Skin Deep: Microscopic Anatomy of Normal Finfish Integument

Diane G Elliott Scientist Emeritus

Western Fisheries Research Center U.S. Geological Survey, Seattle, Washington USA Fish Integument

• Separates and protects fish from its environment

– Also provides means through which most contacts with environment are made • Continuous with lining of all body openings

– Also covers the fins Fish Integument Has Many Functions publicdomainpictures.net • Some important functions include: – Protection – Communication and sensory perception – Locomotion – Respiration, ion regulation, and excretion

publicdomainpictures.net Form Follows Function • More than 33,000 fish species identified • Structure of integument of each species highly adapted to carry out various functions • Adaptations vary widely among species, but some structural similarities

publicdomainpictures.net publicdomainpictures.net publicdomainpictures.net General Fish Skin Structure

• As in other vertebrates, fish skin consists Idahoof State U two basic layers: epidermis and dermis

Drawing by Stewart Alcorn from Elliott 2000b Lamprey skin Epidermis

Dermis (scaleless fish)

Hypodermis From Elliott 2011b 100 µm Salmonid skin Epidermis wrapped around scales (scales in loose connective tissue of dermis) Dense connective tissue of dermis

100 µm Hypodermis From Elliott 2011b Epidermis Pacific lamprey (Lampetra tridentata) • Unlike land vertebrates, all epidermal layers in Idaho State U most fishes comprised of living cells From Elliott 2011b

– Epidermal thickness varies by Goldfish (Carassius auratus) species and body region, and can also vary by age and sex – Surface cells sloughed when dead and replenished from Basement membrane lower cell layers Chinook salmon (Oncorhynchus tshawytscha) – Epidermis separated from dermis by acellular basement membrane (basal lamina)

From Elliott 2011b Pacific lamprey (Lampetra tridentata) Epithelial Cell

• Basic cellular element of Idaho State U fish epidermis – Only element common in all species From Elliott 2011b – Also known by other Goldfish (Carassius auratus) names. Examples: Surface epithelial cell • Malpighian cell Epithelial cell

• Epidermal cell Basal epithelial cell • Filament-containing cell Basement membrane • Principal cell Chinook salmon (Oncorhynchus tshawytscha) • Keratinocyte • Polygonal cell • Mucous cell (jawless fishes) From Elliott 2011b Pacific lamprey (Lampetra tridentata) Epithelial Cell • Metabolically active throughout epidermal Idaho State U layers in most fishes

– Capable of mitotic division in From Elliott 2011b all layers in teleosts but not jawless fishes Goldfish (Carassius auratus) Surface epithelial cell – Generally smaller than most

other epidermal cell types Epithelial cell

Basal epithelial cell – Key element in wound repair Basement membrane via rapid migration from wound edges (Elliott 2000b, Chinook salmon (Oncorhynchus tshawytscha) 2011a) – Capable of phagocytic activity From Elliott 2011b Epithelial Cell • Shape of cell (ec) varies by location Idaho State U – Basal cells (bec) cuboidal to columnar – Surface cells (sec) often squamous (flattened) – Shape of nucleus roughly corresponds to shape of cell

Chinook salmon (Oncorhynchus tshawytscha)

From Elliott 2011b Epithelial Cell

• Microridges or micropapillae on exterior Idaho State U surface of teleost superficial epithelial cells • May help to hold mucous Chinook salmon (Oncorhynchus tshawytscha) secretions on skin surface • May provide some mechanical protection against trauma • Increase absorptive surface area of 10 µm skin From Elliott 2000b Mucous Cuticle • Mucous secretions produced largely by epithelial cells (in most species) form a protective cuticle on skin surfaceIdaho State U – Surface epithelial cell secretions in cuticle mixed with and modified by secretions from other secretory cell types – Avoidance of mechanical damage and special fixation (e.g. glutaraldehyde-alcian blue) often required to preserve cuticle

Blue gourami (Trichogaster trichopterus), PAS stain c =cuticle ep = epidermis arrows = scales lct = loose connective tissue ch = chromatophore

100 µm Photo: Elena Catap Photo: Elena Catap 10 µm From Elliott 2000b From Elliott 2000b Mucous Cuticle • Cuticle continuously sloughed and renewed Idaho State U to help keep surface clear of debris and bacteria – Thickness varies (<1-50 µm) by species and area 10 µm on body, and may be affected by other factors such as environment

Juvenile Chinook salmon (Oncorhynchus tshawytscha) a) Skin surface without fixation to preserve cuticle b) Skin surface with cuticle preserved (Glutaraldehyde-alcian blue fixative) 10 µm From Elliott 2011b Mucous Goblet Cell

• Unicellular exocrine gland Idaho State U common to most animal groups

– Reported in most fishes, gc including: • Jawless fishes: hagfish • Selachians (sharks, skates and

rays) Goldfish gc epidermis, methylene blue- • Relict bony fishes: dipnoans azure II stain (lungfish) and Polypteriformes (bichirs and reedfish) • Most teleosts Mucous Goblet Cell

Idaho State U • Goblet cells reported absent from: • Jawless fishes: lampreys • Relict bony fishes: Polydon (paddlefish) • Teleost fishes: Periophthalmus (mudskipper)

Pacific lamprey epidermis From Elliott 2011b

In lampreys, mucus is produced by epithelial cells (also called mucous cells) Mucous Goblet Cell • Most frequently recognized in middle to outer layers of epidermis Idaho State U • May differentiate from epithelial cells in lower epidermis • Differentiated goblet cells do not undergo further mitotic division • Goblet cells may enlarge as move toward epidermal surface and numbers of secretory vesicles increase • Nucleus and organelles often become displaced basally • Goblet cell secretions: mucous glycoproteins and other components, depending on species (most weakly basophilic or unstained in routine histological sections) Hagfish (Myxine sp.) Pacific lamprey ec epidermis gc From Elliott 2011b ec gc gc In lampreys,gc mucus is produced by epithelial cells (also called From Elliott 2011bmucous cells) Chinook salmon (Oncorhynchus tshawytscha) From Elliott 2000b Mucous Goblet Cell

• At epidermal surface, Idaho State U goblet cell emerges (usually between adjacent epithelial cells) • Apical goblet cell membrane ruptures to release cell contents, and

cell dies From Elliott 2011b 10 µm

• In some fishes such as Chinook salmon (Oncorhynchus tshawytscha); apical ends of goblet cells at salmonids, greatest epidermal surface (arrows) volume of epidermal secretions from goblet cells Mucous Goblet Cell • Goblet cell mucus has variety of functions Idaho State U – e.g. lubrication, chemical and physical protection, and possible regulatory functions • Goblet cell numbers affected by many factors within a species: – Body region (even specific area of epidermis covering a Goldfish (Carassius auratus) epidermis showing single scale) goblet cells (gc) (Methylene blue-azure II stain) – Fish sex – Season gc – Processes such as larval gc metamorphosis, sexual maturation or adaptation to gc seawater

– Environmental changes or gc exposure to irritants or other From Elliott stressors 2000b Club Cell

wikimedia.org Idaho State U • Secretory cell characteristic of certain taxa of ray-finned (actinopterygian) fishes. Present in:

– Polypteriformes (relict bony fishes) – Anguilliformes (eels) – Fishes of superorder Ostariophysi (Gonorhynchiformes, Cypriniformes, Characiformes, Siluriformes and Gymnotiformes)

• Superorder contains nearly 25% of all known fish species and 75% of all freshwater species Club Cell

Club cells (cc) in short-fin eel (Anguilla • Usually large and round to oval or wikimedia.orgaustralis)). club-shaped, with 1-2 centrally located nuclei with prominent nucleoli

– Generally in middle layers of epidermis (no openings to epidermal surface) – Club cells in eels have cytoplasmic secretory vacuoles (arrows); ostariophysan club cells do not

Ostariophysan club cells in goldfish (Carassius auratus)

cc cc

Photo: Barbara Nowak 10 µm From Elliott (2000b) Ostariophysan club Ostariophysan Club Cells cells in goldfish (Carassius auratus) • Contain alarm pheromone that iswikimedia.org released into water when epidermis cc is damaged cc – Alarm pheromone detected by other fish of same species, which perform H&E stain species-specific anti-predatory defensive behaviour – Club cells may be lost during spawning season in species with abrasive

spawning behaviour (e.g. fathead cc

minnow Pimephales promelas) cc – Club cell products may have additional functions methylene blue- azure II stain Club cells (cc) in short-fin eel (Anguilla australis) have secretory vacuoles Eel-Type Club Cells (arrows). Other abbreviations: ec = epithelial cell, gc = goblet mucous cell, bl = basal layer epitelial cells, lct = loose wikimedia.orgconnective tissue (dermis), sca = scale • Eel and polypterid club cells do not appear to be involved in alarm reaction • Bioactive substances identified in club cells suggest other functions in ostariophysans and non-ostariophysans: – Toxins – Anti-pathogenic agents – Specific factors that may be involved in epidermal cell regulation Photo: Barbara Nowak 10 µm From Elliott (2000b) Other Epidermal Cells • Serous cells: Secretory cells

reported in a variety of fish taxa Idaho State U From Elliott 2011b – Basally located nucleus and Chinook salmon acidophilic staining cytoplasm (Oncorhynchus tshawytscha); epidermal – Two types (serous goblet cells and secretory serous cell (s) that may be a serous sacciform cells); may only be goblet cell or sacciform distinguishable by electron microscopy cell • Serous goblet cells: Reported from some bony fish taxa – Usually less numerous than mucous goblet cells in species that have them – Vesicles contain basic proteins – Increased numbers of serous goblet cells associated with skin irritation Other Epidermal Cells 100 µm • Sacciform cells: Reported from some cartilaginous and bony fish Idaho State U taxa – Most mature cells open at skin surface by apical pore, but some do not open at epidermal surface – Sacciform cells in Gadiformes are distinctive: enormous and cyst-like Epidermis of pollock (Pollachius virens) showing enormous, cyst-like sacciform cells (sc) typical of – Possible functions of Gadiformes. Note peripheral nuclei of individual cells. Other abbreviations: predominantly proteinaceous s = solitary sensory cell secretions: Toxic/repellent gc = goblet mucous cell me = melanophore substances, regulatory peptides bl = basal lamina (basement membrane) or alarm pheromones Drawing by Mary Whitear From Elliott 2000b Other Epidermal Cells: Examples • Ionocytes (chloride cells): monovalent ion regulation • Epithelial sensory organs and supporting cells; solitary sensory cells. Examples: – Neuromasts (mechanoreceptors or electroreceptors) – Taste buds (chemoreceptors) • Paraneuronal cells (Merkel cells and others): receptosensory cells that release signal substances • Luminescent organs (photophores): Contain self-luminous cells or symbiotic luminous bacteria • Venom cells and supporting cells – Venom glands often associated with spines

Free neuromast in Ionocytes in epidermis of goldfish opercular (Carassius auratus) epidermis of a killifish (Fundulus heteroclitus). From Elliott 2000b Other Epidermal Cells • Leukocytes: Lymphocytes most Eosinophilic lymphocyte common, but macrophages and granulocyte Idaho State U granulocytes also observed • Increased abundance during healing of skin injury, exposure to infectious lymphocyte agents or other stressors

• Dendritic cells (Langerhans cells): H&E stain Antigen presenting cells Epidermis of goldfish (Carassius auratus) with skin infection of • Rodlet cells: (many teleosts) atypical Aeromonas salmonicida (no bacteria shown) • Pear-shaped, fibrillar “capsule” beneath plasma membrane, rodlet cells (arrows) acidophilic “rodlets” oriented longitudinally within membrane-bound cytoplasmic vesicles • May have defensive function; often more numerous in fish exposed to damage by parasites or other insults Methylene blue-azure II stain Other Epidermal Cells • Thread cell: Secretory cell in hagfish – Elongated oval cell with basal nucleus – Dense thread-like component in peripheral portions of cell; central mass containing fine granules or globules • “Thread” is complex biopolymer up to 60 cm long X 3 µm thick – Thread cell contents and mucus produced by other cells combine to form copious slime discharged by irritated hagfish (“slime eel”)

ec

ec Hagfish (Myxine sp.) thread tc cells (tc), goblet cell (gc) and tc gc epithelial (mucous) cells (ec) tc all produce mucous substances

From Elliott 2000b Other Epidermal Cells • Granular cell (lamprey only): Function(s) unknown – Located in middle to upper layers of epidermis (do not open at epidermal surface) – Large cell with centrally located nucleus, prominent nucleolus, numerous cytoplasmic granules, and appendages penetrating into deeper epidermis • Skein cell (lamprey): May provide mechanical support to epidermis – Elongate cells with basal part contacting basement membrane, and extending into upper epidermal layers – Often binucleate; slightly eosinophilic cytoplasm with twisted internal filaments

granular cell

Pacific granular cell lamprey (Lampetra tridentata) epidermis skein cell

H&E stain Methylene blue-azure II stain Dermis • Dermis of most fishes divided into two layers Idaho State U – Stratum spongiosum (stratum laxum): upper (outer) layer of loose collagenous connective tissue – Stratum compactum: lower layer of dense connective tissue consisting primarily of orthogonal collagen bands

Skin of steelhead trout (Oncorhynchus mykiss)

anterior posterior

Epidermis wrapped around scales (scales in stratum spongiosum of dermis)

Stratum compactum of dermis 100 µm From Elliott 2011b Hypodermis Dermis • Relative thickness of stratum spongiosum and stratum

compactum depends largely on presence or absence of Idaho State U scales – Separation between dermal layers may be indistinct in non-scaled areas – In lampreys and some primitive actinopterygians, dermis consists primarily of dense connective tissue

Skin of Epidermis Pacific lamprey (Lampetra tridentata) Dermis (dense connective tissue)

Hypodermis From Elliott 2011b 100 µm Dermis: Stratum Spongiosum • Contiguous with basement membrane; many of its finer fibers insert there (basement membrane is a dermal element)Idaho State U • Contains scales and a variety of vascular and neural elements • Cellular elements include: – Fibroblasts – Pigment cells – Leukocytes – Cells of scale synthesizing tissues

Skin of coho salmon (Oncorhynchus kisutch) Drawing by Stewart Alcorn from Elliott 2000b Dermis: Stratum Compactum • Always present from an early stage in ontogeny • Major component is collagen bundle, which forms a denseIdaho State U matrix above the hypodermis – Collagen fibers usually highly ordered in series of layers at right angles to each other – Dermal fibroblasts distributed between collagen fibers (appear as small darkly staining cells in mature fish) – Stratum compactum pierced at regular intervals by vertical columns of collagen Stratum compactum bearing nerves and blood vessels up to stratum spongiosum

Skin of yellow perch (Perca flavescens) From Elliott 2000a Hypodermis • Hypodermis (subcutis or subcutaneous layer) separates dermis from underlying skeletal muscleIdaho State U – Upper portion of hypodermis generally contains chromatophores (pigment cells), blood vessels and nerve bundles – Beneath are adipose cells, loose connective tissue and vessels.

Skin of a short-fin eel (Anguilla australis). Photo: Barbara Nowak From Elliott 2000a

Abbreviations: ep = epidermis ch = chromatophore lct = loose connective tissue dct = dense connective tissue hyp = hypopdermis mu = muscle 100 µm Specialized Dermal Elements: Chromatophores • Integumentary colors primarily dependent on chromatophores (pigment cells) and their patterns of arrangement Idaho State U – Chromatophores of fishes occur in dermis (stratum spongiosum) or hypodermis or both – In stratum spongiosum of non-scaled skin, chromatophores located in upper portion • In stratum spongiosum of scaled skin, chromatophores both above and beneath scales – Chromatophores may occur sporadically in epidermis

Chromatophores (in stratum spongiosum)

100 µm Chromatophores (in hypodermis) Chinook salmon (Oncorhynchus tshawytscha) Pacific lamprey (Lampetra tridentata) dermis and hypodermis. From Elliott 2011b Chromatophores

• Most chromatophores are dp dendritic, with multiple branched or Idaho State U unbranched processes • Melanophores are most common dendritic chromatophores in fish – Occur almost everywhere in skin where signs of dark shade n recognizable – Usually largest of dendritic chromatophores (up to 200 µm) – Uninucleate or binucleate – Chromatosomes (melanosomes) contain deposits of melanoprotein dp – Dark brown or black coloration makes scale 10 µm melanophores readily visible in routine Melanophore in fin of smooth dogfish shark (Mustelus canis). From Elliott 2000b. n = histological sections nucleus, dp = dendritic process containing melanosomes (brown pigment organelles) Chromatophores • Other common dendritic chromatophores Idaho State U (chromatosome color): – Erythrophore (red or yellowish) – Xanthophore (primarily yellowish) – Leucophore (colorless, light- reflective) • Iridophores (usually) are non- dendritic light-reflective chromatophores – Common in dermis wherever there are whitish or slivery areas of skin – Contain large crystalline platelets

that tend to form stacks in Common dendritic and non-dendritic cytoplasm (platelets composed chromatophores as viewed from above the plane of the skin of a flounder (Paralychthys sp.). From mainly of guanine) Elliott 2000b. Chromatophores • Rapid changes in hue and pattern of coloration primarily controlled by nervous and endocrine systems Idaho State U – Rapid changes effected by aggregation or dispersion of chromatosomes in dendritic chromatophores – Cytoskeletal systems involving microtubules, actin, and perhaps other elements involved in aggregation and dispersion of chromatosomes – Melanophores often play leading role in rapid color changes • Usually show highest degree of rapid aggregation and dispersion of chromatosomes • Morphological (slow) color changes effected by changes in number and size of chromatophores and amount of pigment

Pigment-dispersion response (left) and pigment-aggregation response of chromatosomes in a dendritic chromatophore. From Elliott 2000b. Specialized Dermal Elements: Scales

• Scales of fishes differ from those of other vertebrates Idaho State U – Scales of other vertebrates consist largely of keratinized epidermal tissue (ectodermal origin) – Fish scales are mineralized dermal structures (mesenchymal origin) • Scales are important component of dermal skeleton of fishes From Elliott • Scales evolved 2011b a) Placoid scale, spiny dogfish shark (Squalus acanthias). independently in b) Ganoid scale (transparency), actinopterygian (longnose gar, Lepisosteus osseus). cartilaginous and bony c) Elasmoid scale (cycloid type) from a teleost. fishes d) Elasmoid scale (ctenoid type) from a teleost. e) Scute (bony plate) from a teleost (armored catfish, Corydoras aeneus). Placoid Scale (Chondrichthyes) • Tooth-like scale (odontode) with basal plate anchored directly in upper dermis by collagen fibers (Sharpey’s fibers) Idaho State U • Spine or cusp projects backward, often through epidermis – Spine comprised of a cap or cone of dentine covered with a layer of hard, transparent enameloid – Spine encloses a pulp cavity; blood vessels, nerves and presumptive channels of the secondary vascular system enter cavity through one or more openings in basal plate • Placoid scales do not exhibit indefinite growth; replaced when worn out or lost Placoid scale (odontode) of chondrichthyan. Abbreviations: sf – Sharpey’s fibers b = basal plate ep = epidermis de = dentine ed = enameloid pu = pulp cavity From Elliott 2011b. Placoid Scale (Chondrichthyes) • Some placoid scales may be Idaho State U completely covered by epidermis – Epidermis may be damaged during sampling • Placoid scales are usually non- overlapping, except where they protect 100 µm

lateral line canal Fin tissue of adult female smooth dogfish shark (Mustelus canis), showing placoid scales beneath epidermis and remnants of epidermis (arrows). From Elliott 2011b. Ganoid Scale (Some Primitive Actinopterygii)

• Rigid plates (often rhomboidal or diamond-shaped)Idaho State U that may slightly overlap and articulate with each other by peg-and-socket joints at margins (often bound together by Sharpey’s fibers)

From Elliott 2000a

Histological section of slightly overlapping ganoid scales of a gar (Lepisoteidae) Ganoid Scale (Some Primitive Actinopterygii) • Bichirs (Polypteridae) have

ganoid scales closest to ancestral Idaho State U form): – Thick osseous basal plate lies beneath a stack of thin, incompletely mineralized collagenous layers (elasmodine) arranged in plywood-like pattern – Elasmodine covered by layer of Ganoid scale of a vascular dentine that is capped by polypterid. layer of enamel-like, Abbreviations: sf – Sharpey’s fibers hypermineralized ganoine b = basal plate vc = vascular canal – Scales penetrated by vascular el = elasmodine canals supplying exterior tissues de = dentine g = ganoine ep = epidermis From Elliott 2011b. Ganoid Scale (Some Primitive Actinopterygii) • Scales of gars (Lepisosteidae) have no dentine layer but still have a ganoine layer Idaho State U – Small tooth-like odontodes (denticles) may project from surface of scales of some bichirs and gars (may be transient in gars) • Dermal ossification of sturgeons (Acipenseridae) and paddlefish (Polydontidae) reduced to bony denticles without dentine or ganoine – Large articulated bucklers (scutes) and small denticles in sturgeon – Isolated vestigial denticles in paddlefish

From Elliott U.S. Army Corps of Engineers From Elliott 2011b 2000a Rows of bony plates on Atlantic sturgeon Scales of a gar (Lepisosteus osseus) (Acipenser oxyrhynchus oxyrhynchus) showing odontodes on scale surface Elasmoid Scale (Teleosts) • Lacks dentine and ganoine; usually reduced to relatively thin,

largely transparent, flexible structure comprised of: Idaho State U – Basal plate (thickest layer) composed of several layers of partially mineralized collagen (elasmodine) organized in a plywood-like pattern – External layer: thin, well-mineralized layer comprised of a network of interwoven collagen fibrils – Outer limiting layer: hyper-mineralized layer lacking collagen, deposited at scale surface in non-overlapped (posterior) region close to epidermis – Anchoring Elasmoid scale of a collagen fibers: teleost (cichlid). Abbreviations: Sparse collagen el = elasmodine fiber bundles ex – external layer arising from non- ci = circulus overlapped ol = outer limiting layer superficial surface af = anchoring collagen fiber of scale; anchor ep = epidermis mature scales From Elliott 2011b. anterior posterior Elasmoid Scale (a) • Most often overlapping (imbricated) Steelhead trout (Oncorhynchus mykiss) From Elliott 2011bIdaho State U (a) In (usually) pelagic species, both the scale and the covering epidermis protrude and overlap the epidermis that covers the scale behind (b) In some other fishes, the posterior edge of the scale Minnow (Phoxinus phoxinus) approaches the epidermis, From Elliott 2000a which forms only a slight indentation or tuck beneath the (c) scale (c) Certain fishes such as burbots (Lota) and immature freshwater eels (Anguilla) have mosaic, rather than overlapping, patterns of scalation (sca = scale) Short-fin eel (Anguilla australis). Photo by Barbara Nowak. From Elliott 2000b Elasmoid Scale (Teleosts) • Growth of elasmoid scales generally continues throughout life – Circuli mark successive stages of growth of scale Idaho State U – Most teleosts lose their scales at least once in their lifetime, but lost scales and overlying epidermis are usually quickly regenerated unless the integumental injury is severe Abbreviations: Juvenile Chinook salmon (Oncorhynchus tshawytscha) Dermis: epidermis and upper dermis. From Elliott 2011b lct = loose connective tissue sca = scale bp = basal plate (elasmodine) ex – external layer ol = outer limiting layer ci = circulus Arrowheads = cells involved in scale growth ch = chromatophore bm = basement membrane Epidermis: ec = epithelial cell bec = basal epithelial cell sec = surface epithelial cell gc = goblet cell 10 µm s = serous cell l = lymphocyte (a) Modified Scales • Placoid or elasmoid scales modified as “Toothbrush” of spinous scales, male fish spines. Examples: Idaho State U (b) sca = non-spinous scale spi = spinous scale – Tail “stinger” of sting rays (Dasyatidae) lct – loose connective tissue dct = dense connective tissue – Dorsal spines of spiny dogfish (Squalus) ep = epidermis and chimaeroids (Chimaera, Hydrolagus) – Knife-like spines at base of tail of surgeonfish (Acanthurus) 100 µm – Bony spines of porcupine fishes (c) (Diodontidae) that stand erect when fish inflates • Perforated/tubulated scales provide spi = spinous scale surface outlets for lateral line canal ep = epidermis • Scales modified to form bony plates or 100 µm encasements. Examples: seahorses Toothbrush leatherjacket filefish (Acanthaluteres vittiger). Drawing (a) (Syngnathidae), armored catfishes (e.g. from MuseumsVictoria.com.Au. Callichthyidae), trunkfishes (Ostraciidae) Photos (b) and (c) by Barbara Nowak (from Elliott 2000b). In fishes, beauty really is “skin deep”

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publicdomainpictures.net Literature Cited

Idaho State U • Elliott D.G. 2000a. Integumentary system: Gross functional anatomy. Pages 95-108 In G.K. Ostrander (ed.) The Laboratory Fish. Academic Press, London. • Elliott D.G. 2000b. Integumentary system: Microscopic functional anatomy. Pages 271-306 In G.K. Ostrander (ed.) The Laboratory Fish. Academic Press, London. • Elliott D.G. 2011a. The many functions of fish integument. Pages 471-475 In A.P. Farrell (ed.) Encyclopedia of Fish Physiology: From Genome to Environment. Academic Press, San Diego. • Elliott D.G. 2011b. Functional morphology of the integumentary system in fishes. Pages 476-488 In A.P. Farrell (ed.) Encyclopedia of Fish Physiology: From Genome to Environment. Academic Press, San Diego.