Publisher Summary This chapter describes the general anatomy of the gills in fish. The gills form a highly characteristic feature of fishes and their presence has a marked effect on the anatomy and functioning of the rest of the animal. A gill septum separates two adjacent gill pouches and a series of filaments is attached to its surface. In the most primitive groups, the septum forms a complete partition between the pharynx and the outer body wall. Its extension forms a flap-valve for the next posterior slit. In more advanced groups, there is a progressive reduction in the septum and the consequent freeing of the filaments at their tips. Filaments form the most distinctive respiratory structure of fish gills and are sometimes referred to as “primary lamellae.” In adult fish, the number of filaments does not increase so markedly as during the juvenile growth period, but there is a very significant increase in the length of each of them as the fish grows. The lamellae are the most important units of the gill system from the point of view of gas exchange. The rest of the basic anatomy is directed to providing a suitable support for these structures and to enable the water and blood to come into close proximity.

1 General Anatomy of the Gills

Life originated in anoxia, but many organisms came to depend upon oxygen for survival, independently evolving diverse respiratory systems for acquiring oxygen from the environment. Ambient oxygen tension (PO2) fluctuated through the ages in correlation with biodiversity and body size, enabling organisms to migrate from water to land and air and sometimes in the opposite direction. Habitat expansion compels the use of different gas exchangers, for example, skin, gills, tracheae, lungs, and their intermediate stages, that may coexist within the same species; coexistence may be temporally disjunct (e.g., larval gills vs. adult lungs) or simultaneous (e.g., skin, gills, and lungs in some salamanders). Disparate systems exhibit similar directions of adaptation: toward larger diffusion interfaces, thinner barriers, finer dynamic regulation, and reduced cost of breathing. Efficient respiratory gas exchange, coupled to downstream convective and diffusive resistances, comprise the "oxygen cascade"-step-down of PO2 that balances supply against toxicity. Here, we review the origin of oxygen homeostasis, a primal selection factor for all respiratory systems, which in turn function as gatekeepers of the cascade. Within an organism's lifespan, the respiratory apparatus adapts in various ways to upregulate oxygen uptake in hypoxia and restrict uptake in hyperoxia. In an evolutionary context, certain species also become adapted to environmental conditions or habitual organismic demands. We, therefore, survey the comparative anatomy and physiology of respiratory systems from invertebrates to vertebrates, water to air breathers, and terrestrial to aerial inhabitants. Through the evolutionary directions and variety of gas exchangers, their shared features and individual compromises may be appreciated.

Evolution of air breathing: oxygen homeostasis and the transitions from water to land and sky.

Publisher Summary This chapter presents an overview of the internal morphology of gills. Each gill arch skeleton is jointed with the posterior skull dorsally and with the copula ventrally. The septum contains nerves and blood vessels and bears the filaments. Two rows of filaments are generally inserted on each gill arch. This whole forms the so-called holobranchs. The mode of the insertion of filaments on the gill arch depends on the morphology of the septum and varies with the group of fish. In some groups (Chondrostei and the holostean Lepisosteus), certain arches—the hyoidean and the mandibular only—bear a single row of filaments, called “hemibranchs.” In other groups or species, these arches are completely devoid of filaments or absent, such as in Amia (Holostei), which has a mandibular but no hyoidean arch. The presence of lamellae on gill arches corresponds with the distribution of true aortic arches. The pattern of branchial arch organization is relatively constant among teleosts, but differs significantly in lower groups. In Myxinoidei, the variable number of gill slits correspond to a series of pouches connected by narrower tubes with the gut and body surface.

2 Gill Internal Morphology

A model describing passive accumulation of organic chemicals from the aqueous environment and contaminated food in fish is developed. This model considers both biological attributes of the fish and...

Modelling Bioaccumulation of Organic Pollutants in Fish with an Application to PCBs in Lake Ontario Salmonids

Over the past 20 years, a variety of models have been developed to simulate the bioconcentration of hydrophobic organic chemicals by fish. These models differ not only in the processes they address but also in the way a given process is described. Processes described by these models include chemical diffusion through the gill's interlamellar water, epithelium, and lamellar blood plasma; advective chemical transport to and from the gill by ventilation and perfusion, respectively; and internal chemical deposition by thermodynamic partitioning to lipid and other organic phases. This article reviews the construction and associated assumptions of 10 of the most widely cited fish bioconcentration models. These models are then compared with respect to their ability to predict observed uptake and elimination rates using a common database for those model parameters that they have in common. Statistical analyses of observed and predicted exchange rates reveal that rates predicted by these models can be calibrated almost equally well to observed data. This fact is independent of how well any given model is able to predict observed exchange rates without calibration. The importance of gill exchange models and how they might by improved are also discussed.

A review and comparison of models for predicting dynamic chemical bioconcentration in fish.

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.

S.V. Lewis

Papers

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

A scanning electron study microscope of the gills of the lamprey, Lampetra fluviatilis (L.)