General Points

• Blood of all marine fishes (except hagfish) is less salty than seawater
• Water therefore lost by osmosis
• Excrete excess salt through kidneys & rectal glands

Chondrichthyes

• Hypertonic to seawater
• Regulate osmotic potential using urea
• Urea toxic because it denatures proteins
• Sharks protect proteins with trimethylamine oxide (TMAO)

Osteichthyes

• Drink large amounts of seawater
• Excrete excess salt from kidneys & rectal glands
• Excrete very small volumes of urine

Sense of Smell

• Very well developed in most
• Olfactory sacs on each side accessible from 1-2 nostrils
• Sharks have most developed, blood at 1 part per 1,000,000
• Salmon can smell their home stream

Sense of Taste

• Taste buds distributed over much of body
• Some fish have taste buds on barbels

Sight

• Eyes similar to terrestrial vertebrates
• Focus by changing distance of lens to object viewed
• Color vision in Osteichthyes, b/w in Chondrichthyes
• Many sharks have a nictitating membrane

Hearing, Balance & Other Senses

• lateral line senses vibrations & water movement
• Ampullae of Lorenzini in Chondrichthyes senses electrical currents
• Inner ear has fluid filled canals with sensory organs like those in lateral line
• Swim bladder can be used to amplify sounds
• Otoliths in ears are similar to statocysts in invertebrates

Territoriality

• Sometimes only during reproduction
• Most common in crowded environments
• Aggressive behaviors seldom lead to "blows"
• A few use sounds as a deterrent
• May be defended alone, in pairs, or sometimes cooperatively

Schooling

• Most common in Osteichthyes
• Schools usually comprise fish of same size
• Stationary schools sometimes mixed
• Highly coordinated by vision, other cues likely used as well
• Proposed functions:

1. Protection from predators (predator swamping)
2. Increased swimming efficiency (not well supported)
3. Enhances mating & feeding

General Migration

• Most often for feeding
• Following the tide to feed
• Vertical
• To temperate waters where food is more abundant (tuna)

Anadromous Fishes

• Examples: sturgeon, lamprey, smelt, salmon
• Pacific salmon spend adult life wandering North Pacific & Arctic
• How they migrate to the right river is unknown
• Once they reach the river they do not feed
• Homing behavior, once in freshwater, depends on smell
• Salmon defend nest for awhile, then die
• After hatching most species return to ocean immediately
• Migrations disrupted by humans: dams, logging, eutrophication
• Most now raised in hatcheries
• Atlantic salmon may survive after spawning

Catadromous Fishes (freshwater eels)

• Both American & European eels migrate to Sargasso Sea
• Spawn at depths of 400-700 m
• Eggs hatch to leptocephalus larvae
• American larvae spend a year and metamorphose, European 2-3 years
• Young go to freshwater and return to spawn after 10-15 years
• Possibly use earth's magnetic field to migrate?

Reproductive & Life History

Reproductive System

• Chondrichthyes have paired ducts from gonads to cloaca
• Agnatha & Osteichthyes have a urogenital opening
• many marine fishes only produce gametes at certain times
• Timing is controlled by hormones
• Hormone release determined by environmental cues
• A few marine fishes are hermaphroditic
• Some sea bass, groupers, parrotfishes & wrasses use sex reversal

Reproductive Behavior

• Most species congregate to spawn, often in specific places
• Color changes often accompany spawning
• Courtship often fairly complex
• Both internal & external fertilization occur, latter most common
• Pair of claspers used in copulation in sharks
• Eggs shed into water column often contain oil droplets (buoyant)
• Grunion lays eggs in sand at high tide
• Fish that lay fewer eggs often guard them
• Male pipefishes and seahorses carry eggs around

Types of Development

• Oviparous
• Ovoviviparous
• Viviparous