Gill Cover Research Articles

Terrestrial locomotion characteristics of climbing perch (Anabas testudineus).

The evolution and utilization of limbs facilitated terrestrial vertebrate movement on land, but little is known about how other lateral structures enhance terrestrial locomotion in amphibian fishes without terrestrialized limb structures. Climbing perch (Anabas testudineus) exhibit sustained terrestrial locomotion using uniaxial rotating gill covers instead of appendages. To investigate the role of such simple lateral structures in terrestrial locomotion and the motion-generating mechanism of the corresponding locomotor structure configuration (gill covers and body undulation), we measured the terrestrial kinematics of climbing perch and quantitatively analysed its motion characteristics. The digitized locomotor kinematics showed a unique body postural adjustment ability that enables the regulation of the posture of the caudal peduncle for converting lateral bending force into propulsion. An analysis of the coordination characteristics demonstrated that the motion of the gill cover is kinematically independent of axial undulation, suggesting that the gill cover functions as an anchored simple support pole while axial undulation actively mediates body posture and produces propulsive force. The two identified feature shapes explained more than 87% of the complex lateral undulation in multistage locomotion. The kinematic characteristics enhance our understanding of the underlying coordinating mechanism corresponding to locomotor configurations. Our work provides quantitative insight into the terrestrial locomotor adaptation of climbing perch and sheds light on terrestrial motion potential of locomotor configurations containing a typical aquatic body and restricted lateral structure.

Structure and Function of Gill Cover and Branchiostegal Rays in Teleostei

Structure and Function of Gill Cover and Branchiostegal Rays in Teleostei

Analysis and Evaluation of Muscle Quality in Different Parts of the Bighead Carp (Aristichthys nobilis).

In this study, the bighead carp (Aristichthys nobilis) was the object of research to compare and analyze the contents of conventional nutrients, amino acids, fatty acids, inosinic acid, and earthy-smelling compounds (geosmin and 2-methylisoborneol) in muscles of its dorsal, belly, tail, opercula, eye socket, and mandible in order to evaluate their quality. The findings could inform recommendations for the consumption and processing of different muscle parts of the bighead carp. The results showed that the water content in the abdominal muscle was significantly lower than that in other parts, and the crude fat content was significantly higher than that in other parts (p < 0.05, the same below). Seventeen kinds of amino acids were detected in the muscles of the six parts of the fish, and the dorsal muscles had the highest umami amino acids, essential amino acids and total amino acids, which were 6.45 g/100 g, 6.82 g/100 g and 17.26 g/100 g, respectively. The total amount of essential amino acids in the muscle was higher than that in the FAO/WHO standard model. According to the AAS standard, the first limiting amino acid in the muscle of the six parts was valine (Val). There were 26 kinds of fatty acids in the abdomen, under the gill cover and in the eye socket muscles, and the content of polyunsaturated fatty acids in the mandibular muscles was the highest (45.41%). The content of inosine in the dorsal muscle was significantly higher than that in other parts. Geosmin was the main substance in the muscle. There was no correlation between the distribution of earthy-smelling compounds and fat content, but the content of earthy-smelling compounds in the muscle of the belly and eye socket was the highest. Therefore, the muscle quality of different parts of the bighead carp has its own characteristics, and targeted development and utilization can make more efficient use of bighead carp resources.

Co-infections of Aeromonas dhakensis and Chryseobacterium indologenes in largemouth bass (Micropterus salmoides)

As one of the most important economic fishes, largemouth bass (Micropterus salmoides) is being widely farmed around the world. With the improper expansion and high-density culture, largemouth bass is susceptible to an increasing number of pathogens. Particularly, co-infection of multiple pathogens is ubiquitous, which has become a main obstacle to the development of largemouth bass industry. In the study, a co-infected case caused by Aeromonas dhakensis (A. dhakensis) and Chryseobacterium indologenes (C. indologenes) in largemouth bass was firstly reported. Diseased fish with clinical signs such as exophthalmia, hemorrhage on gill cover, rot or ulceration on gill, skin and fin were collected for further diagnostics. Tissue lesions of liver, spleen and kidney were observed, such as necrosis, hemorrhage, vacuolization and disintegration. Two bacteria strains were isolated from liver, one appeared as a white or yellowish colony, and the other was characterized by golden yellow color. Based on the conjoint analysis of 16S rRNA and gyrB gene sequences, the isolated strains were identified as A. dhakensis and C. indologenes, respectively. Antibiotic sensitivity testing indicated that both A. dhakensis and C. indologenes were sensitive to flumequine, enrofloxacin, ceftazidime and minocycline, and resistant to penicillin, sulfamethoxine sodium, ampicillin and cefamezin. Largemouth bass showed similar clinical signs and tissue lesions to naturally diseased fish following co-infection with the isolated strains, indicating that the isolated strains fulfill Koch's postulates. Besides, a slightly increased pathogenicity was observed following co-infection compared with single infection. The data firstly reveal a co-infected case caused by A. dhakensis and C. indologenes in largemouth bass, not only enriching the relevant information of largemouth bass diseases, but also promoting the diagnosis, prevention and control of A. dhakensis and C. indologenes co-infection.

Some morphological structures and histological features of the gills of Binni (Mesopotamichthys sharpeyi)

This study aimed to describe some morphological and histological aspects of the binni fish's gills. Fifteen adult male binni were pulled alive from the Al-Forat River at about (24 - 28) months, immediately after death. The gills of binni were situated on each side of the head beneath a gill cover; the operculum and the gills were made of finger-like long filaments joined to the cartilaginous gill bar. From each filament, numerous fragile lamellae that resemble leaves protrude. These lamellae comprised tiny capillaries covered in plain squamous epithelial cells. The epithelium acts as a barrier between the fish's blood and the surrounding water. The first, second, third, and fourth pairs of gill arches make up the gills supported by the bone skeleton. Each arch carries a row of gill filaments on its convex side and two rows of gill rakers on its concave side. On the rostral concave border, the filaments in the two extremes of the gill were short, but the length of all the filaments was the same. The first-gill arch's lateral rakers were longer than the medial rakers, while the fourth gill arch's medial rakers were longer than the lateral rakers. The rakers, bony comb-like projections, filled the rostral region of the gill arches at the rostral concave interior side. There was interdigitation between the gill rakers of the neighboring gill arches. Each gill arch included the lateral gill rakers pointing dorsolaterally and medial gill rakers pointing dorsomedially. Filaments originate from the gill arches and are maintained by the cartilage on the caudal convex border of the primary lamellae, from which the secondary lamellae emerge. The simple squamous epithelium lined the secondary lamellae. Keywords: Binni, Gill, Morphology, Histology

Skeletal deformities in gilthead seabream (Sparus aurata): exploring the association between mechanical loading and opercular deformation

Fish aquaculture is frequently confronted with skeletal abnormalities. In gilthead seabream (Sparus aurata (Linnaeus, 1758)), opercular deformities are one of the most common types of deformities. Many studies point at potential causal factors, mainly genetic or nutritional. However, no clear consensus has surfaced yet, and other factors known to affect bone formation remain unexplored, including mechanical stressors by external forces or muscle contraction. In this study, we investigated whether an altered mechanical use of the gill cover could be associated with opercular deformities, by inducing a change in the respiratory rate and thus gill ventilation. Juvenile seabreams were reared under 80, 100 or 200% dissolved oxygen (DO) to trigger altered respiration behaviour, and the effect on body and opercular shape was analysed. The main hypothesis was that hypoxic conditions would increase opercular ventilation, which would result in a higher prevalence of opercular deformities. The results show that the hypoxic condition (80% DO) did not trigger a significantly higher prevalence of opercular deformations, though the opposite is true for the hyperoxic condition (200% DO). No effect of oxygen treatment was observed on overall body shape, though deformed opercles showed a pronounced, but non-significant difference in shape across treatments. Morphometric results and µCT scans reveal that deformations mainly occur in the dorsocaudal region of the opercular bone. Although no causal link could be demonstrated, we discuss how these results can indirectly suggest that an altered mechanical loading on the operculum could explain its deformation.

A high latitude Gondwanan species of the Late Devonian tristichopterid Hyneria (Osteichthyes: Sarcopterygii).

We describe the largest bony fish in the Late Devonian (late Famennian) fossil assemblage from Waterloo Farm near Makhanda/Grahamstown, South Africa. It is a giant member of the extinct clade Tristichopteridae (Sarcopterygii: Tetrapodomorpha) and most closely resembles Hyneria lindae from the late Famennian Catskill Formation of Pennsylvania, USA. Notwithstanding the overall similarity, it can be distinguished from H. lindae on a number of morphological points and is accordingly described as a new species, H. udlezinye sp. nov. The preserved material comprises most of the dermal skull, lower jaw, gill cover and shoulder girdle. The cranial endoskeleton appears to have been unossified and is not preserved, apart from a fragment of the hyoid arch adhering to a subopercular, but the postcranial endoskeleton is represented by an ulnare, some semi-articulated neural spines, and the basal plate of a median fin. The discovery of H. udlezinye shows that Hyneria is a cosmopolitan genus extending into the high latitudes of Gondwana, not a Euramerican endemic. It supports the contention that the derived clade of giant tristichopterids, which alongside Hyneria includes such genera as Eusthenodon, Edenopteron and Mandageria, originated in Gondwana.

Sharks are the preferred scraping surface for large pelagic fishes: Possible implications for parasite removal and fitness in a changing ocean.

Mutualistic and commensal interactions can have significant positive impacts on animal fitness and survival. However, behavioural interactions between pelagic animals living in offshore oceanic environments are little studied. Parasites can negatively effect the fitness of their hosts by draining resources and diverting energy from growth, reproduction, and other bodily functions. Pelagic fishes are hosts to a diverse array of parasites, however their environment provides few options for removal. Here we provide records of scraping behaviour of several pelagic teleost species, a behaviour that is likely used for parasite removal. These records span three ocean basins and, to the best of our knowledge, include the first records of scraping interactions involving tunas, blue sharks, and mako sharks as well as the first records of intraspecific scraping. We found that scrapers preferred scraping their head, eyes, gill cover, and lateral surfaces, areas where parasites are commonly found and where damage would likely have a substantial impact on fitness. Scraper species varied in their scraping preferences with tunas scraping mostly on the posterior caudal margins of sharks and occasionally conspecifics, while rainbow runner scraped in more varied locations on both sharks and conspecifics. Lengths of scrapers and scrapees were positively correlated and fish scraping on sharks were larger than those scraping on conspecifics, suggesting that risk of predation may be a limiting factor. We show that pelagic teleosts prefer to scrape on sharks rather than conspecifics or other teleosts and suggest that this behaviour may have a positive impact on teleost fitness by reducing parasite loads. The decline of shark populations in the global ocean and the reduction in mean size of many species may limit these interactions, eroding possible fitness benefits associated with this behaviour, and consequently placing more pressure on already highly targeted and vulnerable species.

Turbine blade strike tests for evaluation and optimization of fish-friendly turbine

Widening the knowledge for fish injury and mortality mechanism caused by pressure, shear stress and blade strikes inside turbine was constructive for optimizing fish-friendly turbine design and turbine operation regime. A special designed test equipment was applied for simulating the rotating blade inside turbine passage, anesthetized fish (crucian carp) was placed on the route of the blade for strike tests. The fish length, blade thickness, strike angle, and strike speed were independent variables during the tests, the injury rate, degree of injury and mortality rate were analysed according to observation or anatomical examination after repeated tests. The injury types were classified into: imbalance, ribs broken, gill cover broken, internal hemorrhage and death. The injury severity was directly affected by strike speed under same blade thickness and fish length. The ratio of blade thickness and fish length also affect the injury rate. Multiple injuries can be observed on one fish after tests, and combined injuries are more likely to lead to death. In the fish-friendly turbine design, blade thickness in the leading edge and blade numbers shall be carefully considered and the rotational speed shall be calculated.

Evaluation of Age-Dependent Changes in the Coloration of Male Killifish Nothobranchius Guentheri Using New Photoprocessing Methods.

Simple SummaryThis paper proposes a new methodology for evaluating fish coloration, which allows us to identify differences in the intensity of coloration of specific areas of the body. Changes in fish coloration occur during growth and under the influence of environmental factors. Male fish belonging to the family Nothobranchius are characterized by extremely diverse coloration, depending on the age of the fish, environmental factors, and social hierarchical status. As the lifespan of this genus of fish is very short (12–14 months), studies on age-dependent changes are possible. In this study, we demonstrate correlations between the coloration of particular body zones of male Nothobranchius guentheri and age using new photofixation methods and image processing software. This methodology can be applied to other fish with unique coloration patterns, for example, family Cichlidae and order Cyprinodontiformes.Fish as model objects have found wide applications in biology and fundamental medicine and allow studies of behavioral and physiological responses to various environmental factors. Representatives of the genus Nothobranchius are one of the most convenient objects for such studies. Male fish belonging to the family Nothobranchiidae are characterized by extremely diverse coloration, which constantly changes, depending on the age of the fish, environmental factors, and social hierarchical status. These fish species are characterized by a short life cycle, which allows changes in coloration, an indicator of the ontogenesis stage, to be estimated. Existing methods of fish color assessments do not allow the intensity of coloration of particular body zones to be clearly differentiated. In the present study, we suggest a method of two-factor assessment of specific fish body zones using modified methods of photofixation and image processing software. We describe the protocol of the method and the results of its application to different-aged groups of male Nothobranchius guentheri. The coloration of selected areas (i.e., red spot on the gill cover (RSGC), black border on the caudal fin (BBCF), and white border on the dorsal fin (WBDF)) differed significantly according to the size and age of the fish (p < 0.05). The data obtained suggest that N. guentheri can be a model for studying aging by the intensity of body coloration in males.

Balancing between Artemia and microdiet usage for normal skeletal development in zebrafish (Danio rerio).

Targeting in zebrafish fast growth, high survival rates and improved reproductive performance has led over the last years in variable feeding regimes between different facilities. Despite its significance on fish function and welfare, normal skeletal development has rarely been evaluated in establishing the best feeding practices for zebrafish. The aim of this study was to establish a protocol for normal skeletal development, growth and survival of zebrafish larvae through live feed-to-microdiet transition at an appropriate rate. Four feeding regimes including feeding exclusively on Artemia nauplii (A) or dry microdiet (D), and feeding on both Artemia and microdiet at two different transition rates (slow (B) or fast (C)) were applied from 5 to 24 dpf (days post-fertilization). Results demonstrated a significant effect of feeding regimes on the incidence of skeletal abnormalities (gill cover, fins and vertebral column, p<.05) in zebrafish larvae. The A and B experimental groups presented the highest (88±3 and 84±17%, respectively), but the C and D the lowest (18±14 and 11±2%, respectively), rates of normal fish (fish without any abnormality). Similarly, growth rate was comparatively elevated in A and B groups. No significant differences were observed in fish survival between A, B and C groups. However, D group presented a significantly lower survival rate. To our knowledge, this is the first study to show that the live feed-to-microdiet transition rate influences larval growth, survival and abnormality rates in a non-homogenous pattern.

Evaluation of the effects of shear stress on crucian carps passing through turbines

Turbine-passed fish is inevitably exposed to one or more physical forces, such as fast pressure change, blade strikes and shear stress, which can lead to injury or fatalities. The injury mechanisms of these forces to fish and related criteria are necessary for fish-friendly-turbine design and optimization. The effects of shear stress on crucian carps are studied in a specially designed experimental system. Crucian carps are exposed to a submerged water jet at velocities ranging from 5m/s to 20m/s. High-speed, high-resolution digital videos of the exposure process was recorded for motion-tracking analysis. Fish sizes, flow directions and water jet velocities are key factors considered during the tests. Injury rate, degree of severity, site of trauma, and death rate of crucian carps after tests were studied according to repeated tests with different factors. Injuries such as shedding of fish scales, tearing of gill cover, laceration of eyeballs are observed at different water jet velocity and with different size of crucian carps. The injury mechanisms were concluded and design criteria for fish-friendly-turbine are proposed according to the study.

Opportunistic Bacteria Isolated from Trypauchen Vagina Fish that Infected with Protozoan from Iraq Marine Water

Opportunistic bacteria are microorganisms that have been isolated from a host suffering from stress as a result of parasitic infestation. In this study, two bacterial species, namely Aeromonas hydrophila and Rhizobium radiobacter, were isolated from Trypauchen vagina fish that were infected with Microsporidian parasite cysts from Iraqi marine waters. The infection were mass implanted in the internal and external skeletal muscles and under the gill cover. Many symptoms of bacterial infection were observed in the fish, including flatulence with haemorrhagic fluids, skin disscolouration blackening, separation of crusts, and deformities in the area. The two bacterial species were characterised morphologically and identified using Vitek II system (Biomerieux - USA). To control infection, antibiotic susceptibility was performed using six antibiotics: nalidixic acid, nitrofurantoin, gentamicin, streptomycin, tetracycline and amoxicillin, both bacteria showed high antibiotic resistance. Identification of opportunistic bacteria has a pivotal role in preventing stress factors in fish, treating infections and alleviating their impact on other aquatic organisms.

Ontogenic Development Of Gills In Pre To Post-Flexion Stages Larvae Of Himalayan Snow Trout Schizothorax Plagiostomus (Heckel)

The ontogenic development of gills, including gill arch, gill filaments, and gill rakers in pre to postflexion stages larvae of Himalayan Snow Trout Schizothorax plagiostomus were studied with the objectives that this study could serve as a base for further studies about the early embryonic development and organogenesis in various fish species inhabiting hill stream environments. To obtain the pre to post-flexion stages larvae, an artificial breeding experiment was conducted during September- October on the bank of snow-fed river Alaknanda by stripping method. Further, the development of the gill apparatus was studied histologically, using light microscopy. Hatchling takes place 124-130 hours after fertilization at the incubation temperature of 19-200C. On second dph (day post-hatching), gill arches, gill filaments, and branchiostegal membrane began to differentiate. By the third dph, blood channels were observed in gill filaments as well as in pseudobranch. Formation of secondary lamellae, branchial arteries, elongation of the gill cover, cartilaginous rod formation in the gill arch, and the branchiostegal membrane was discernible by fourth-fifth dph. Pillar cells and afferent and efferent arteries with RBCs in primary and secondary lamellae were observed on the seventh dph. Around the onset of exclusive exogenous feeding (twelfth dph), gill rakers and a group of pillar cells with blood channels were recognizable. Well-organized and functional gill structures with increased number and size of secondary gill lamellae were present among the fifteenth-seventeenth dph larvae. The general pattern of structural and functional development of both the natural site and laboratory-reared larvae was similar, except a large amount of mucous and clustered epithelial cells among the laboratory-reared larvae, which may be due to the physiological as well as environmental stress posed by the adverse physicochemical conditions.

Evolution of vertebrate gill covers via shifts in an ancient Pou3f3 enhancer

Whereas the gill chambers of jawless vertebrates open directly into the environment, jawed vertebrates evolved skeletal appendages that drive oxygenated water unidirectionally over the gills. A major anatomical difference between the two jawed vertebrate lineages is the presence of a single large gill cover in bony fishes versus separate covers for each gill chamber in cartilaginous fishes. Here, we find that these divergent patterns correlate with the pharyngeal arch expression of Pou3f3 orthologs. We identify a deeply conserved Pou3f3 arch enhancer present in humans through sharks but undetectable in jawless fish. Minor differences between the bony and cartilaginous fish enhancers account for their restricted versus pan-arch expression patterns. In zebrafish, mutation of Pou3f3 or the conserved enhancer disrupts gill cover formation, whereas ectopic pan-arch Pou3f3b expression generates ectopic skeletal elements resembling the multimeric covers of cartilaginous fishes. Emergence of this Pou3f3 arch enhancer >430 Mya and subsequent modifications may thus have contributed to the acquisition and diversification of gill covers and respiratory strategies during gnathostome evolution.

Survey on Some Ectoparasitosis in Moolgarda seheli from Suez Canal area, Egypt

This survey study has been applied to a total number of 200 Moolgarda seheli that collected randomly and seasonally from different areas of the Suez Canal at Ismailia Governorate. Infected fish showed no obvious clinical abnormalities except in case of heavy infestation with the ectoparasites, where some infested fishes revealed respiratory manifestation and surface swimming. Other fishes showed hemorrhagic areas on the gill cover, abdomen, and fins. The results of this survey revealed 2 phyla of economically important fish parasites which were three copepods (Lernanthropus species, Caligus lichiae, and Lepeophtheirus lichiae) isolated from the gills and one monogenean (Benedenia sekii ) isolated from the gill and the buccal cavity of all examined fish. The total prevalence of infestation was 31% where Lernanthropus, Caligus species and monogenea were 24.5%, 5% and13%; respectively while the mixed infestation between crustaceans with monogenea was 7%. The seasonal prevalence of ectoparasitic infestation was the highest in winter (67.5%) followed by spring then autumn and the lowest was summer (10%). The highest infestation ofLernanthropus species was recorded in the winter (50%) and the lowest in summer (10%). On other hand, the highest infestation of Caligus was highest in the spring (33.33%) and disappeared in the other seasons. The seasonal prevalence of monogenean infestation was the highest in the spring (43.33%) and disappeared in the autumn and summer. Molecular identification of Benedenia species using PCR analysis of the 28SrRNA(mix of C1/D2 pairs) generated larger sequence fragments which were amplicons at approx. 800 bp. The partial sequencing and phylogenetics of the positive sample revealed that our sample was closer related to Benedenia sekii followed by Benedenia sciaenae from Australia which were 97.9% and 97.2%; respectively. The histopathological changes of infested fish with ectoparasite revealed severe atrophy of the gill lamellae along with mechanical destruction, other fish revealed focal infiltrations with leukocytes, in addition to congestion of the gill arch.

Male Mate Preference for Female Coloration in a Cyprinid Fish, Puntius titteya.

In the traditional view of sexual selection, females are the choosier sex, and males of many species often develop exaggerated ornaments. Recently, however, the evolution and maintenance of female ornaments has also attracted significant attention. In the present study, we examined the function of a female ornament, i.e., red coloration of the area around gill cover, in the context of male mate preferences in the cyprinid fish Puntius titteya. The result of a dichotomous choice experiment showed that males preferred females with redder coloration. Together with the results of a previous study, these findings suggest that males and females in this species mutually select each other based on red coloration. In addition, females with higher color saturation spawned larger eggs. With supplementation of carotenoid-rich foods, females exhibited redder coloration and higher color saturation. These results imply that, by choosing females based on carotenoid-based coloration, males might obtain high quality mates and offspring.

Potential physical effects of suspended fine sediment on lotic macroinvertebrates

This study investigates the potential for physical damage caused by suspended fine sediment on gills of three macroinvertebrate species, Hydropsyche siltalai, Ephemera danica and Ecdyonurus venosus. Macroinvertebrate cadavers were exposed to three suspended sediment concentrations (control 3.5, low 83.7 and high 404.0 mg l−1) at two velocities (low 0.19 m s−1 and high 0.37 m s−1), for 6 h in a recirculating flume. Tracheal gill surfaces were subsequently examined for evidence of physical damage using scanning electron microscopy (SEM) images. Physical damage predominantly consisted of fine sediment coverage of gill surfaces, appearing as a deposited layer of sediment obscuring and potentially clogging the gill. For E. venosus, suspended sediment concentration influenced gill cover, but velocity had no significant effect. Coverage of H. siltalai gill surfaces increased significantly between low and high sediment concentrations but only at the higher flow velocity. Gill coverage of E. danica did not differ across any sediment concentration. The results were consistent with reported species sensitivities to fine sediment, despite the use of cadavers. However, we found limited evidence of physical abrasion as a direct physical effect of fine sediment under the experimental conditions used.

Kültür koşullarında Nil Tilapyası ve Melez Kırmızı Tilapya (Cichlidae, Oreochromis spp.)’da görülen gaz kabarcığı hastalığı

The supersaturated gases in water can cause gas bubble disease in fish and this can result in death. At the present study a case of gas bubble disease resulted from the gas supersaturation were investigated in Nile tilapia (Oreochromis niloticus) and hybrid red tilapia (Oreochromis sp.). Macroscopic and histological findings were obtained from the culture conditions to find more knowledge on this disease which is frequently seen in fish farming. Study was conducted as two steps in summer (August) and winter (December) months with experimentally cultured fish. According to clinical and histological examinations gas bubbles were found in the dorsal, pectoral and caudal fin, and petechial bleeding were found on pectoral, dorsal, caudal fins and the gill cover. The necropsy were revealed that the stomach and intestines were filled with gas bubbles and the gills were hemorrhagic. According to the histological findings hyperplasia and edema in the gills and hemorrhage in the liver and spleen were found, and gastrointestinal mucosa destruction was observed. The clinical findings were similar for both fishes except for a few special cases and it was observed that the effects of supersaturation were severe for both fish species.

Developmental tuning of mineralization drives morphological diversity of gill cover bones in sculpins and their relatives

The role of osteoblast placement in skeletal morphological variation is relatively well understood, but alternative developmental mechanisms affecting bone shape remain largely unknown. Specifically, very little attention has been paid to variation in later mineralization stages of intramembranous ossification as a driver of morphological diversity. We discover the occurrence of specific, sometimes large, regions of nonmineralized osteoid within bones that also contain mineralized tissue. We show through a variety of histological, molecular, and tomographic tests that this “extended” osteoid material is most likely nonmineralized bone matrix. This tissue type is a significant determinant of gill cover bone shape in the teleostean suborder Cottoidei. We demonstrate repeated evolution of extended osteoid in Cottoidei through ancestral state reconstruction and test for an association between extended osteoid variation and habitat differences among species. Through measurement of extended osteoid at various stages of gill cover development in species across the phylogeny, we gain insight into possible evolutionary developmental origins of the trait. We conclude that this fine‐tuned developmental regulation of bone matrix mineralization reflects heterochrony at multiple biological levels and is a novel mechanism for the evolution of diversity in skeletal morphology. This research lays the groundwork for a new model in which to study bone mineralization and evolutionary developmental processes, particularly as they may relate to adaptation during a prominent evolutionary radiation of fishes.