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Finfish Evolution, Anatomy & Physiology – Comprehensive Study Notes

Learning Objectives

  • Identify scenarios where veterinarians encounter aquatic patients

  • Recognize aquaculture’s role in global food security

  • Trace fish evolution from jawless forms to modern teleosts

  • Summarize key teleost adaptations (gills, swim bladder, lateral line, etc.)

  • Describe external & internal fish anatomy

  • Explain integument (skin, scales, chromatophores) and the lateral line

  • Understand otolith structure/function

  • Outline muscle types, locomotion mechanics, and skeletal organization

  • Detail general physiology: respiratory, circulatory, thermoregulation, digestion, renal function, gas bladder types

Veterinary Interaction With Aquatic Cases

  • Clinical contexts

    • Companion fish / ornamental aquaria

    • Public aquaria & zoo exhibits

    • Wildlife conservation & health monitoring (state & federal agencies)

    • Emerging environmental threats: temperature rise, pollution, invasive species

    • Aquaculture production units (finfish, crustaceans, molluscs, aquatic plants)

  • Unique professional skills

    • Water-quality & toxicology diagnostics (ammonia, nitrite, nitrate, pH, salinity, dissolved O$_2$)

    • Recognition/treatment of aquatic-specific pathogens & parasites

    • Bacteria, viruses, true fungi, oomycetes (water molds)

    • Monogeneans, crustacean ectoparasites, ciliates, myxozoans / myxosporeans

Aquaculture & Global Food Security

  • Human population growth (UN 2015 projection)

    • \text{2030: } 8.5\,\text{billion}

    • \text{2050: } 9.7\,\text{billion}

    • \text{2100: } 11.2\,\text{billion}

  • 2012 global animal-protein output

    • Seafood (harvest + farmed): \approx 120\,\text{MMT} \,(36\%)

    • Pork: 90\,\text{MMT} \,(26\%)

    • Poultry: 59\,\text{MMT} \,(16\%)

    • Beef/veal: 57\,\text{MMT} \,(16\%)

    • Others (lamb/mutton, misc.): 25\,\text{MMT} \,(7\%)

  • FAO definition: “Farming of aquatic organisms…with ownership & husbandry intervention (stocking, feeding, predator control, etc.)”

  • Fastest-growing food sector (>10\% per yr); already >30\% of all fish consumed

  • OIE prediction: by 2050 \; \tfrac12 of all animal protein eaten by humans will be aquaculture-derived

Evolutionary Overview

  • Early vertebrates

    • Jawless, fin-less, body covered in bony plates; dominated for \approx 100\,\text{my}

  • Major extinction \sim 400\,\text{mya} ➔ rise of sharks (Chondrichthyes) & bony fish (Osteichthyes)

  • Ray-finned fishes (Actinopterygii) diversification timeline (selected clades)

    • Herrings, tarpon, eels, salmonids

    • Cyprinids (carp), catfish, gars, arapaimas

    • Lophiiformes (anglerfish), Gadidae (cods)

    • Percomorpha (largest order; perch-like incl. tilapia, bass, cichlids)

  • Lobe-finned fishes (Sarcopterygii) ➔ tetrapods; modern genetics show unexpected similarity to “primitive” fishes

  • Gadidae family evolutionary split examples

    • Atlantic cod Gadus morhua (8.5\,\text{mya})

    • Pacific counterparts, hakes, pollock, burbot, etc.

Key Teleost Adaptations

  • Gills: high-surface lamellae for water–blood gas exchange

  • Backbone & segmented musculature for efficient undulation

  • Single-loop circulation: \text{Heart} \to \text{Gills} \to \text{Body} \to \text{Heart} (low-pressure systemic flow)

  • Swim bladder

    • Buoyancy control via CO$_2$ (and other gases) secretion/absorption

    • Resonating chamber (sound production/detection)

  • Lateral line system: neuromast hair cells detect vibration, pressure gradients

  • Operculum: bony gill cover that pumps water across gills; allows resting ventilation

  • Sense organs

    • Nares/olfactory rosettes (chemoreception); independent of mouth

    • Otolith inner-ear stones for balance, orientation & age determination (annuli)

External Anatomy & Fins

  • Body plan: fusiform with head, trunk (viscera), tail (locomotion)

  • Unpaired fins: dorsal (1–3), anal (1–2), caudal; provide stability

  • Paired fins: pectoral & pelvic; their relative position reflects evolutionary lineage (e.g.

    • Pelvic abdominal in salmonids \approx 145\,\text{mya}

    • Thoracic/jugular in perch-like recent forms \approx 50\,\text{mya})

  • Specialized appendages: barbels, elongated rostra, tubular snouts; feeding ecology indicators

Mouth/Snout Variants

  • Superior mouth (lower jaw projects): surface feeders

  • Tubular snout: suction feeders

  • Inferior mouth (snout overhangs): bottom dwellers

  • Extended upper jaw (billfish), terminal jaws, etc.

Fin Arrangement Examples

  • Gadidae: three dorsal + two anal fins (cod, haddock, pollock)

  • Elopomorpha superorder: tarpons, eels, bonefishes; share leptocephalus larva

Integument: Skin, Scales & Chromatophores

  • Multifunctional interface: protection, immunity, osmoregulation, excretion, limited respiration, sensory

  • Two layers

    • Epidermis: living cells, abundant mucous (goblet) cells; mucus = antimicrobial, osmotic barrier, hydrodynamics

    • Dermis: vascular, houses scales & chromatophores; structural collagen matrix

  • Scale types (cycloid, ctenoid, ganoid)

    • Growth rings (circuli/radii) allow age estimation

    • Lateral-line scales perforated ➔ neuromast canal exposure

  • Chromatophore classes

    • Melanophores (black-brown), lipophores/xanthophores (yellow-orange), iridophores (guanine crystals; iridescence)

Lateral Line System

  • Canal with neuromasts (hair-cell clusters) coursing mid-flank & into head canals

  • Functions

    • Detects low-frequency vibrations, pressure gradients

    • Schooling alignment, predator avoidance, prey localization, spatial mapping in turbid/dark water

Otoliths (“Earstones”)

  • Calcium-carbonate concretions (aragonite) in inner-ear chambers

    • Utriculus, sacculus, lagena

  • Provide inertial mass for acceleration & gravity sensing; secondary hearing role

  • Species-specific shape; annual rings (annuli) used in age/stock assessments

  • Forensic/archaeological/ecological value (microchemistry traces environmental history)

Muscle Types & Locomotion

  • White muscle: bulk, anaerobic burst swimming

  • Red muscle: narrow band along lateral line; rich myoglobin, aerobic sustained swimming

  • Pink muscle: intermediate, recruited at moderate speeds

  • Myomeres (W-shaped) separated by myosepta; sequential contraction creates body wave travelling tail-ward

  • Unpaired fins (esp. caudal) convert body wave into thrust; paired fins fine-tune manoeuvring, braking, hovering

Skeleton Highlights

  • Axial: cranium, vertebral column, ribs; cranium articulates mandibles, gill arches, opercula

  • Appendicular: pectoral & pelvic girdles; fin rays (lepidotrichia) support membranes

Respiratory & Circulatory Systems

  • Four-chamber serial heart (sinus venosus → atrium → ventricle → bulbus arteriosus); single circulation per beat

  • Ventral aorta → afferent branchial arteries (de-O$_2$) → gill lamellar capillaries → efferent branchial arteries → dorsal aorta (oxygenated)

  • Counter-current exchange maximizes O$2$ uptake & CO$2$ elimination

Thermoregulation

  • Most fishes poikilothermic; some lamnid sharks & tunas exhibit regional endothermy

    • Counter-current heat exchangers (retia mirabilia) conserve metabolic heat in red muscle block

    • Example bluefin tuna core \approx 73^\circ\text{F} while ambient water \approx 47^\circ\text{F}

Digestive System

  • Components: mouth → buccal cavity → pharynx → oesophagus → stomach (if present) → pyloric caeca → intestine → vent

  • Accessory organs: liver–pancreas complex, gall bladder, spleen

  • Morphological diversity correlates with diet

    • Carnivores: pronounced stomach, short intestine, few pyloric caeca (e.g., snook)

    • Herbivores/omnivores: reduced/absent stomach, elongated coiled intestine, gizzard-like modifications (e.g., carp)

  • Pyloric caeca (1–1000): secretory & absorptive functions

Renal / Excretory & Osmoregulation

  • Kidneys originally paired; variable fusion ➔ continuous organ along dorsal body wall

    • Anterior (pronephros/head kidney): haematopoietic, endocrine (interrenal, chromaffin)

    • Posterior (mesonephros/body kidney): nephrons with glomeruli, renal tubules, collecting ducts

  • Functions

    • Osmoregulation: marine fish excrete divalent ions, conserve water; freshwater fish excrete excess water, reabsorb ions

    • Nitrogenous waste (primarily NH$_3$) excreted via gills & kidneys

  • Urinary ducts may terminate in urinary bladder prior to vent

Swim (Gas) Bladder

  • Provides neutral buoyancy; secondary sound roles

  • Types

    1. Physostomous – pneumatic duct connects to alimentary canal; gas exchange via gulping/burping atmospheric air (e.g., salmonids, eels)

    2. Physoclistous – duct absent; gas secretion/resorption via gas gland + rete mirabile + oval window

  • Rete mirabile = intertwined arterial/venous capillaries; counter-current mechanism concentrates gases (mainly O$_2$)

  • Deep-water physoclists (e.g., Sebastes sp.) risk barotrauma if surfaced rapidly (gas expansion ➔ everted stomach/gills)

Water Quality & Pathogen Considerations (Clinical Relevance)

  • Critical parameters: temperature, dissolved O$2$, pH, total ammonia (\text{NH}3 + \text{NH}_4^+), nitrite, nitrate, hardness, salinity, chlorine/chloramine, heavy metals

  • Toxicological issues unique to aquatics (e.g., unionized ammonia toxicity increases with pH & T)

  • Common disease categories

    • Bacterial: Aeromonas, Vibrio, Edwardsiella, etc.

    • Viral: Koi herpesvirus, ISA virus, VHSV

    • Fungal/oomycete: Saprolegnia

    • Parasitic: Ichthyophthirius (Ich), Gyrodactylus/Dactylogyrus (monogeneans), crustacean lice (Argulus), Myxozoa (Whirling disease)

(End of Notes)