Fish Physiology BZO6T16

Syllabus topics (RTMNU)

• Digestive system: comparative anatomy and histology of alimentary canal and digestive glands with respect to diet
• Physiology of digestion: enzymatic digestion in herbivorous, carnivorous and omnivorous fishes; absorption and assimilation; hormonal regulation of feeding and digestion
• Excretory system: organs in fishes and their significance; evolutionary aspects
• Osmoregulation: freshwater, marine and euryhaline fishes; role of skin, gills, kidneys and gastrointestinal tract

Digestive system — comparative anatomy and histology in relation to diet

The fish alimentary canal: mouth → buccopharynx → oesophagus → stomach (absent in some) → intestine → rectum → anus (cloaca in elasmobranchs). Digestive glands: liver (often with gall bladder), pancreas (often diffuse, "hepatopancreas" in carps). Pyloric caeca (finger-like outgrowths at the start of the intestine) increase surface area in many carnivores and omnivores (absent in carps).

Histology of the gut wall: mucosa (epithelium — stratified with mucous cells in oesophagus, columnar with gastric glands in stomach, columnar absorptive cells with brush border and goblet cells in intestine), submucosa, muscularis (circular and longitudinal; striated in the oesophagus of many fish), serosa. Elasmobranchs have a spiral valve in the intestine to increase absorptive area.

Physiology of digestion

• Carnivores: strong gastric digestion — HCl and pepsin (protein to peptides, pH about 2–4), followed by pancreatic trypsin, chymotrypsin, carboxypeptidases, lipase and intestinal peptidases; low amylase activity; bile from the liver emulsifies fats.
• Herbivores: no acid stomach; digestion mainly in the long intestine at neutral/alkaline pH; high amylase, maltase and sucrase activity; mechanical grinding by pharyngeal teeth breaks plant cell walls; limited cellulase (from gut microbes in some species).
• Omnivores: balanced protease, amylase and lipase activity; enzyme levels adapt to diet.

Absorption and assimilation: amino acids and sugars absorbed by active transport across the intestinal epithelium into portal blood to the liver; fatty acids and monoglycerides re-formed into fats and lipoproteins (chylomicron-like particles); pyloric caeca and spiral valve help absorption. Absorbed nutrients are used for growth, energy (fish rely heavily on protein and lipids rather than carbohydrates) and stored as glycogen and fat in liver and muscle.

Hormonal regulation of feeding and digestion: appetite — stimulated by ghrelin (stomach), NPY and orexin (brain); reduced by leptin, CCK and CART. Digestion — gastrin (gastric acid), cholecystokinin (CCK) (pancreatic enzymes and gall bladder contraction), secretin (bicarbonate), somatostatin (inhibitory), insulin and glucagon (metabolism of absorbed nutrients); growth hormone and thyroid hormones promote growth.

Excretory system and its evolution in fishes

Fish are mostly ammonotelic — about 80–90% of nitrogen is excreted as ammonia by diffusion across the gills; some urea through gills and kidneys; elasmobranchs are ureotelic (retain urea for osmotic balance).

Organs: paired, elongated kidneys lying dorsally under the vertebral column — head kidney (anterior; mainly lymphoid/haemopoietic with interrenal and chromaffin tissue) and trunk kidney (excretory). Kidney type: pronephros (functional in larvae and some adult teleosts as head kidney) and mesonephros/opisthonephros (adult). Each nephron has a glomerulus (Malpighian body), neck, proximal tubule, (distal tubule in freshwater fish) and collecting duct leading to the mesonephric (archinephric) duct, urinary bladder and urinary pore. Other excretory routes: gills, skin, gut, liver (bile).

Evolutionary aspects: the vertebrate kidney is thought to have evolved in freshwater (glomerulus to remove excess water). Freshwater teleosts have large, numerous glomeruli and distal tubules (dilute urine). Marine teleosts show reduced glomeruli, and some (e.g. toadfish Opsanus, pipefish, seahorse) are aglomerular — urine formed only by tubular secretion to conserve water. Elasmobranchs have large glomeruli and complex tubules that reabsorb urea, plus a rectal gland for NaCl excretion.

Osmoregulation in fishes

Marine elasmobranchs retain urea and TMAO (trimethylamine oxide — protects proteins from urea) in blood, becoming slightly hyperosmotic to sea water, so water enters slowly; excess NaCl removed by the rectal gland.

Euryhaline fishes (salmon, eel, tilapia, Mugil) move between fresh and salt water: they switch the direction of ion transport in gill ionocytes, start or stop drinking, and change kidney filtration. Hormones: prolactin is the freshwater-adapting hormone (reduces permeability, retains Na⁺); cortisol, growth hormone and IGF-1 are seawater-adapting (increase Na⁺/K⁺-ATPase and chloride cells); stanniocalcin and urotensins also help. Role of tissues: gills — main site of ion exchange; kidney — water and divalent ion balance; gut — water absorption in sea; skin — barrier and in larvae ion transport; urinary bladder — further ion reabsorption.

Likely exam questions

1. (Long, 8 marks) Compare the alimentary canal of carnivorous, herbivorous and omnivorous fishes with reference to diet.
2. (Long, 8 marks) Explain osmoregulation in freshwater and marine fishes.
3. (Short, 4 marks) What are pyloric caeca? Give their function.
4. (Short, 4 marks) Write a note on aglomerular kidneys.
5. (Short, 4 marks) How do euryhaline fishes adjust to changes in salinity?

Syllabus topics (RTMNU)

• Structure and functional organisation of fish heart: anatomy and histology; chambers; conus arteriosus and bulbus arteriosus; cardiac physiology
• Blood circulation in fishes: single circulation; pathway; countercurrent exchange and adaptations
• Branchial respiration: structure of gills; mechanism of respiration and gas exchange
• Accessory respiratory organs: cutaneous, buccopharyngeal, intestinal, air bladder, labyrinth organ and dendriform organ

Structure and functional organisation of the fish heart

The fish heart is a venous heart — it receives only deoxygenated blood and pumps it to the gills. It lies in the pericardial cavity ventral to the pharynx, behind the gills, and consists of chambers arranged in series:

1. Sinus venosus — thin-walled sac receiving blood from the ducts of Cuvier (common cardinals) and hepatic veins; contains the pacemaker.
2. Atrium (auricle) — single, thin-walled, large chamber.
3. Ventricle — thick, muscular main pump; myocardium may be fully spongy (trabecular, supplied by venous blood in the lumen) or have an outer compact layer with coronary vessels (active fishes like tuna).
4. Conus arteriosus (in elasmobranchs and primitive bony fishes) — contractile, with cardiac muscle and several rows of semilunar valves; or bulbus arteriosus (in teleosts) — non-contractile, elastic thickening of the ventral aorta made of smooth muscle and elastic tissue, with no valves inside (only one pair at its base).

Valves (sinu-atrial, atrio-ventricular, conal/bulbar) prevent backflow. Histology: epicardium, myocardium (cardiac muscle) and endocardium; bulbus rich in elastin.

Cardiac physiology: myogenic heart — impulse begins in pacemaker tissue at the sinu-atrial junction, spreads to atrium and ventricle; heart rate about 20–100 beats per minute, varying with temperature and activity; controlled by the vagus nerve (inhibitory, cholinergic) and by adrenaline/noradrenaline (excitatory) — elasmobranch hearts receive only vagal fibres, while teleosts also get adrenergic fibres through the vagosympathetic trunk; cardiac output increased mainly by stroke volume (Frank–Starling). The bulbus/conus acts as a windkessel, smoothing pulsatile flow and protecting delicate gill capillaries from pressure peaks.

Blood circulation in fishes

Fishes have single circulation — blood passes through the heart only once per complete circuit:

Heart (sinus venosus → atrium → ventricle → conus/bulbus) → ventral aorta → afferent branchial arteries → gill capillaries (oxygenation) → efferent branchial arteries → dorsal aorta → body organs → veins (anterior and posterior cardinals, hepatic portal, renal portal) → ducts of Cuvier → sinus venosus.

Blood pressure falls after the gills, so systemic circulation is at low pressure — a limitation; fish compensate by low metabolic demands, high capillary density in active muscles, and accessory pumps (caudal heart in hagfish). Lungfishes show a partially double circulation (pulmonary vein, partially divided atrium).

Countercurrent exchange and adaptations: in the gill lamellae blood flows opposite to the flow of water, so a diffusion gradient for O₂ is maintained along the whole lamella — fish can extract up to about 80% of the O₂ in water (versus about 50% in concurrent flow). Countercurrent heat exchangers (rete mirabile) in tuna and lamnid sharks keep red muscles warm (regional endothermy); rete mirabile in the swim bladder (gas gland) concentrates O₂; red blood cells with high haemoglobin in active fish; Root and Bohr effects help unload O₂; Antarctic icefish have no haemoglobin but large hearts and blood volume.

Branchial respiration — structure of gills and mechanism

Structure: teleosts have four pairs of gills on each side in the branchial chambers, covered by a bony operculum. Each gill has a bony/cartilaginous gill arch bearing gill rakers on the inner side (filter food, protect filaments) and two rows of gill filaments (primary lamellae, a hemibranch each) on the outer side. Each filament bears numerous thin, plate-like secondary lamellae — the actual respiratory surface — made of a thin epithelium over pillar cells enclosing blood channels (diffusion distance about 1–10 µm). Elasmobranchs have 5–7 pairs of gill slits with interbranchial septa and no operculum.

Mechanism of ventilation (teleosts) — double pump:

1. Buccal pump: mouth opens, buccal floor lowers → pressure falls, water enters mouth (opercular valves closed).
2. Opercular (suction) pump: opercula swing outward, opercular cavity enlarges → pressure falls below buccal pressure, water drawn across gills.
3. Mouth closes (oral valves prevent backflow), buccal floor rises → water forced over gills; opercula move in and the opercular valve (branchiostegal membrane) opens → water exits.

The pressure in the buccal cavity is kept slightly higher than in the opercular cavity throughout, so water flows almost continuously and unidirectionally over the gills. Fast swimmers (tuna, mackerel, some sharks) use ram ventilation — swimming with mouth open.

Gas exchange: O₂ diffuses from water into blood of secondary lamellae and CO₂ out by countercurrent flow; water holds about 1/30 of the O₂ of air and is dense and viscous, so a large surface, thin barrier and continuous flow are vital. Gills also excrete ammonia and regulate ions.

Accessory respiratory organs and their adaptive significance

Fishes living in stagnant, warm, oxygen-poor waters or that come out of water (air-breathing fishes, common in India) have accessory organs to breathe air:

Adaptive significance: survival in oxygen-deficient, warm, swampy waters and during drought (aestivation of lungfish in mud cocoons); ability to move over land (Anabas, mudskippers, Clarias); some are obligatory air-breathers and drown if denied air (Anabas, Channa, lungfish). Such fishes are hardy and suited to aquaculture in derelict waters; evolutionary link to the origin of tetrapod lungs.

Likely exam questions

1. (Long, 8 marks) Describe the structure of the teleost heart and the course of blood circulation in fishes.
2. (Long, 8 marks) Describe the accessory respiratory organs of fishes and their adaptive significance.
3. (Short, 4 marks) Distinguish between conus arteriosus and bulbus arteriosus.
4. (Short, 4 marks) Explain the countercurrent principle in fish gills.
5. (Short, 4 marks) Describe the mechanism of branchial respiration in a teleost.

Syllabus topics (RTMNU)

• Nervous system: organisation of CNS in fishes — brain and spinal cord
• Peripheral and autonomic nervous system: cranial and spinal nerves; sympathetic and parasympathetic systems
• Neuroendocrine regulation: hypothalamo–pituitary axis
• Endocrine system: pituitary, thyroid, interrenal tissue, chromaffin tissue and corpuscles of Stannius

Central nervous system of fishes — brain and spinal cord

The fish brain is small relative to body size, enclosed in the cranium, and divided into:

• Forebrain (prosencephalon): telencephalon — olfactory bulbs, olfactory tracts and cerebral hemispheres (mainly olfactory in function; large in sharks with a keen sense of smell; teleost pallium is everted); diencephalon — thalamus, epithalamus with the pineal organ (photosensitive, melatonin), and hypothalamus with the infundibulum and pituitary, saccus vasculosus (in deep-sea fishes and sharks).
• Midbrain (mesencephalon): large paired optic lobes (tectum) — main visual and integration centre in teleosts; tegmentum.
• Hindbrain (rhombencephalon): cerebellum (well developed — coordination of swimming and balance; huge in active fishes and electric fishes), and medulla oblongata — centres for respiration, heart rate, osmoregulation; vagal and facial lobes (taste) prominent in carps and catfishes; large Mauthner cells for the rapid escape (C-start) reflex.

Spinal cord: a cylindrical tube in the neural canal of the vertebral column, extending almost to the tail; central canal surrounded by grey matter (H-shaped) and outer white matter; gives rise to segmental spinal nerves; caudal neurosecretory system (urophysis) at its posterior end secretes urotensins (osmoregulation). Meninges: a single primitive meninx.

Peripheral and autonomic nervous system

Cranial nerves: fishes have 10 pairs (I olfactory, II optic, III oculomotor, IV trochlear, V trigeminal, VI abducens, VII facial, VIII auditory/statoacoustic, IX glossopharyngeal, X vagus) plus terminal nerve (0); lateral line nerves (anterior and posterior) are associated with VII and X. The vagus is large, supplying gills, heart, gut and the lateral line.

Spinal nerves: one pair per body segment; each arises by a dorsal (sensory, with ganglion) and ventral (motor) root, which join and divide into dorsal, ventral and visceral branches supplying skin, muscles and fins.

Autonomic nervous system: controls visceral organs.

• Elasmobranchs: sympathetic ganglia are segmental but not joined into chains; no sympathetic supply to the head or skin; heart has only vagal (parasympathetic) control.
• Teleosts: paired sympathetic chains of ganglia extend into the head, linked to cranial nerves; supply blood vessels, chromatophores (colour change), swim bladder, gut, heart (via vagosympathetic trunk) — mainly adrenergic, causing vasoconstriction, pigment aggregation and increased heart rate.
• Parasympathetic: cranial outflow mainly through the oculomotor (eye — pupil) and vagus (heart slowing, gut motility, gills) — cholinergic. No sacral parasympathetic outflow in most fishes (spinal autonomic supply to the hind gut is mainly sympathetic).

Neuroendocrine regulation — hypothalamo–pituitary axis

The hypothalamus links the nervous and endocrine systems. In teleosts there is no hypophyseal portal system (in most species); instead, hypothalamic neurosecretory fibres directly innervate the cells of the adenohypophysis, releasing hormones close to target cells (elasmobranchs and some primitive fishes have a portal system).

• Hypothalamic hormones: GnRH (gonadotropins), CRH (ACTH), TRH (TSH, also prolactin), GHRH and somatostatin (growth hormone), dopamine (inhibits gonadotropin release in cyprinids and prolactin release), MCH, neuropeptide Y.
• Neurohypophyseal hormones (made in preoptic nucleus, stored in neurohypophysis): arginine vasotocin (water/ion balance, blood pressure, spawning behaviour) and isotocin (smooth muscle, reproductive behaviour).
• Roles of the axis: growth (GH–IGF), reproduction (HPG axis), stress and osmoregulation (HPI axis — cortisol), metabolism (HPT axis), colour change (MSH, MCH), smoltification in salmon.

Endocrine system of fishes

• Pituitary gland: lies below the diencephalon, attached by the infundibulum. Neurohypophysis (pars nervosa — interdigitates with the adenohypophysis) and adenohypophysis — rostral pars distalis (prolactin, ACTH), proximal pars distalis (GH, TSH, FSH-like GTH-I and LH-like GTH-II), and pars intermedia (MSH, somatolactin). Functions: growth, reproduction (gonadotropins), freshwater adaptation (prolactin), stress (ACTH), colour change (MSH). Carp pituitary extract is used for induced breeding (hypophysation) in aquaculture.
• Thyroid gland: in most teleosts not a compact gland but scattered follicles along the ventral aorta and afferent branchial arteries (sometimes in kidney, spleen); compact in elasmobranchs and some teleosts (parrotfish). Secretes thyroxine (T₄) and T₃ — growth, metamorphosis (e.g. flatfish eye migration, eel larvae), smoltification, metabolism, pigmentation, reproduction.
• Interrenal tissue (homologous to the adrenal cortex): cells embedded in the head kidney around the posterior cardinal veins (separate interrenal body in elasmobranchs); secretes cortisol (teleosts; 1α-hydroxycorticosterone in elasmobranchs) — stress response, glucose mobilisation, seawater adaptation (chloride cells), immune suppression.
• Chromaffin tissue (homologous to the adrenal medulla): cells in the head kidney near the interrenal and along cardinal veins (separate segmental bodies in elasmobranchs); secrete adrenaline and noradrenaline — fight-or-flight: increased heart rate, gill blood flow, glycogenolysis, O₂ uptake.
• Corpuscles of Stannius: small, whitish, round bodies on or embedded in the kidneys of holostean and teleost fishes (unique to them). Secrete stanniocalcin (hypocalcin), which lowers blood calcium by reducing Ca²⁺ uptake from water at the gills and gut — important in sea water rich in calcium; removal (stanniectomy) causes hypercalcaemia.
• Others: ultimobranchial gland (calcitonin), endocrine pancreas (Brockmann bodies — insulin, glucagon), urophysis (urotensins), pineal (melatonin), gonads (sex steroids), gut hormones.

Likely exam questions

1. (Long, 8 marks) Describe the structure of the brain of a fish and the functions of its parts.
2. (Long, 8 marks) Describe the endocrine glands of fishes: pituitary, thyroid, interrenal, chromaffin tissue and corpuscles of Stannius.
3. (Short, 4 marks) Write a note on the hypothalamo-pituitary axis in teleosts.
4. (Short, 4 marks) What are corpuscles of Stannius?
5. (Short, 4 marks) Name the cranial nerves of fishes.

Syllabus topics (RTMNU)

• Male reproductive system: structure and functions of testes; spermatogenesis
• Female reproductive system: structure and functions of ovaries; vitellogenesis
• Neuroendocrine control of reproduction: hypothalamo–pituitary–gonadal (HPG) axis
• Modes of reproduction: oviparity, viviparity and ovoviviparity; sex reversal and factors influencing it

Male reproductive system — testes and spermatogenesis

Structure: paired, elongated, creamy-white testes suspended by the mesorchium in the body cavity, dorsal to the gut; size increases greatly in the breeding season (gonadosomatic index rises). Sperm leave through a sperm duct (vas deferens) opening at the urogenital papilla (teleosts — separate from the kidney duct) or through the kidney duct (elasmobranchs, where males have claspers — modified pelvic fins for internal fertilisation). Live-bearing teleosts (guppy) have a gonopodium (modified anal fin).

Two types of teleost testis: lobular (most teleosts — lobules containing cysts of germ cells; sperm released into the lobule lumen) and tubular (atherinomorphs such as guppy — germ cells restricted to the distal end of tubules). Spermatogenesis is cystic: each cyst is formed by Sertoli cells enclosing a clone of germ cells all at the same stage; Leydig (interstitial) cells between lobules secrete androgens — mainly 11-ketotestosterone (the main fish androgen) and testosterone.

Spermatogenesis: spermatogonia (mitosis) → primary spermatocytes → meiosis I → secondary spermatocytes → meiosis II → spermatids → spermiogenesis (formation of head, midpiece and flagellum; most teleost sperm lack an acrosome — they enter the egg through the micropyle) → spermatozoa. Spermiation (release) and milt hydration are stimulated by LH and the progestin 17α,20β-dihydroxyprogesterone (DHP). Sperm are immotile in seminal plasma and are activated by the change in osmolality on release into water, staying motile for only seconds to a few minutes.

Female reproductive system — ovaries and vitellogenesis

Structure: paired, sac-like ovaries (fused in some), suspended by the mesovarium. Most teleosts have cystovarian ovaries — a hollow ovary whose lumen continues into a short oviduct, so eggs are shed into the lumen and pass directly out through the genital pore. Salmonids and eels have gymnovarian ovaries — eggs released into the body cavity and leave via genital pores (as in elasmobranchs, which have true oviducts with a shell gland). Ovigerous lamellae bear oocytes at different stages, each enclosed by a follicle (granulosa + theca layers).

Stages of oocyte development: chromatin-nucleolus → perinucleolar (primary growth) → cortical alveolus stage → vitellogenic (yolk accumulation; secondary growth) → maturation (germinal vesicle migration and breakdown, completion of meiosis I) → ovulation; mature egg has a thick chorion (zona radiata) with a micropyle.

Vitellogenesis — formation and deposition of yolk:

1. Pituitary FSH (GTH-I) stimulates the follicle to produce oestradiol-17β (theca makes testosterone, granulosa converts it by aromatase).
2. Oestradiol acts on the liver, which synthesises vitellogenin (Vtg) — a large phospholipo-glycoprotein carrying lipids, phosphorus and calcium.
3. Vtg is secreted into blood and taken up by growing oocytes by receptor-mediated endocytosis.
4. Inside the oocyte it is cleaved into yolk proteins — lipovitellin and phosvitin — stored in yolk granules/globules; the oocyte grows enormously.

Vtg in the blood of male fish is used as a biomarker of exposure to oestrogenic pollutants (endocrine disruptors).

Neuroendocrine control of reproduction — HPG axis

• Environmental cues (photoperiod, temperature, monsoon rains/flooding, water current, lunar cycle, presence of mates and substrate) are sensed via the brain and pineal (melatonin).
• Hypothalamus secretes GnRH (several forms, e.g. salmon GnRH, chicken GnRH-II) that stimulates gonadotropin release; in cyprinids dopamine inhibits LH release (hence induced breeding uses GnRH analogues plus a dopamine blocker such as domperidone — e.g. Ovaprim). Kisspeptin acts upstream of GnRH.
• Pituitary gonadotropins: FSH (GTH-I) — gametogenesis, vitellogenesis and spermatogenesis; LH (GTH-II) — final oocyte maturation, ovulation, spermiation.
• Gonads produce steroids: oestradiol-17β (vitellogenesis), 11-ketotestosterone (spermatogenesis, secondary sex characters), and the maturation-inducing steroid (MIS) — 17α,20β-DHP (in many teleosts) — which triggers final oocyte maturation (via maturation-promoting factor) and spermiation; prostaglandins aid ovulation and act as sex pheromones.
• Feedback: sex steroids exert positive and negative feedback on the brain and pituitary. Thyroid hormones, GH and cortisol also influence reproduction.
• Application: induced breeding of Indian major carps by hypophysation (carp pituitary extract injection — pioneered in India by H. L. Chaudhuri, 1957) or synthetic GnRH analogue formulations.

Modes of reproduction — oviparity, viviparity, ovoviviparity

• Oviparity (most bony fishes): eggs laid and fertilised externally in water (some oviparous sharks lay eggs fertilised internally in horny cases — "mermaid's purse", e.g. dogfish Scyliorhinus). Large numbers of small eggs (lakhs in carps); little or no parental care, though some build nests or guard eggs (stickleback, cichlids, mouth-brooding tilapia; male seahorse broods eggs in a brood pouch).
• Ovoviviparity (aplacental viviparity / lecithotrophy): fertilisation internal; eggs retained in the oviduct/ovary and embryos develop on their own yolk, without a placental connection; young born alive — e.g. many sharks (Squalus, spiny dogfish), rays, coelacanth, rockfishes; in some sharks embryos eat other eggs (oophagy) or siblings.
• Viviparity (matrotrophy): internal fertilisation with the mother supplying nutrition — yolk-sac placenta in hammerhead and blue sharks (Scoliodon in India), uterine milk (histotrophy) in stingrays, follicular placenta in poeciliids (Poecilia guppy, Gambusia, Xiphophorus), and embedded embryos in surfperches; fewer, larger, well-developed young with better survival.

Sex reversal and factors influencing it

Fishes show remarkable sexual plasticity. Hermaphroditism: synchronous (both gonads functional at once — Serranus, Kryptolebias, which can self-fertilise) and sequential — sex changes during life:

• Protandry (male first, then female) — clownfish (Amphiprion): the largest fish of a group is the female; when she dies, the dominant male becomes female. Also Lates calcarifer (barramundi/bhetki), sea breams.
• Protogyny (female first, then male) — wrasses (Thalassoma, Labroides cleaner wrasse), groupers (Epinephelus), parrotfishes: when the dominant male disappears, the largest female becomes male within days.
• Bidirectional sex change in some gobies.

Factors influencing sex reversal/determination: social (dominance hierarchy, sex ratio, presence/absence of a dominant individual), size and age (size-advantage model), temperature (temperature-dependent sex determination in some species, e.g. Atlantic silverside, tilapia — high temperature favours males), pH, density, hormones — a fall in oestrogen (aromatase inhibition) and rise of 11-ketotestosterone drives female-to-male change; cortisol (stress) can masculinise; genetic sex-determining genes (XX/XY or ZZ/ZW systems; e.g. dmy in medaka); and environmental pollutants (endocrine disruptors causing feminisation of males).

Applied: hormonal sex reversal in aquaculture — feeding tilapia fry with 17α-methyltestosterone produces all-male populations (faster growth, no unwanted breeding); oestradiol for all-female stocks in species where females grow faster or for caviar; YY "supermales" for monosex seed.

Likely exam questions

1. (Long, 8 marks) Describe the structure of the fish ovary and explain the process of vitellogenesis.
2. (Long, 8 marks) Explain the neuroendocrine control of reproduction in fishes through the HPG axis.
3. (Short, 4 marks) Distinguish between oviparity, ovoviviparity and viviparity with examples.
4. (Short, 4 marks) What is sex reversal? Give examples of protandry and protogyny.
5. (Short, 4 marks) Write a note on spermatogenesis in teleosts.