First evidence of sexual dimorphism in olfactory organs of deep-sea lanternfishes (Myctophidae).

Finding a mate is of the utmost importance for organisms, and the traits associated with successfully finding one can be under strong selective pressures. In habitats where biomass and population density is often low, like the enormous open spaces of the deep sea, animals have evolved many adaptations for finding mates. One convergent adaptation seen in many deep-sea fishes is sexual dimorphism in olfactory organs, where, relative to body size, males have evolved greatly enlarged olfactory organs compared to females. Females are known to give off chemical cues such as pheromones, and these chemical stimuli can traverse long distances in the stable, stratified water of the deep sea and be picked up by the olfactory organs of males. This adaptation is believed to help males in multiple lineages of fishes find mates in deep-sea habitats. In this study, we describe the first morphological evidence of sexual dimorphism in the olfactory organs of lanternfishes (Myctophidae) in the genus Loweina. Lanternfishes are one of the most abundant vertebrates in the deep sea and are hypothesized to use visual signals from bioluminescence for mate recognition or mate detection. Bioluminescent cues that are readily visible at distances as far as 10 m in the aphotic deep sea are likely important for high population density lanternfish species that have high mate encounter rates. In contrast, myctophids found in lower density environments where species encounter rates are lower, like those in Loweina, likely benefit from longer-range chemical or olfactory cues for finding and identifying mates.

The three-dimensionally articulated oral apparatus of a Devonian heterostracan sheds light on feeding in Palaeozoic jawless fishes.

Attempts to explain the origin and diversification of vertebrates have commonly invoked the evolution of feeding ecology, contrasting the passive suspension feeding of invertebrate chordates and larval lampreys with active predation in living jawed vertebrates. Of the extinct jawless vertebrates that phylogenetically intercalate these living groups, the feeding apparatus is well-preserved only in the early diverging stem-gnathostome heterostracans. However, its anatomy remains poorly understood. Here, we use X-ray microtomography to characterize the feeding apparatus of the pteraspid heterostracan Rhinopteraspis dunensis (Roemer, 1855). The apparatus is composed of 13 plates arranged approximately bilaterally, most of which articulate from the postoral plate. Our reconstruction shows that the oral plates were capable of rotating around the transverse axis, but likely with limited movement. It also suggests the nasohypophyseal organs opened internally, into the pharynx. The functional morphology of the apparatus in Rhinopteraspis precludes all proposed interpretations of feeding except for suspension/deposit feeding and we interpret the apparatus as having served primarily to moderate the oral gape. This is consistent with evidence that at least some early jawless gnathostomes were suspension feeders and runs contrary to macroecological scenarios that envisage early vertebrate evolution as characterized by a directional trend towards increasingly active food acquisition.

Macro- and micro morphology of the olfactory organ of African bonytongue, Heterotis niloticus (Cuvier 1829), compared with other species of the family Osteoglossidae (Teleostei).

Osteoglossiformes (bonytongue fishes) possess many morphological specializations associated with functions such as airbreathing, feeding, and electroreception. The olfactory organ also varies among species, notably in the family Osteoglossidae. Herein, we describe the olfactory organ of an osteoglossid, Heterotis niloticus, to compare it with the olfactory organs of other osteoglossiforms. We demonstrate the presence of an olfactory rosette within the olfactory chamber. This structure consists of a short median raphe surrounded by olfactory lamellae, which possess dorsal lamellar processes. On the surface of the olfactory lamellae, there are secondary lamellae formed by the olfactory epithelium. Within the olfactory epithelium, two zones can be distinguished: parallel brands of sensory cells located in the cavities between the secondary lamellae and a nonsensory area covering the remaining part of the olfactory lamellae. The olfactory epithelium is formed by ciliated and microvillus olfactory sensory neurons, supporting cells, goblet cells, basal cells and ciliated nonsensory cells. Additionally, rodlet cells were observed. The results confirm large variability in terms of the olfactory organ of Osteoglossiformes, particularly of Osteoglossidae, and support the secondary lamellae evolution hypothesis within this family.

Comparative digestive biology between the ponyfishes from the Pranburi River estuary, Thailand.

We investigated the digestive biology of two prevalent leiognathid species in Pranburi River estuary, Thailand: the decorated ponyfish (Nuchequula gerreoides) and the splendid polyfish (Eubleekeria splendens). A total of 632 samples collected from February to April and September to November 2017 were analysed using morphological and histological approaches. The overall structures were similar between the species: a short mucous-cell-rich oesophagus region, a well-developed gastric gland uniformly present across the stomach's mucosal layer, and three finger-like pyloric caeca between the stomach and intestine. However, there were marked differences in the mouth, gill raker, and intestinal coefficient (IC). N. gerreoides had a relatively longer mouth, smoother gill rakers, and an IC of 1.08 ± 0.01, similar to those of other carnivorous fish. In contrast, the gill raker of E. splendens had more villiform teeth that can filter-feed better, and their IC was 2.16 ± 0.02 (i.e., longer intestine). Although digestive structures were generally similar between the ponyfishes, these differences suggest that N. gerreoides is relatively carnivorous with stronger suction, whereas E. splendens may be an omnivorous or herbivorous filter-feeder.

Diving into dual functionality: Swim bladder muscles in lionfish for buoyancy and sonic capabilities.

Although the primary function of the swim bladder is buoyancy, it is also involved in hearing, and it can be associated with sonic muscles for voluntary sound production. The use of the swim bladder and associated muscles in sound production could be an exaptation since this is not its first function. We however lack models showing that the same muscles can be used in both movement and sound production. In this study, we investigate the functions of the muscles associated with the swim bladder in different Pteroinae (lionfish) species. Our results indicate that Pterois volitans, P. radiata and Dendrochirus zebra are able to produce long low-frequency hums when disturbed. The deliberate movements of the fin spines during sound production suggest that these sounds may serve as aposematic signals. In P. volitans and P. radiata, hums can be punctuated by intermittent louder pulses called knocks. Analysis of sonic features, morphology, electromyography and histology strongly suggest that these sounds are most likely produced by muscles closely associated with the swim bladder. These muscles originate from the neurocranium and insert on the posterior part of the swim bladder. Additionally, cineradiography supports the hypothesis that these same muscles are involved in altering the swim bladder's length and angle, thereby influencing the pitch of the fish body and participating in manoeuvring and locomotion movements. Fast contraction of the muscle should be related to sound production whereas sustained contractions allows modifications in swim bladder shape and body pitch.

Comparative ontogeny and phylogenetic relationships of eight lizardfish species (Synodontidae) from the Northwest Pacific, with a focus on Trachinocephalus monophyly.

Lizardfish (Aulopiforms: Synodontidae), distributed broadly in temperate to tropical waters, are represented globally by 83 species across four genera, with 10 species in Korea. Despite these numbers, few studies have been conducted on the early life history of lizardfishes compared to their adult counterparts. Thus, we conducted molecular identification of 123 Synodontidae larvae collected from the Northwest Pacific (Korea Strait, Yellow Sea, East China Sea, and East Sea) between June 2017 and July 2021, using mitochondrial DNA COI and 16S rRNA sequences. Significant morphological differences were observed in the larvae and juvenile, including variation in melanophore, count, morphometric characteristics, and body shape. The morphological traits of eight species (Harpadon nehereus, Saurida macrolepis, Saurida wanieso, Saurida sp., Synodus hoshinonis, Synodus kaianus, Synodus macrops, and Trachinocephalus trachinus) served as vital data for interpreting the phylogenetic relationships within the Northwest Pacific Synodontidae. Ultimately, the identification key revealed by this study will enable accurate identification of Synodontid larvae and juveniles, and further facilitate our understanding of the phylogenetic relationships within this family.

True grit? Comparative anatomy and evolution of gizzards in fishes.

Gut morphology frequently reflects the food organisms digest. Gizzards are organs of the gut found in archosaurs and fishes that mechanically reduce food to aid digestion. Gizzards are thought to compensate for edentulism and/or provide an advantage when consuming small, tough food items (e.g., phytoplankton and algae). It is unknown how widespread gizzards are in fishes and how similar these structures are among different lineages. Here, we investigate the distribution of gizzards across bony fishes to (1) survey different fishes for gizzard presence, (2) compare the histological structure of gizzards in three species, (3) estimate how often gizzards have evolved in fishes, and (4) explore whether anatomical and ecological traits like edentulism and microphagy predict gizzard presence. According to our analyses, gizzards are rare across bony fishes, evolving only six times in a broad taxonomic sampling of 51 species, and gizzard presence is not clearly correlated with factors like gut length or dentition. We find that gizzard morphology varies among the lineages where one is present, both macroscopically (presence of a crop) and microscopically (varying tissue types). We conclude that gizzards likely aid in the mechanical reduction of food in fishes that have lost an oral dentition in their evolutionary past; however, the relative scarcity of gizzards suggests they are just one of many possible solutions for processing tough, nutrient-poor food items. Gizzards have long been present in the evolutionary history of fishes, can be found in a wide variety of marine and freshwater clades, and likely have been overlooked in many taxa.

What is missing from the sturgeon jaw: Developmental morphology of the upper jaw in Acipenser.

Sturgeons belong to the family Acipenseridae, the most species-rich extant family of Acipenseriformes, a basal actinopterygian group of key importance in assessing the early radiations of the actinopterygians. At the same time, acipenseriforms display unique specializations in the morphology of the snout and jaws which make them a valuable model for studying evolutionary novelties. However, despite a long history of research, the homologies of the snout and the mandibular arch of acipenseriforms remain uncertain preventing further studies on the evolutionary origin of their unique snout and jaw structure, and in particular, of the upper jaw symphysis, the key apomorphy of the group and the preoral snout. In the present study, a detailed description of the upper jaw morphology and development in sturgeons is provided in order to address its composition in terms of the common actinopterygian archetype. Based on the obtained results, the upper jaw of acipenseriforms is assumed to have lost the autopalatine portion, which most likely is represented by the separate cartilages supporting the tentacles. Also, the conventional interpretation of the sturgeon's maxilla as dermopalatine is rejected on the grounds of this bone structure and development. Paedomorphosis is proposed to be the most likely mechanism explaining the evolutionary origin of the upper jaw symphysis and supposed modifications of the snout in sturgeons.

Morphology of the cornea and iris in the Australian lungfish Neoceratodus forsteri (Krefft 1870) (Dipnoi): Functional and evolutionary perspectives of transitioning from an aquatic to a terrestrial environment.

The Australian lungfish, Neoceratodus forsteri (Krefft 1870), is the sole extant member of the Ceratodontidae within the Dipnoi, a small order of sarcopterygian (lobe-finned) fishes, that is thought to be the earliest branching species of extant lungfishes, having changed little over the last 100 million years. To extend studies on anatomical adaptations associated with the fish-tetrapod transition, the ultrastructure of the cornea and iris is investigated using light and electron (transmission and scanning) microscopy to investigate structure-function relationships and compare these to other vertebrate corneas (other fishes and tetrapods). In contrast to previous studies, the cornea is found to have only three main components, comprising an epithelium with its basement membrane, a stroma with a Bowman's layer and an endothelium, and is not split into a dermal (secondary) spectacle and a scleral cornea. The epithelial cells are large, relatively low in density and similar to many species of non-aquatic tetrapods and uniquely possess numerous surface canals that contain and release mucous granules onto the corneal surface to avoid desiccation. A Bowman's layer is present and, in association with extensive branching and anastomosing of the collagen fibrils, may be an adaptation for the inhibition of swelling and/or splitting of the stroma during its amphibious lifestyle. The dorsal region of the stroma possesses aggregations of pigment granules that act as a yellow, short wavelength-absorbing filter during bright light conditions. Desçemet's membrane is absent and replaced by an incomplete basement membrane overlying a monocellular endothelium. The iris is pigmented, well-developed, vascularised and contractile containing reflective crystals anteriorly. Based upon its ultrastructure and functional adaptations, the cornea of N. forsteri is more similar to amphibians than to other bony fishes and is well-adapted for an amphibious lifestyle.

Study of some morphometric and meristic characteristics of Alepes vari (Cuvier, 1833) collected from the Arabian coast / Estudo de características morfométricas e merísticas de Alepes vari (Cuvier, 1833) coletados na costa da Arábia

The present study examines the correlations between fifteen morphometric and ten meristic characters and total length (TL) of males, females, and combined sexes of Alepes vari (Cuvier, 1833) collected from Karachi fish harbor, West Wharf of Karachi Coast. Statistical analyses of linear regression relationships show mostly strong correlations (r≥0.70; p<0.05) between total length (TL) and most morphometric characters in males, females, and combined sexes, except the height of pectoral-fin (PFH), and pelvic-fin base length (PelFL); whereas, meristic characters were found to be constant and indicate weak or negative type correlations (r≤0.50; p>0.05) with total length (TL). Hence, according to our present results, there is a direct relationship between the total length of fish and all morphometric characters, which were found to be the best indicators of positive allometric pattern growth in fish. Moreover, analysis of the 2-sample t-test revealed (t-test; p>0.05) that no sexual dimorphism was reported in Alepes vari. Thus, our present study could be valuable in systematic classification, sexual dimorphism, and management of this species on the Karachi coast.

O presente estudo examina as correlações entre 15 caracteres morfométricos e 10 caracteres merísticos e comprimento total (CT) de machos, fêmeas e sexos combinados de Alepes vari (Cuvier, 1833), coletados do porto de Karachi, West Wharf, na costa de Karachi. As análises estatísticas das relações de regressão linear mostraram, principalmente, correlações fortes (r ≥ 0,70; p < 0,05) entre o CT e a maioria dos caracteres morfométricos em machos, fêmeas e sexos combinados, exceto a altura da nadadeira peitoral e o comprimento da base da nadadeira pélvica, enquanto os caracteres merísticos foram constantes, indicando correlações fracas ou negativas (r ≤ 0,50; p > 0,05) com o CT. Portanto, de acordo com nossos resultados, existe uma relação direta entre o CT dos peixes e todos os caracteres morfométricos, que foram considerados os melhores indicadores de crescimento do padrão alométrico positivo em peixes. Além disso, a análise do teste t de duas amostras revelou (teste t; p > 0,05) que nenhum dimorfismo sexual foi relatado em A. vari.

Histología testicular y morfología de la vejiga natatoria del pez neotropical Plagioscion magdalenae (Acanthuriformes: Sciaenidae): implicaciones en su taxonomía y distribución / Testicular histology and morphology of the swim bladder of the neotropical fish Plagioscion magdalenae (Acanthuriformes: Sciaenidae): implications in taxonomy and distribution

Introducción: La Pácora (Plagioscion magdalenae) es una especie nativa de Colombia y en categoría de casi amenazada a nivel del país, de la que poco se conoce acerca de las estructuras con aporte etológico en su historia de vida. Objetivo: Analizar el desarrollo gonadal relacionado con la espermatogénesis de P. magdalenae y las implicaciones taxonómicas a partir de la vejiga natatoria. Métodos: Realizamos siete muestreos en dos periodos climáticos: el primero en octubre de 2019 a febrero de 2020 de recolectas bimensuales y la segunda de octubre 2020 a enero 2021 de recolectas mensuales; con el fin de abarcar cada momento del pulso de inundación de la cuenca baja del Magdalena. Describimos la histomorfología testicular y las características macroscópicas de la vejiga natatoria de la especie. Además, analizamos aspectos como relaciones morfométricas, clases de tallas por sexos, la proporción sexual y la época de madurez sexual. Resultados: Analizamos 142 ejemplares y 4 intervalos de tallas fueron establecidos (entre 145 y 575 mm LE), 66 fueron machos y 73 hembras con una proporción sexual global 1.1:0.9 (hembra-macho) sin diferencias. Los machos tienen testículos de tipo tubular, protegidos por el peritoneo que se encuentra recubierto por la musculatura sónica de la vejiga natatoria, la cual posee forma de ''zanahoria'' sin apéndices, con dos bandas laterales de músculos intrínsecos (promedio entre 6.75 cm de largo y 1.48 cm de ancho) unidos por una aponeurosis y que solo se encuentran en machos sexualmente maduros. Conclusión: Los testículos poseen una organización de tipo lobular irrestricto y la espermatogénesis se da en lóbulos seminíferos. La vejiga natatoria de la Pácora es simple y no tiene estructuras accesorias. La distribución geográfica de P. magdalenae se restringe a la cuenca del río Magdalena en Colombia.

Introduction: The Pácora (Plagioscion magdalenae) is a species native to Colombia and in near-threatened category at the country level, of which little is known about the structures with ethological contribution in its life history. Objective: To analyze the gonadal development related to spermatogenesis of P. magdalenae and the taxonomic implications from the swim bladder. Methods: We collected seven samples in two climatic periods: the first from October 2019 to February 2020 of bimonthly collections, and the second from October 2020 to January 2021 of monthly collections; to cover every moment of the flood pulse of the lower Magdalena basin. We described the testicular histomorphology and macroscopic characteristics of the swim bladder of the species. In addition, we analyzed aspects such as morphometric relationships, size classes by sex, sex ratio and time of sexual maturity. Results: We analyzed 142 specimens and established 4 size intervals (between 145 and 575 mm SL), 66 were males and 73 females with an overall sex ratio of 1.1:0.9 (female-male) with no differences. Males have tubular testes protected by the peritoneum covered by the sonic musculature of the swim bladder, which has a ''carrot'' shape without appendages, with two lateral bands of intrinsic muscles (average between 6.75 cm long and 1.48 cm wide) connected by an aponeurosis and found only in sexually mature males. Conclusion: The testes have an unrestricted lobular organization and spermatogenesis occurs in the seminiferous lobes. The swim bladder of the Pácora is simple and has no accessory structures. The geographic distribution of P. magdalenae is restricted to the Magdalena River basin in Colombia.

Fossil evidence for a pharyngeal origin of the vertebrate pectoral girdle.

The origin of vertebrate paired appendages is one of the most investigated and debated examples of evolutionary novelty1-7. Paired appendages are widely considered as key innovations that enabled new opportunities for controlled swimming and gill ventilation and were prerequisites for the eventual transition from water to land. The past 150 years of debate8-10 has been shaped by two contentious theories4,5: the ventrolateral fin-fold hypothesis9,10 and the archipterygium hypothesis8. The latter proposes that fins and girdles evolved from an ancestral gill arch. Although studies in animal development have revived interest in this idea11-13, it is apparently unsupported by fossil evidence. Here we present palaeontological support for a pharyngeal basis for the vertebrate shoulder girdle. We use computed tomography scanning to reveal details of the braincase of Kolymaspis sibirica14, an Early Devonian placoderm fish from Siberia, that suggests a pharyngeal component of the shoulder. We combine these findings with refreshed comparative anatomy of placoderms and jawless outgroups to place the origin of the shoulder girdle on the sixth branchial arch. These findings provide a novel framework for understanding the origin of the pectoral girdle. Our evidence clarifies the location of the presumptive head-trunk interface in jawless fishes and explains the constraint on branchial arch number in gnathostomes15. The results revive a key aspect of the archipterygium hypothesis and help reconcile it with the ventrolateral fin-fold model.

Post-testicular sperm maturation in ancient holostean species.

Fish speciation was accompanied by changes in the urogenital system anatomy. In evolutionarily modern Teleostei, male reproductive tracts are fully separated from the excretory system, while in evolutionarily ancient Chondrostei and Holostei, the excretory and reproductive tracts are not separated. Sturgeon post-testicular sperm maturation (PTSM) occurring as a result of sperm/urine mixing is phenomenologically well described, while, in holosteans, functional intimacy of seminal ducts with kidney ducts and the existence of PTSM still need to be addressed. In Lepisosteus platostomus (Holostei), sperm samples were collected from testes (TS), efferent ducts (EDS), and Wolffian ducts (WDS). While WDS was motile, no motility was found in TS and EDS. The existence of PTSM was checked by in vitro PTSM procedure. After TS and EDS incubation in seminal fluid from WDS, no more than 5% motile spermatozoa were observed in TS, whereas in EDS the motility percentage was up to 75%. Experimental dyeing of urogenital ducts in gars and sturgeons revealed some differences in the interconnection between sperm ducts and kidneys. It is concluded that post-testicular sperm maturation occurs in gars and suggests that infraclass Holostei occupies an intermediate evolutionary position between Teleostei and Chondrostei in the anatomical arrangement of the urogenital system.

Gill structure and respiratory ability of Euchiloglanis kishinouyei (Osteichthyes: Siluriformes: Sisoridae).

Glyptosternoid fishes are distributed in the torrent environment of alpine canyons, where they often leave the water to climb rocky cliffs. As one of the most primitive species of glyptosternoid fishes, Euchiloglanis kishinouyei was examined in the current study to analyse its gill microstructure and respiratory ability. We first found that the oxygen consumption rate was relatively high and negatively correlated with body mass and that the average oxygen consumption at night was higher than during the day. The asphyxiation point of E. kishinouyei (5.05 ± 0.22 g) was c. 1.93 mg/L. Subsequently, the surface morphology, gross gill tissue structure, and ultra-microstructure of gill lamellae were investigated using optical microscopy and SEM. The gills showed an overall trend of regression, with five pairs of gill arches in each gill cavity. The adjacent gill filaments had large gaps, and the gill lamellae were thick. The gill filaments were closely arranged on the gill arches, their folded respiratory surface was highly vascularized with no tiny crest, and there were obvious tiny crests, grooves, pits, and pores on the nonrespiratory surface. The gill lamellae were closely embedded on both sides of gill filaments, which were composed of flat epithelial cells, basement membrane, pillar cells, and mucous cells. The gill total respiratory area correlated positively with body mass and length, whereas the gill relative respiratory area correlated negatively with body mass. We comprehensively analysed the gill microstructure and respiratory capacity of E. kishinouyei to provide fundamental data for the adaptive evolution of the gill structures of bimodally respiring fishes and offer insights into further study on the accessory air-breathing function of skin.

Are vertebrates constrained to two sets of paired appendages? The morphology, development, and evolution of pre-pelvic claspers in the Holocephali.

Holocephalans exhibit auxiliary appendages called pre-pelvic claspers (PPCs) that are located anterior to the pelvic fins, while pelvic claspers are pelvic fin modifications located posteriorly as modified metapterygia. Articulation points of the PPCs have not previously been imaged or evaluated in a comparative context, therefore, they may represent modified pelvic fin structures if they articulate with the propterygium. Alternatively, they could represent the only example of an independent third set of paired appendages in an extant taxon, if they articulate independently from any pelvic fin basal cartilages, challenging the current paradigm that extant jawed vertebrates are constrained to two sets of paired appendages. Two extinct groups, including Placoderms and Acanthodians, exhibit variation in the number of paired appendages, suggesting this may be a plesiomorphic trait. We evaluated PPC developmental growth rates, morphology, and articulation points in spotted ratfish (Hydrolagus Colliei, Holocephali). We also compared variation in PPC morphology among representatives of the three extant holocephalan families. Both, the pre-pelvic and pelvic claspers exhibit a dramatic surge in growth at sexual maturity, and then level off, suggesting synchronous development via shared hormonal regulation. While mature females are larger than males, pelvic fin growth and development is faster in males, suggesting a selective advantage to larger fins with faster development. Finally, microcomputed tomography scans revealed that PPCs are not modified propterygia, nor do they articulate with the propterygium. They articulate with the anterior pre-pelvic process on the anterior puboischiadic bar (or pelvic girdle), suggesting that while they are associated with the pelvic girdle, they may indeed represent a third, independent set of paired appendages in extant holocephalans.

Skeletal anatomy of the pectoral fin in mudskipper species from terrestrial and aquatic habitats.

Mudskippers are a group of amphibious fishes in the family Oxudercidae, whose species inhabit a range of habitats from mostly aquatic to mostly terrestrial. Most of our understanding about habitat preference comes from natural history observations, particularly where they are collected (i.e., low intertidal vs. high intertidal regions). Mudskippers have undergone several morphological changes to accommodate a terrestrial life, including major changes to the pectoral and pelvic girdles. These changes result in a novel crutching gait, which mudskippers use to move over land. Though the appendicular morphology and crutching gait of mudskippers have been described in some species, few studies have compared skeletal structures across the family. In our study, we use microcomputed tomography (µCT) scans to compare the skeletal anatomy of 16 species of aquatic and terrestrial mudskippers. Linear discriminant analysis is used to analyze measurements obtained through geometric morphometrics (landmarks). We found bone structures of the pectoral region in the terrestrial group were significantly longer and wider than those in the aquatic group. Furthermore, a significant difference in anatomy is shown between terrestrial and aquatic genera with both axial and appendicular elements contributing to the separation between groups. This work describes the differences in skeletal morphology associated with terrestriality in mudskippers and provides valuable insights into specific anatomical characteristics contributing to their adaptation to novel environments.

Beaks promote rapid morphological diversification along distinct evolutionary trajectories in labrid fishes (Eupercaria: Labridae).

The upper and lower jaws of some wrasses (Eupercaria: Labridae) possess teeth that have been coalesced into a strong durable beak that they use to graze on hard coral skeletons, hard-shelled prey, and algae, allowing many of these species to function as important ecosystem engineers in their respective marine habitats. While the ecological impact of the beak is well understood, questions remain about its evolutionary history and the effects of this innovation on the downstream patterns of morphological evolution. Here we analyze 3D cranial shape data in a phylogenetic comparative framework and use paleoclimate modeling to reconstruct the evolution of the labrid beak across 205 species. We find that wrasses evolved beaks three times independently, once within odacines and twice within parrotfishes in the Pacific and Atlantic Oceans. We find an increase in the rate of shape evolution in the Scarus+Chlorurus+Hipposcarus (SCH) clade of parrotfishes likely driven by the evolution of the intramandibular joint. Paleoclimate modeling shows that the SCH clade of parrotfishes rapidly morphologically diversified during the middle Miocene. We hypothesize that possession of a beak in the SCH clade coupled with favorable environmental conditions allowed these species to rapidly morphologically diversify.

The evolution of fast-growing coral reef fishes.

Individual growth is a fundamental life history trait1-4, yet its macroevolutionary trajectories have rarely been investigated for entire animal assemblages. Here we analyse the evolution of growth in a highly diverse vertebrate assemblage-coral reef fishes. We combine state-of-the-art extreme gradient boosted regression trees with phylogenetic comparative methods to detect the timing, number, location and magnitude of shifts in the adaptive regime of somatic growth. We also explored the evolution of the allometric relationship between body size and growth. Our results show that the evolution of fast growth trajectories in reef fishes has been considerably more common than the evolution of slow growth trajectories. Many reef fish lineages shifted towards faster growth and smaller body size evolutionary optima in the Eocene (56-33.9 million years ago), pointing to a major expansion of life history strategies in this Epoch. Of all lineages examined, the small-bodied, high-turnover cryptobenthic fishes shifted most towards extremely high growth optima, even after accounting for body size allometry. These results suggest that the high global temperatures of the Eocene5 and subsequent habitat reconfigurations6 might have been critical for the rise and retention of the highly productive, high-turnover fish faunas that characterize modern coral reef ecosystems.

The origin of blinking in both mudskippers and tetrapods is linked to life on land.

Blinking, the transient occlusion of the eye by one or more membranes, serves several functions including wetting, protecting, and cleaning the eye. This behavior is seen in nearly all living tetrapods and absent in other extant sarcopterygian lineages suggesting that it might have arisen during the water-to-land transition. Unfortunately, our understanding of the origin of blinking has been limited by a lack of known anatomical correlates of the behavior in the fossil record and a paucity of comparative functional studies. To understand how and why blinking originates, we leverage mudskippers (Oxudercinae), a clade of amphibious fishes that have convergently evolved blinking. Using microcomputed tomography and histology, we analyzed two mudskipper species, Periophthalmus barbarus and Periophthalmodon septemradiatus, and compared them to the fully aquatic round goby, Neogobius melanostomus. Study of gross anatomy and epithelial microstructure shows that mudskippers have not evolved novel musculature or glands to blink. Behavioral analyses show the blinks of mudskippers are functionally convergent with those of tetrapods: P. barbarus blinks more often under high-evaporation conditions to wet the eye, a blink reflex protects the eye from physical insult, and a single blink can fully clean the cornea of particulates. Thus, eye retraction in concert with a passive occlusal membrane can achieve functions associated with life on land. Osteological correlates of eye retraction are present in the earliest limbed vertebrates, suggesting blinking capability. In both mudskippers and tetrapods, therefore, the origin of this multifunctional innovation is likely explained by selection for increasingly terrestrial lifestyles.