Gross structure and dimensions of the gill in an air-breathing estuarine goby,Pseudapocryptes lanceolatus

The gills of the air-breathing estuarine goby,Pseudapocryptes lanceolatus, are reduced owing to the development of a specialized organ of O₂ uptake from air. In the first gill arch, the filaments of the outer hemibranch are reduced to nearly one-half in comparison to those of its inner hemibranch. A smaller number of secondary lamellae per mm (27.6) occurring on one side of the gill filament reduces the gill surface area. A bilogarithmic plot of the gill area and the body weight indicates a curve with two significantly different components, one (b = 0.924) related to the fish weighing up to 6 g and the other (b = 0.405) to the fish weighing 8 g and above.     

Gill morphology and morphometry of the facultative air-breathing armoured catfish,Corydoras paleatus,in relation on aquatic respiration

The Neotropical armoured catfish Corydoras paleatus is a facultative air-breathing teleost commonly exported as ornamental fish. In this species, air breathing enables it to survive and inhabit freshwater environments with low oxygen levels. Therefore, it is important to analyse the gills from a morphological aspect and its dimensions in relation to body mass with reference to aquatic respiration. For that, the gills were analysed using a stereoscopic microscope for morphometric studies, and structural and ultrastructural studies were carried out to compare the four branchial arches. Furthermore, two immunohistochemical techniques were used to locate and identify the presence of a Na⁺/K⁺ pump. The characterization of the potential for cell proliferation of this organ was assessed using an anti-PCNA antibody. The results show that gills of C. paleatus present some characteristics related to its diet and lifestyle, such as the limited development of gill rakers and the abundance of taste buds. In addition, other special features associated with the environment and bimodal breathing were observed: scarce and absent mucous cells (MCs) in the gill filaments and branchial lamellae, respectively, and the localization of mitochondria-rich cells (MRCs) covering the basal third of the branchial lamellae, which reduces the gill respiratory area. A peculiar finding in the gill epithelium of this armoured catfish was the presence of mononuclear cells with sarcomeres similar to myoid cells, whose functional importance should be determined in future studies. Finally, in C. paleatus, the interlamellar space of gill filaments is an important site for cell turnover and ionoregulation; the latter function is also performed by the branchial lamellae.     

Fish gill morphology: inside out

In this short review of fish gill morphology we cover some basic gross anatomy as well as in some more detail the microscopic anatomy of the branchial epithelia from representatives of the major extant groups of fishes (Agnathans, Elasmobranchs, and Teleosts). The agnathan hagfishes have primitive gill pouches, while the lampreys have arch-like gills similar to the higher fishes. In the lampreys and elasmobranchs, the gill filaments are supported by a complete interbranchial septum and water exits via external branchial slits or pores. In contrast, the teleost interbranchial septum is much reduced, leaving the ends of the filaments unattached, and the multiple gill openings are replaced by the single caudal opening of the operculum. The basic functional unit of the gill is the filament, which supports rows of plate-like lamellae. The lamellae are designed for gas exchange with a large surface area and a thin epithelium surrounding a well-vascularized core of pillar cell capillaries. The lamellae are positioned for the blood flow to be counter-current to the water flow over the gills. Despite marked differences in the gross anatomy of the gill among the various groups, the cellular constituents of the epithelium are remarkably similar. The lamellar gas-exchange surface is covered by squamous pavement cells, while large, mitochondria-rich, ionocytes and mucocytes are found in greatest frequency in the filament epithelium. Demands for ionoregulation can often upset this balance. There has been much study of the structure and function of the branchial mitochondria-rich cells. These cells are generally characterized by a high mitochondrial density and an amplification of the basolateral membrane through folding or the presence of an intracellular tubular system. Morphological subtypes of MRCs as well as some methods of MRC detection are discussed.     

THE STRUCTURE OF FISH GILLS IN RELATION TO THEIR RESPIRATORY FUNCTION

1. The general structure of the gills of different fishes is compared and it is concluded that, though essentially the same, there are certain differences by which they can be recognized. Possible ways in which they may have evolved from one another are considered. 2. A detailed account is given of the structure of the secondary lamellae, where gaseous exchange takes place, and it is shown that two epithelial sheets are separated by a vascular axis mainly composed of pillar cells overlain by a basement membrane on each side. Blood pathways through the gills are discussed in relation to their respiratory function. 3. The embryonic development of gills is described and evidence regarding homo-logies of different structures, particularly the pillar cells, is reviewed. 4. The gills of fish having different modes of life show variations in (a) the number of arches, (b) the number and length of the gill filaments, and (c) the size and frequency of the secondary lamellae. Ways in which measurements of gill area may be carried out and some of the complications involved are reviewed and a summary given of measurements made for a wide variety of species. Measurements of the thickness of the water-blood barrier are also discussed; the more active fish generally have thinner water-blood barriers and larger gill areas. 5. The different mechanisms of gill ventilation are summarized and characteristics of gill resistance in elasmobranchs and teleosts are compared. Gas exchange is discussed in relation to available techniques and the current terminology and symbols, and to indicate the value of analogies between gill exchangers and systems studied by engineers. 6. It is outlined how studies of the functioning of gills during coughing, parasitic infection, and in polluted waters add to knowledge of their role in respiration.     

Bystrow’s Paradox – gills,fossils, and the fish‐to‐tetrapod transition

Schoch, R.R. and Witzmann, F. 2011. Bystrow’s Paradox – gills, fossils, and the fish‐to‐tetrapod transition. —Acta Zoologica (Stockholm) 92 : 251–265. The issue of which breathing mechanism was used by the earliest tetrapods is still unsolved. Recent discoveries of stem tetrapods suggest the presence of internal gills and fish‐like underwater breathing. The same osteological features were used by Bystrow to infer a salamander‐like breathing through external gills in temnospondyl amphibians. This apparent contradiction – here called Bystrow’s Paradox – is resolved by reviewing the primary fossil evidence and the anatomy of the two gill types in extant taxa. Rather unexpectedly, we find that internal gills were present in a range of early crown tetrapods (temnospondyls), based on the anatomy of gill lamellae and location of branchial arteries on the ventral side of gill arch elements (ceratobranchials). Although it remains to be clarified which components are homologous in external and internal gills, both gill types are likely to have been present in Palaeozoic tetrapods – internal gills in aquatic adults of some taxa, and external gills in the larvae of these taxa and in larvae of numerous forms with terrestrial adults, which resorbed the external gills after the larval phase. Future developmental studies will hopefully clarify which mechanistic pathways are involved in gill formation and how these might have evolved.     

Flagellate Cryptobia branchialis (Bodonida: Kinetoplastida), ectoparasite of tilapia from the Salton Sea

An infestation of young tilapia, Oreochromis mossambicus Peters, by the flagellate Cryptobia branchialiswas observed at the Salton Sea, California, in September, 1997. This is the first report of C. branchialis in a highly saline water-body (43 g l^–1). Ultrastructure of C. branchialis as well as its effect on the gills of tilapia were studied using the scanning and transmission electron microscopy. No direct effect of C. branchialis on the epithelial cells of fish gills was observed. However, alterations of gill general structure, such as deposition of copious mucus on the gill surface, swelling of filaments, reduction of respiratory lamellae and their transformation into short club-shaped structures were found in infected fish. This suggests mortality of young tilapia may arise from decreased gill function in response to Cryptobia infestation.     

鲻和鲮鳃丝的扫描电镜比较观察

对鲻（Mugil cephalus）和鲮（Cirrhina molitorella）的鳃丝表面结构进行了扫描电镜比较观察,结果表明,鲻鳃丝杆状部比鲮粗．鲻鳃小片高度比鲮低;两者鳃丝表面分泌孔洞口径和密度不同;鲻和鲮细胞外被不同,鲻细胞外被稀疏,鲮的则致密复杂;鳃小片细胞和鳃丝表皮细胞的表面形态存在差异,文章还描述了鳃丝表皮形态特异的细胞。     

Surface morphology and functions of pharyngeal structures in the larval lamprey Petromyzon marinus

To gain insight into the multiple functions of a complex biological structure, the morphology of the pharynx of the larva (ammocoete) of the lamprey Petromyzon marinus was investigated with scanning electron microscopy and histochemistry (PAS and Alcian blue). Features studied include the gills, the parabranchial chambers external to the gills, intrapharyngeal ciliary tracts, the ridged pharyngeal roof, the floor, and the intrapharyngeal taste buds. Significant findings are: (1) All (nonciliated) cells lining these structures are covered with microvilli or microridges. The pattern and packing density of these membrane features vary among different pharyngeal structures. The lumenar membranes of pharyngeal lining cells overlie a mucous prosecretion in the apical cytoplasm, suggesting that the microvilli/ridges on these membranes function to anchor mucus. (2) Patterns of microvilli/ridges on the gill respiratory lamellae differ among ammocoetes of different species. (3) Pharyngeal osmo-regulatory cells (“chloride cells”) could not be identified on the basis of the microvillus/ridge pattern. (4) Two types of ciliary tracts are present within the pharynx. One has tall (x = 13 μm) and densely packed cilia, whereas the other has shorter (x = 7 μm) and less densely packed ones. Because mucus covers both types of tracts their function appears to involve the transport of mucus. (5) Food particles were found on the lateral surfaces of the gill filaments and on the surfaces of the parabranchial chambers. It appears that goblet cells in the epithelia of these regions secrete mucus in which the particles are trapped.     

Gills of the medaka (Oryzias latipes): A scanning electron microscopy study

The general morphology and surface ultrastructure of the gills of adult and larvae medaka (Oryzias latipes) were studied in freshwater and seawater using scanning electron microscopy. The gills of all examined fish were structurally similar to those of other teleosts and consisted of four pairs of arches supporting (i) filaments bearing lamellae and (ii) rakers containing taste buds. Three cell types, specifically pavement cells, mitochondria‐rich cells (MRCs), and mucous cells, constituted the surface layer of the gill epithelium. Several distinctive characteristics of medaka gills were noted, including the presence of regularly distributed outgrowth on the lamellae, enlarged filament tips, the absence of microridges in most pavement cells in the filament and lamellae and the presence of MRCs in the arch at the filament base. A rapid mode of development was recorded in the gills of larval fish. At hatching, the larvae already had four arches with rudimentary filaments, rakers, and taste buds. The rudimentary lamellae appeared within 2 days after hatching. These results suggest the early involvement of larval gills in respiratory and osmoregulation activities. The responses of the macrostructures and microstructures of gills to seawater acclimation were similar in larvae and adult fish and included modification of the apical surface of MRCs, confirming the importance of these cells in osmoregulation. The potential roles of these peculiarities of the macrostructures and microstructures of medaka gills in the major functions of this organ, such as respiration and osmoregulation, are discussed.     

The mucous coat on gill lamellae of rainbow trout (Oncorhynchus mykiss)

The existence of a layer of mucus covering the gill lamellae of healthy rainbow trout (Oncorhynchus mykiss) was investigated. Using cryo-scanning electron microscopy, a smooth, undulating, thin layer was observed which completely covered gill filaments and lamellae, thereby obscuring epithelial microridges. After processing cryopreserved gill arches in glutaraldehyde for conventional scanning electron microscopy, the layer was no longer present and epithelial microridges were clearly visible. The identity of this layer was investigated using cryopreserved gills which were treated in one of two ways. First, gills were incubated with a rabbit antiserum to gill mucus, with normal rabbit serum, or with phosphate-buffered saline. Following fixation in glutaraldehyde and processing, only the gill tissue incubated with the mucus-specific antiserum was still covered with the smooth layer. The layer was also retained on the gills of fish anesthetized in a solution containing mucusspecific antiserum and then processes in glutaraldehyde for conventional scanning electron microscopy. The tenacious nature of the mucous layer was demonstrated by its stability following exposure to formalin and a cationic detergent. Second, the presence of this layer was confirmed on gill tissue which was cryopreserved, followed by freeze-substitution and vapor fixation, and then examined by transmission electron microscopy.     

Functional differentiation in the anterior gills of the aquatic air-breathing fish, <Emphasis Type="Italic">Trichogaster leeri</Emphasis>

The fish gill is a multifunctional organ responsible for gas exchange and ionic regulation. It is hypothesized that both morphological and functional differentiation can be found in the gills of the aquatic air-breathing fish, Trichogaster leeri. To test this, we used the air-breathing fish, Trichogaster leeri, to investigate various morphological/functional parameters. First, we evaluated the importance of performing the aquatic surface respiration behavior in T. leeri. A reduced survival rate was observed when fish were kept in the restrained cages in hypoxic conditions. On the gross anatomy of gills, we found evidence of both morphological and functional modification in the first and the second gills and are responsible for ionic regulation. There were large-bore arterioarterial shunts in the fourth gill arch. It is specialized for the transport of oxygenated blood and is less responsive to environmental stress. In addition, the anterior and the posterior gills differed in the Na⁺, K⁺-ATPase activity upon ionic stresses. That is, only the Na⁺, K⁺-ATPase activity of the anterior two gills was up-regulated significantly in the deionized water. Lastly, we found that the number of mitochondria-rich cells in the first and the second gills increased following ionic stress and no difference was found in the third and the fourth gills following such an exposure. These results supported the hypothesis that there are morphological and functional differences between anterior and posterior gill arches within the air-breathing Trichogaster leeri. In contrast, no significant difference was found among gills in gross anatomy, filament density and Na⁺, K⁺-ATPase activity in the non-air-breather, Barbodes schwanenfeldi.     

Functional morphology of the gills of the shortfin mako,Isurus oxyrinchus,a lamnid shark

This study examines the functional gill morphology of the shortfin mako, Isurus oxyrinchus, to determine the extent to which its gill structure is convergent with that of tunas for specializations required to increase gas exchange and withstand the forceful branchial flow induced by ram ventilation. Mako gill structure is also compared to that of the blue shark, Prionace glauca, an epipelagic species with lower metabolic requirements and a reduced dependence on fast, continuous swimming to ventilate the gills. The gill surface area of the mako is about one‐half that of a comparably sized tuna, but more than twice that of the blue shark and other nonlamnid shark species. Mako gills are also distinguished from those of other sharks by shorter diffusion distances and a more fully developed diagonal blood‐flow pattern through the gill lamellae, which is similar to that found in tunas. Although the mako lacks the filament and lamellar fusions of tunas and other ram‐ventilating teleosts, its gill filaments are stiffened by the elasmobranch interbranchial septum, and the lamellae appear to be stabilized by one to two vascular sacs that protrude from the lamellar surface and abut sacs of adjacent lamellae. Vasoactive agents and changes in vascular pressure potentially influence sac size, consequently effecting lamellar rigidity and both the volume and speed of water through the interlamellar channels. However, vascular sacs also occur in the blue shark, and no other structural elements of the mako gill appear specialized for ram ventilation. Rather, the basic elasmobranch gill design and pattern of branchial circulation are both conserved. Despite specializations that increase mako gill area and efficacy relative to other sharks, the basic features of the elasmobranch gill design appear to have limited selection for a larger gill surface area, and this may ultimately constrain mako aerobic performance in comparison to tunas. J. Morphol. 271:937–948, 2010. © 2010 Wiley‐Liss, Inc.     

Gill morphometrics in relation to gas transfer and ram ventilation in high‐energy demand teleosts: Scombrids and billfishes

This comparative study of the gill morphometrics in scombrids (tunas, bonitos, and mackerels) and billfishes (marlins, swordfish) examines features of gill design related to high rates of gas transfer and the high‐pressure branchial flow associated with fast, continuous swimming. Tunas have the largest relative gill surface areas of any fish group, and although the gill areas of non‐tuna scombrids and billfishes are smaller than those of tunas, they are also disproportionally larger than those of most other teleosts. The morphometric features contributing to the large gill surface areas of these high‐energy demand teleosts include: 1) a relative increase in the number and length of gill filaments that have, 2) a high lamellar frequency (i.e., the number of lamellae per length of filament), and 3) lamellae that are long and low in profile (height), which allows a greater number of filaments to be tightly packed into the branchial cavity. Augmentation of gill area through these morphometric changes represents a departure from the general mechanism of area enhancement utilized by most teleosts, which lengthen filaments and increase the size of the lamellae. The gill design of scombrids and billfishes reflects the combined requirements for ram ventilation and elevated energetic demands. The high lamellar frequencies and long lamellae increase branchial resistance to water flow which slows and streamlines the ram ventilatory stream. In general, scombrid and billfish gill surface areas correlate with metabolic requirements and this character may serve to predict the energetic demands of fish species for which direct measurement is not possible. The branching of the gill filaments documented for the swordfish in this study appears to increase its gill surface area above that of other billfishes and may allow it to penetrate oxygen‐poor waters at depth. J. Morphol. 2010. © 2009 Wiley‐Liss, Inc.     

Structural adaptations for ram ventilation: Gill fusions in scombrids and billfishes

For ram‐gill ventilators such as tunas and mackerels (family Scombridae) and billfishes (families Istiophoridae, Xiphiidae), fusions binding the gill lamellae and filaments prevent gill deformation by a fast and continuous ventilatory stream. This study examines the gills from 28 scombrid and seven billfish species in order to determine how factors such as body size, swimming speed, and the degree of dependence upon ram ventilation influence the site of occurrence and type of fusions. In the family Scombridae there is a progressive increase in the reliance on ram ventilation that correlates with the elaboration of gill fusions. This ranges from mackerels (tribe Scombrini), which only utilize ram ventilation at fast cruising speeds and lack gill fusions, to tunas (tribe Thunnini) of the genus Thunnus, which are obligate ram ventilators and have two distinct fusion types (one binding the gill lamellae and a second connecting the gill filaments). The billfishes appear to have independently evolved gill fusions that rival those of tunas in terms of structural complexity. Examination of a wide range of body sizes for some scombrids and billfishes shows that gill fusions begin to develop at lengths as small as 2.0 cm fork length. In addition to securing the spatial configuration of the gill sieve, gill fusions also appear to increase branchial resistance to slow the high‐speed current produced by ram ventilation to distribute flow evenly and optimally to the respiratory exchange surfaces. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Immunohistological classification of ionocytes in the external gills of larval Japanese black salamander,Hynobius nigrescens Stejneger

In this cytological and immunohistological study, we clarified the localization of the membrane transporters Na⁺, K⁺‐ATPase (NKA), vacuolar‐type H⁺‐ATPase (VHA), and epithelial sodium channel (ENaC) and distinguished ionocyte subtypes in the gill of the Japanese salamander (Hynobius nigrescens). In larvae (IY stages 43–65), NKA immunoreactivity was observed on the basolateral plasma membrane in more than 60% cells and less than 20% cells in the primary filaments and secondary lamellae of the external gills, respectively. VHA immunoreactivity was observed on the apical membrane of some epithelial cells in the secondary lamellae of the external gills. High ENaCα immunoreactivity was widely observed on the apical cell membrane of a population of squamous cells, presumably pavement cells (PVCs), and mitochondria‐rich cells (MRCs), in the primary filaments and secondary lamellae of the external gills. Using double immunofluorescence microscopy, epithelial cell types involved in ionic regulation were characterized and divided into three ionocyte types: NKA‐, NKA‐ and ENaC‐, and VHA‐positive cells. VHA‐immunoreactive cells as well as NKA‐positive cells were observed during IY stages 43–65 of the salamander larvae. During late stages of metamorphosis, NKA, VHA, and ENaCα immunoreactivities in the external gills decreased and finally disappeared during the completion of metamorphosis (IY stage 68). PVCs and MRCs in the external gills are probably involved in acid–base balance regulation and osmoregulation in urodele amphibian larvae. The results are discussed in relation to the ionocytes previously reported in fish gills and the frog skin epithelium. J. Morphol., 2011. © 2011Wiley‐Liss, Inc.     

黄斑篮子鱼和金钱鱼鳃的扫描电镜观察

对两种鲈形目鱼类黄斑篮子鱼(Siganus oramin)和金钱鱼(Scatophagus argus)的鳃结构进行扫描电镜观察。结果表明,黄斑篮子鱼和金钱鱼鳃的表面结构及微细结构与其他硬骨鱼类基本相似,鳃丝表面都具有规则或不规则分布的环形微嵴、沟、坑、孔等结构。黄斑篮子鱼的鳃片中部鳃丝表皮有大量凸起,而端部鳃丝表皮的凹凸程度明显较低,黄斑篮子鱼的鳃小片高度较金钱鱼鳃小片高。黄斑篮子鱼和金钱鱼鳃上皮的扁平上皮细胞、氯细胞和黏液细胞的形态结构及数量分布存在细微的差异。黄斑篮子鱼鳃片鳃丝的端部和中部表面有黏液细胞,金钱鱼鳃丝表面的黏液细胞很难观察到,与大多数淡水鱼类相似。黄斑篮子鱼鳃丝表面分布的氯细胞数量多于金钱鱼,这可能与两种鱼生活环境、生活习性的长期演变相关。     

Gross structure and dimensions of the gills in a hill-stream sisorid catfish,Glyptothorax pectinopterus

Gross structure and dimensions of the gills have been examined in a hill-stream sisorid catfish,Glyptothorax pectinopterus, which remains adhered to rocks by means of an adhesive organ developed on the ventral side of the thorax. The fish shows a greater weight-specific gill area and greater length of the gill filaments by comparison with other hill-stream fishes. Adaptation for life in a hill-stream habitat is shown by the presence of additional filaments on the gills and patches of specialised cells on the filament epithelium.     

Interspecific variations in the surface ultrastructure of the gills of freshwater mullets

SEM studies were made on the gills of freshwater mullets,Rhinomugil corsula andSicamugil cascasia, to correlate surface ultrastructure of various gill units with their probable functions. Two types of lamellated gill rakers of the former fish are suited for plankton feeding and the short, stumpy and transversely beaded gill rakers of the latter reflect the varied food and feeding habit of the fish.R. corsula has numerous mucous glands on the epithelium covering the gill arch and gill filaments,S. cascasia has fewer. In accordance with the differences in the density and distribution of the mucous glands, the microridged epithelial cells also show variations in their architectural plan. In both species the epithelium of the secondary lamellae is smooth, probably an adaptation for better gaseous exchange.     

Gill Morphometrics of the Lampreys Lampetra fluviatilis (L.) and Lampetra planeri (Bloch)

Morphometric measurements have been made on various gill components of different stages in the life cycle of the anadromous parasitic lamprey, Lampetra fluviatilis, and its nonparasitic derivative Lampetra planeri. The total gill area, expressed in terms of body weight, of both larval (1462–2717 mm² g^–1) and adult (1402–2337 mm² g^–1) L. fluviatilis are greater than those previously recorded in the rather meagre literature on lamprey gill measurements and are comparable with those found in the most active teleosts. The gills of the two Lampetra species are apparently identical in the larval stages and those of metamorphosing and adult L. planeri are similar to those of metamorphosing L. fluviatilis. Although the pharyngeal arrangement of lampreys differs greatly from that of teleosts, there are many features of the gills indicative of convergence between the two groups. Thus, in a given stage in the life cycle of lampreys, the secondary lamellae on either side of the filaments also alternate, become more widely spaced as the filament length increases and increase in area as the body weight becomes greater. Furthermore, the fractional cumulative increase in secondary lamellae area along a line following the presumed direction of water flow is also represented by a sigmoid curve. While at metamorphosis the pharynx becomes considerably modified to accommodate the change from a unidirectional to a tidal respiratory water flow, the total gill areas of the ammocoete are similar to those of metamorphosing stages which have attained adult characteristics. However, there are clearly differences in some of the components that influence and contribute towards the total gill area. Thus, in terms of body weight, the number and total length of the filaments and the total number of secondary lamellae, together with the number of secondary lamellae found on a given distance of filament, are greater in late metamorphosing stages, while the reverse is true for the average bilateral area of the secondary lamellae which is considerably greater in ammocoetes.     

Morphology and distribution of blood fluke eggs and associated pathology in the gills of cultured Pacific bluefin tuna, Thunnus orientalis

Infestations of blood flukes of the genus Cardicola have been observed in juvenile Pacific bluefin tuna (PBT) cultured in Japan. Infected fish harbor large numbers of parasite eggs in their gills. Although the link between blood fluke infection and juvenile mortality is not clear, accumulation of parasite eggs appears to be pathogenic to the fish. We investigated the origins, general morphology/distribution, and histopathology of these eggs in artificially produced 0 yr old PBT. Dead and live fish were sampled on several occasions from two culture facilities in Wakayama prefecture, Japan. The number of eggs in each gill filament was enumerated under a microscope. In addition, we estimated the total number of eggs by dissolving the gills in a weak NaOH solution. We observed two morphologically distinct egg types in the gill filaments, smaller, oval shaped eggs in the gill lamellae and larger, crescent shaped eggs that occurred primarily in the filamentary arteries. Based on the ITS2 sequence, the ovoid and crescent shaped eggs were identified as C. orientalis and C. opisthorchis, respectively. Eggs of the former species were more abundant (maximum: 6400 per filament) than the latter (maximum: 1400), but the number was highly variable among filaments. The eggs of the latter species were relatively evenly distributed among the filaments. In a heavily infected individual, we estimated a total of >4.5 million eggs were present in the gills on one side of the fish. The number of eggs from the two species was positively correlated to each other and the dead fish tended to harbor more eggs than the live fish. Histological observation revealed host responses around the eggs, including encapsulation by fibroblasts and nodule formation, as seen in response to other aporocotylid eggs. In addition, we observed widespread fusion of gill lamellae and blockage of the filamentary arteries in some instances. Our results provide information that can be used for routine diagnosis of Cardicola blood flukes in cultured tuna and suggest they represent a risk to juvenile PBT.