Dogfish Dissection Squalus acanthias

Classification Kingdom – Animalia Phylum – Chordata SubPhylum - Vertebrata Class – Chondrichthyes (cartilaginous fish) Subclass – Elasmobranchii (sharks and rays) Order – Squaliformes (dogfish sharks) Family - Squalidae Genus - Squalus Species- acanthias

Phylum Chordata 4 major characteristics seen at some point during development Notochord – stiff rod along back of the embryo during development (vertebral column in humans) Dorsal nerve cord (hollow) – nerves attached (spinal cord in humans) Pharyngeal slits – openings along wall of pharynx (gills) Post-anal tail – extends beyond anus

Sub phylum Vertebrates Chordates with a backbone which gives support and protection of spinal cord Allows for larger growth in organism (compare to size of invertebrates) Also share: segmentation, bilateral symmetry, 2 pair of appendages, cephalization, complex brain and sense organs, true coelom, closed circulatory system, chambered heart, higher level of cellular organization

Evolution of fish/sharks 1st chordates – 550 mya Sharks – 400 mya Special adaptations of sharks for survival Streamlined – faster in water Paired fins supported by spines – finer control Development of jaws for grasping prey

Class: Chondrichthyes sharks, rays and skates Sharks – cartilaginous fish – skeleton made of cartilage strengthened by calcium carbonate, light but strong Superior design for swimming Teeth are modified scales, 6 – 10 rows Respiration through gills with gill slits

Reproduction Oviparous – internal fertilization following release of eggs Ovoviviparous – internal fertilization and development without nutrition from mother, eggs retain in body until hatched SHARKS Viviparous – internal fertilization and development with nourishment (mammals)

Sharks detect prey 3 well developed senses Ability to sense electric currents in water Lateral line system – sense pressure caused by a fish or other animal swimming nearby Keen sense of smell, detect 1 drop of blood in 25 gallons of water.

External Dogfish 3 main body regions – cranial, trunk, caudal Placoid scales – cone shaped, rough texture Countershading – dark gray above, light under Lateral line – carries impulses to CNS Ampullae of Lorenzini – openings around eyes, snout and nostrils to sense temp change, water pressure, electrical fields and salinity. Fusiform – body shape of shark, streamlined for least resistance

External Head Rostrum – snout Nostrils – nares – openings of external nostrils, underside of rostrum, anterior to jaws Incurrent – sense of smell Strong jaws Eyes – transparent cornea protects eye Upper and lower lids – protection Conjunctiva – inner membrane inside lower lid

External Spiracles – posterior and dorsal to eyes Reduced 1st gill, respiration while sharks mouth is closed or eating Gill slits – most have 5 external Water taken in passes over internal gills, oxygen removed, carbon dioxide excreted, water leaves via gill slits Endolymphatic pores – top of head, between spiracles, serve as primary equilibrium organ.

External Fins – 2 dorsal – anterior (larger) and posterior, spines lie anterior to each dorsal fin, defense, contains toxins Caudal fin – asymmetric, 2 lobes Pectoral fins – deflect water to maintain shark moving in horizontal direction Pelvic fins – on either side of cloacal opening Cloaca – ventral surface, between pelvic fins, receives products of intestine, urinary and genital ducts Claspers – male organs used for reproduction, fertilization is internal, inserted in oviduct of female