(a) Explain types of fins in fishes and their role in locomotion. (20 marks) (b) Describe the functions of various gonadal…

(a) Types of fins in fishes and their role in locomotion (20 marks)

Types of fins. Fins are of two kinds.

• Median (unpaired) fins: dorsal (one to three), caudal (tail) and anal fins, and in some teleosts a small fleshy adipose fin (salmon, catfish).
• Paired fins: the pectoral and pelvic fins, homologous with the limbs of tetrapods, supported by the pectoral and pelvic girdles.

Support. The fin is supported by basal cartilages or bones (basals and radials) and by dermal fin rays: horny ceratotrichia in sharks and bony lepidotrichia in bony fishes, and spiny rays in some.

Types of caudal fin.

• Protocercal: symmetrical with the notochord straight to the tip, as in lampreys and larvae.
• Heterocercal: the vertebral column turns into a larger upper lobe, as in sharks.
• Homocercal: apparently symmetrical, but the column ends in the urostyle at the base, as in teleosts (Labeo).
• Diphycercal: symmetrical, with the column straight to the tip, as in lungfishes.

Role in locomotion.

• The caudal fin gives the main propulsive thrust, and the body is thrown into waves, in which the caudal peduncle pushes water backward. The shape of the caudal fin fits the speed and habit; the heterocercal tail of a shark gives lift.
• The dorsal and anal fins act as keels that prevent rolling and yawing and give stability.
• The pectoral fins steer, brake, control depth and pitch, and give lift. The pelvic fins keep the body balanced and brake, and both paired fins act as hydroplanes.
• The adipose fin gives stability.

Special functions. The pectoral fins of flying fishes act as wings for gliding, and those of rays are used for propulsion. Anglerfish and mudskippers use them to crawl, and pelvic fins form a sucker in gobies and a clasper in male sharks. The anal fin forms the gonopodium in Gambusia and Poecilia.

(b) Gonadal hormones in humans (15 marks)

Testis.

• Testosterone, from the Leydig cells under LH, develops the male reproductive tract, maintains spermatogenesis (with FSH) and the secondary sexual characters, such as beard, deep voice and muscle, and promotes libido and protein synthesis.
• Sertoli cells secrete inhibin, which suppresses FSH, androgen-binding protein, which concentrates testosterone in the tubules, and anti-Müllerian hormone (AMH), which regresses the Müllerian ducts in the male foetus.

Ovary.

• Oestrogens (mainly oestradiol), from the granulosa and theca cells of the follicle, develop the female tract and secondary sexual characters, stimulate the proliferative phase of the endometrium, and produce the LH surge for ovulation by positive feedback. They also protect bone.
• Progesterone, from the corpus luteum, causes the secretory phase of the endometrium, maintains pregnancy, suppresses uterine contractions and prepares the mammary gland.
• Inhibin suppresses FSH, and relaxin, from the corpus luteum and placenta, relaxes the pelvic ligaments and cervix for birth. AMH from the granulosa cells regulates follicle growth.

Control. The hypothalamus releases GnRH, which stimulates the pituitary to release FSH and LH, and the gonadal steroids feed back to control them.

(c) Honey bees: general features, distribution and social organisation (15 marks)

General features. Honey bees (genus Apis, family Apidae, order Hymenoptera, class Insecta, phylum Arthropoda) are social insects that build wax combs and store honey.

• The body has a head, thorax and abdomen, with compound eyes and ocelli, antennae, and chewing and lapping mouthparts (a tongue, the proboscis).
• Hind legs carry a pollen basket (corbicula), the abdomen has wax glands, and the workers have a barbed sting (a modified ovipositor). Two pairs of wings are hooked together.

Distribution. Apis dorsata (the rock bee), A. cerana indica (the Indian bee, domesticated) and A. florea (the little bee) occur in India, and A. mellifera (the European bee) was introduced. Apis mellifera is kept worldwide. Beekeeping is done in Himachal Pradesh, Punjab, Kashmir, Kerala and Karnataka, among others.

Social organisation. A colony has three castes.

• Queen: the single fertile diploid female that lays up to 1,500 or more eggs a day, and lives 2–5 years. She is raised on royal jelly.
• Drones: haploid males from unfertilised eggs, whose only role is to mate with the queen.
• Workers: sterile diploid females from fertilised eggs, who work in order of age: cleaning, nursing, wax-building, guarding and foraging.

Caste determination. Sex is determined by ploidy (haplodiploidy), and queen or worker by the diet of the larva. Communication. Foragers dance a round dance for nearby food and a waggle dance for food far away, giving distance and direction (von Frisch), and pheromones, such as the queen substance, keep the colony together.