity among them. However, significant genetic differentiation was detected among all populations, with pigeons from La Palma and El Hierro exhibiting the closest affinity. Bayesian clustering supported population separation between islands, and also detected fine-scale structure within the Tenerife and La Gomera populations. Our results suggest that, despite columbids have a high movement ability, they can show signature of genetic divergence among populations, particularly on oceanic islands. Geological history of the islands and distribution range of habitats could have close influence on the evolutionary trajectories of these birds. This approach can provide practical tools to implement appropriate conservation measures for range-restricted species and their habitat.

During early development, fishes undergo significant changes that influence external morphology and the functioning of internal organs and systems. This often results in gradual variation of the morphological traits of individuals across developmental stages. The investigation of larval and juvenile fish development and growth patterns has pertinent implications for the systematic and ecological elucidation of species. Bryconops gracilis is a medium-sized fish, omnivorous that inhabits lotic and lentic environments with acidic and transparent waters in the Amazon basin. In this study, the early development of B. gracilis is described, until recently a practically unknown species. In terms of development, we used morphological, meristic, and morphometric data to characterize the larvae and juveniles. The individuals were collected in the Curuá-Una River, Amazon basin, Brazil. Fifty-four specimens were examined. Samples include individuals with 3.39-21.79 mm SL. Yolk-sac larvae have two attachment organs on the dorsal surface of head and body. The larvae of B. gracilis are considered altricial, with a fusiform body, and the intestine reaches the median region of the body. Initially, the mouth is subterminal and becomes isognathic from the postflexion stage on. During the postflexion stage, the most relevant morphological changes occur (e.g., presence of all fins, mouth position similar to adults, increased body pigmentation), making individuals more specialized to explore new habitats and diets and maximize their chances of survival. Furthermore, vertebrae and myomeres are compared and assist with differentiating some Bryconops species at early life stages that occur in sympatry in the Amazon basin. Our results contribute to knowledge about the external morphology of neotropical freshwater fishes, enabling the identification of larvae and juveniles through traditional taxonomy and broadening the perspective on the ontogenetic study of the adipose fin in Characoidei.

For small non-hibernating mammals, a high thermogenic capacity is important to increase activity levels in the cold. It has been previously reported that lactating females decrease their thermogenic activity of brown adipose tissue (BAT), whereas their capacity to cope with extreme cold remains uncertain. In this study we examined food intake, body temperature and locomotor behavior, resting metabolic rate, non-shivering thermogenesis, and cytochrome c oxidase activity, and the rate of state 4 respiration of liver, skeletal muscle, and BAT in striped hamsters (Cricetulus barabensis) at peak lactation and non- breeding hamsters (controls). The lactating hamsters and non- breeding controls were acutely exposed to -15°C, and several markers indicative of thermogenic capacity were examined. In comparison to non-breeding females, lactating hamsters significantly increased food intake and body temperature, but decreased locomotor behavior, and the BAT mass, indicative of decreased BAT thermogenesis at peak lactation. Unexpectedly, lactating hamsters showed similar body temperature, resting metabolic rate, non-shivering thermogenesis with non-breeding females after acute exposure to -15°C. Furthermore, cytochrome c oxidase activity of liver, skeletal muscle and BAT, and serum thyroid hormone concentration, and BAT uncoupling protein 1 expression, in lactating hamsters were similar with that in non-breeding hamsters after acute extreme cold exposure. This suggests that lactating females have the same thermogenic capacity to survive cold temperatures compared to non-breeding animals. This is particularly important for females in the field to cope with cold environments during the period of reproduction. Our findings indicate that the females during lactation, one of the highest energy requirement periods, do not impair their thermogenic capacity in response to acute cold exposure.

Sexual dimorphism (SD), the divergence of secondary sexual traits between males and females within a species, can arise from diverse evolutionary forces, such as natural selection, mate choice, and intrasexual competition. Allometric scaling patterns of dimorphic traits are related to their functional roles and the different selective pressures that affect each sex. Generally, traits that threaten rivals involved in intrasexual competition tend to exhibit the highest allometric slopes. Conversely, non-sexual traits often display isometric scaling, while genitalia and traits in direct contact between the sexes during courtship and copulation typically show hypoallometry. A good approach to study patterns of SD and allometry is to complement interspecific studies with analyzes of case studies, where the functional aspect is known in detail. Here, we review the occurrence of SD and evaluation of allometry in the Order Scorpiones, allowing us to compare general trends in a broader comparative framework within the group. In addition, we examined SD and allometric slopes of multiple traits (including somatic traits used in sexual and non-sexual interactions, as well as genitalia) in adult individuals of the scorpion Timogenes elegans (Scorpiones, Bothriuridae). We found that at an interspecific level there was a variation in SD between species and morphological traits, with most traits showing a male-biased SD, except for the chelicerae, which were found to be wider in females. Regarding SD studies, we found relatively few reports of functional allometry showing differences in allometric patterns between species. The results in T. elegans follow some of the general patterns found in other scorpions. We found hypoallometry in genital traits and hyperallometry in the pedipalps of both sexes, with steeper allometric slopes observed for pedipalp height in males. These results suggest that genital traits are under stabilizing selective pressure, while pedipalps in both sexes may be under natural and sexual selective pressure. Understanding allometric patterns and their relationship to function in scorpions provides significant insights into the evolutionary pressures driving the divergence of morphological traits used in both sexual and non-sexual contexts.

Compound eyes undoubtedly represent the widespread eye architecture in the animal kingdom. The insects' compound eye shows a wide variety of designs, and insects use their visual capacity to accomplish several tasks, including avoiding enemies, searching for food and shelter, locating a mate, and acquiring information about the environment and its surroundings. Broad literature data support the concept that visual ability lies in the way the eyes are built. Since the resolution and sensitivity of the compound eye are partly determined by the density of the ommatidia and the size of the facets. Morphological parameters of the compound eyes could influence the function of the visual organ and its capacity to process information, also representing a sensitive indicator of different habitat demands. In this study, we compared compound eyes' parameters in four closely related species of tiger beetles to disclose differences arising from different habitats. Furthermore, to investigate whether there are consistent intersexual differences, we also compared the most relevant parameters of the eye in males and females of four selected species. Our results show sex-related and interspecific differences that occur in examined species.

Climbing animals such as geckos and arthropods developed astonishing adhesive mechanisms which are fundamental for their survival and represent valuable models for biomimetic purposes. A firm adhesion to the host surface, in order to successfully lay eggs is necessary for the reproduction of most parasitoid insects. In the present study, we performed a comparative investigation on the attachment ability of four parasitoid species (the egg parasitoid Anastatus bifasciatus (Eupelmidae), the aphid parasitoid Aphidius ervi (Braconidae), the fly pupal ectoparasitoid Muscidifurax raptorellus (Pteromalidae) and the pupal parasitoid of Drosophila Trichopria drosophilae (Diapriidae)) with hosts characterized by a surface having different wettability properties. The friction force measurements were performed on smooth artificial (glass) surfaces showing different contact angles of water. We found that attachment systems of parasitoid insects are tuned to match the wettability of the host surface. Sexual dimorphism in the attachment ability of some tested species has been also observed. The obtained results are probably related to different microstructure and chemical composition of the host surfaces and to different chemical composition of the parasitoid adhesive fluid. The data here presented can be interpreted as an adaptation, especially in the female, to the physicochemical properties of the host surface and contribute to shed light on the coevolutionary processes of parasitoid insects and their hosts.

A major part of the described species is understudied, falling into the Linnean shortfall. This is a major concern for cryptic species, which require integrative approaches to better evaluate their diversity. We conducted morphological analyses using specimens of Stellifer punctatissimus, S. gomezi, and S. menezesi to reassess their taxonomical identity. We evaluated the allometric and sexual components of the morphology of the Stellifer punctatissimus species complex, and tested and discussed species hypotheses. The combined evidence of our work and previous studies agrees with the current morphological hypothesis of three species, as opposed to the two-lineage molecular hypothesis. However, as cryptic species, they overlap in most their traits, especially females and juveniles. Previously unaccounted variation of allometric and sexually dimorphic characters in this species complex revealed a confounding effect that might explain past and current taxonomic errors. Taxonomical practice of using body depth as a diagnostic character had led to juveniles and females being, respectively, described as a different species or wrongfully identified. Hence, taxonomical studies demand better assessment of allometric and sexual dimorphism components. Herein, we present new characters in a key to the Atlantic species of Stellifer, which disclose size and sexual variation unnoticed in previous studies. The contrasting growth patterns among these species could imply distinct habitat use. As a result, it could be suggested that such species are under different threats, which highlights the need of differential management and conservation strategies.

Members of the subfamily Heterocongrinae (Congridae) are a peculiar group of anguilliform eels that construct sandy borrows, form large colonies, and are popularly recognized as garden eels. They live with most of their bodies inside self-constructed borrows exposing their heads and trunk to feed on zooplankton, preferably copepods, that are brought passively by currents. As plankton feeders there was a suspicion that their branchial skeleton would have structures that could aid in the filtering process, such as highly developed or modified branchial rakers, which are observed in other suspension-feeding fishes, such as anchovies and sardines. Branchial rakers, however, were considered to be absent across Anguilliformes (except for Protanguilla). Nonetheless, specimens that were examined using clearing and staining and computed tomography showed, in all cases, branchial rakers associated with their gill arches. Heterocongrines have branchial rakers across their first to fourth branchial arches. These rakers are conical and apparently unossified, but further studies are necessary to attest its degree of ossification or its complete absence. Their pharyngeal tooth plates are reduced, a condition that may reflect their preference for smaller food items. Additionally, they may use crossflow filtering to feed, although detailed studies are necessary to clarify if hydrosol sieving may also aid in food capture. Furthermore, the present study proposes that the presence of branchial rakers should be better investigated in Anguilliformes with similar feeding habits as heterocongrines, considering that these structures may be more widespread within the group than previously considered.

Disproportional changes (i.e. allometry) in shark morphology relative to increasing body size have been attributed to shifts in function associated with niche shifts in life history, such as in habitat and diet. Photographs of blue sharks (Prionace glauca, 26-145 kg) were used to analyze changes in parameters of body and fin morphology with increasing mass that are fundamental to swimming and feeding. We hypothesized that blue sharks would demonstrate proportional changes (i.e. isometry) in morphology with increasing mass because they do not undergo profound changes in prey and habitat type; accordingly, due to geometric scaling laws, we predicted that blue sharks would grow into bodies with greater turning inertias and smaller frontal and surface areas, in addition to smaller spans and areas of the fins relative to mass, which are parameters that are associated with the swimming performance in sharks. Many aspects of morphology increased with isometry. However, blue sharks demonstrated negative allometry in body density, whereas surface area, volume and roll inertia of the body, area, span and aspect ratio of both dorsal fins, span and aspect ratio of the ventral caudal fin, and span, length and area of the mouth increased with positive allometry. The dataset was divided in half based on mass to form two groups: smaller and larger sharks. Besides area of both dorsal fins, relative to mass, larger sharks had bodies with significantly greater turning inertia and smaller frontal and surface areas, in addition to fins with smaller spans and areas, compared to smaller sharks. In conclusion, isometric scaling does not necessarily imply functional similarity, and allometric scaling may sometimes be critical in maintaining, rather than shifting, function relative to mass in animals that swim through the water column.

Osteoglossomorpha, the bony tongue fishes, show great variation in morphology, behavioural strategies, reproductive biology and gamete ultrastructure. The order Osteoglossiformes is the only vertebrate taxon, in which four types of sperm (monoflagellate, biflagellate and aflagellate aquasperm and the complex introsperm) have been described. It is also the only vertebrate lineage in which aflagellate spermatozoa exist. The aim of this study was to analyse the structure of the testis and the process of spermiogenesis in the mormyrid Campylomormyrus compressirostris during the breeding season using light and electron microscopy (transmission and scanning). Males of this species have a single testis of the anastomosing tubular type. The tubules of the anterior part of the testis contain cysts with developing germ cells, and this region is much wider than the posterior part, which consists of efferent ducts filled with sperm cells. The cysts are filled with single or mitotic spermatogonia, primary and secondary spermatocytes and early spermatids. At the stage of spermatids with fine granular chromatin, the cysts rupture and successive stages of spermatid differentiation take place in the testicular lumen; we therefore characterise this process as 'extracystic spermiogenesis'. Sperm development in C. compressirostris is extremely simple and involves chromatin condensation in the central region of the nucleus, a slight decrease in nuclear volume, the appearance of numerous vesicles in the cytoplasm that form a tubular-vesicular system at the base of the nucleus. Both centrioles and mitochondria are translocated to the peripheral region of the midpiece, which forms the opposite pole to the nucleus. There are many differences between the types of spermiogenesis described so far in teleosts and that found in C. compressirostris, including the loss of flagellum formation. This unique type of spermiogenesis is restricted to species of the families Mormyridae and Gymnarchidae, all of which possess aflagellate spermatozoa. Our data demonstrate that the spermatid differentiation and existence of the aflagellate spermatozoon are a unique phenomena not only among teleosts but also in the whole vertebrate lineage.