20,000 species (three classes) of marine fishes swim in our oceans: Note: "fishes" is plural when discussing more than one species whereas "fish" is plural for fish within a species.

Agnathans are primitive jawless vertebrates

Modern agnathans are eel-like (they lack paired appendages) and consume soft foods

Hagfish are marine scavengers

Lampreys are ecto-parasites that rasp holes in their hosts.

Like salmon, the larvae of many species of lamprey live in fresh water, adults are marine.

Chondrichthyes are cartilaginous fishes: sharks, rays, and skates.... about 1100 species total...

The cartilaginous skeleton is derived from bony ancestors.... cartilage is lighter and more flexible than bone.

Their skin is covered with denticles or placoid scales, which are different from the scales covering bony fishes

As a rule, this group takes many years to mature and produce few offspring... increasing vulnerability to overfishing.

Rays are carnivores but most lack obvious teeth

Stingrays have bony mouth plates that crush mollusks and urchins

Their "sting" is primarily for defense against predators like sharks

Most live on the bottom... often buried

Manta rays, the largest of the rays consume zooplankton and live in mid-water

Electric (torpedo) rays can generate up to 200 volts of electricity from gel-filled cells that act like batteries

Electricity is used to stun prey and for defense...not communication like electric eels in fresh water.

Though best known for the voracious feeding habits of some larger sharks, sharks come in many shapes and sizes and many are harmless. Their teeth are arranged in rows and those in back replace front teeth as they are lost.

The largest sharks (basking sharks and whales sharks) are planktivores

Sharks are denser than water, despite a large, oil rich liver, and will sink if they stop swimming

Fertilization is internal (male claspers are the intermittent organ); sharks lay eggs (oviparous) or give birth to live young (viviparous)... gestation is often 5-9 months!

Sharks have acute senses

In addition to olfaction (smell) that works over large distances... most sharks can also detect electrical fields generated by muscle contractions of prey at short range

The lateral line system detects small differences in pressure (like ears).... they lack eardrums

Diversity of sharks

Sawsharks (Pristophoriformes) One family: five spp. These easily recognized, benthic sharks are found in warm temperate or tropical seas. They bear live young.

Dogfish Sharks (Squaliformes) Three families, 73 spp. Mostly deep water sharks found worldwide. They bear live young and eat crustaceans, squid and other fishes. Harmless to humans because of their small teeth and small size.

Angel Sharks (Squatiniformes) One family, 13 spp. These flattened, bottom dwelling sharks are found on continental shelves and upper slopes of cold temperate and tropical seas. Often bury themselves. Their sharp, awl-like teeth impale small fish and crustaceans that swim too close

Bullhead or Horn Sharks (Heterodontiformes) One family, 8 spp. Live among cracks and crevices in the Pacific and Indian Oceans where they consume invertebrates.

Gilled Sharks (Hexanchiformes) Two families, five spp. Typically deep-water, bottom-dwelling sharks with a worldwide distribution. These are the only sharks with six or seven gill slits (as opposed to 4-5). They bear live young and eat crustaceans, and other fishes.

Mackerel Sharks (Lamniformes) Seven families, 16 spp. These typically large open ocean predators eat a variety of foods, from zooplankton to squid, fishes, and marine mammals. Found in all seas. This group includes the Mako and Great White Sharks, as well as the plankton eating Megamouth and Basking Sharks.

Carpet Sharks (Otectolobiformes) Seven families, 31 spp. These harmless sharks with flattened heads are benthic except for the whale shark. Most eat small fishes and invertebrates, although the whale shark is plankton feeder. Some bear live young while others lay eggs.

Ground Sharks (Carcharhiniformes) 8 families, 193 spp. A diverse order of sharks. found in temperate and tropical oceans worldwide...they are often found near the coast. They eat a variety of prey including, small invertebrates, squid, other fishes, and marine mammals. This group includes the Tiger shark.

Osteichthyes are bony fishes: the most abundant and diverse vertebrates (about 30,000 species... fresh and salt water)... they range in size from 1 cm to more than 6 m

Their skin is usually covered with scales... mucous "slime" reduces skin friction and provides some protection against infection.

The operculum is a protective flap over the gills

The swim bladder is an internal air sac that helps control buoyancy.

See your text for a discussion of how fish swim

Most bony fishes have external fertilization of pelagic or benthic eggs... though some (e.g. surf perches) have internal fertilization and give birth to live young

From freshwater origins, most bony marine fishes are "ray-finned"... the more primitive fleshy-finned fishes (e.g. lungfish) are fresh water species... however the lobe-finned coelacanth returned to marine habitats... while its ancestors eventually evolved into the amphibian lineage.

Some important groups of bony fish

Eels -Anguilliformes 597 spp

Tarpon -Elopiformes 11 spp

Salmon -Salmoniformes 350 spp

Deep Sea Fish -Stomiiformes 250 spp

Gobies -Gobiesociformes 114 spp

Trumpetfish -Syngnathiformes 257 spp

Flyingfishes -Cyprinodontiformes 845 spp

Silversides -Atheriniformes 235 spp

Squirrelfishes -Beryciformes 164 spp

Scorpionfishes -Scopaeniformes 1160 spp

Flatfish -Pleuronectiformes 538 spp

Triggerfish -Tetraodontiformes 329 spp

Perch Like -Perciformes 7791 spp, largest order

 

Marine fishes offer ecologists and evolutionary biologists many unique opportunities for research. Why?

1) They are often abundant, visible, and easily manipulated

2) Many adult fish have relatively limited home ranges, so individuals can be repeatedly studied for three important proxies of fitness: growth, reproduction, and survivorship

3) Many fish show a variety of behaviorally or morphologically labile responses to different physical or demographic environments, so they respond to experimental manipulations in meaningful, quantifiable ways

Ultimately, understanding the ecology and evolution of marine fishes requires data that relates directly to factors relating to biological fitness.

Fish reproduction

Most fish have external fertilization and spawning occurs in pairs or groups.

Pair spawning occurs when males can control access to resources that females need (excluding other males via territorial behavior)

Monogamy (butterflyfishes)

Polygamy (serial pair spawns by territorial male wrasses and many females)

"Sneakers" mimic females and gain access to females by fooling territorial males

Group spawning occurs when males cannot monopolize mating sites that contain resources for females

Group mating is sometimes promiscuous.... males and females reproduce in mating swarms (e.g. anchovies)

The number of eggs produced by a female reflects several life history tradeoffs

1) Trade off between available current energy levels and expected future reproductive success (the latter is a function of potential for future reproduction, discounted by the likelihood of surviving to the next reproductive bout)

2) Trade off between egg quality and egg number.

With no parental investment after spawning... making lots of "cheap" eggs is often the best strategy

With parental care, usually by males, there are often fewer, more "expensive" eggs....

Most eggs hatch to produce planktonic larvae, though some males care for larvae (e.g. toadfish)

At the extreme (in seahorses and pipefish) the male carries the eggs and then the larvae until they have matured

Hermaphrodites and sex change

Many fish are both male and female: either simultaneously (at one time) or serially (different sexes at different times.

Simultaneous hermaphrodites can fertilize more eggs per available energy, if there is "cooperation"

Serial hermaphrodites are either protandrous (male first, change to female) or protogynous (change from female to male).

Anisogamy (sperm are cheaper than eggs) and the Size-advantage model of sex change

Basic premise: reproductive potential is related to size

Female fecundity is limited by access to resources and body size

Male reproductive success is limited by access to females

 When mates are extremely difficult to find (e.g. in the deep sea where fish densities are low), parasitic dwarf males may evolve

 

Fish growth and survival

Otoliths (ear bones) and scales can be used for estimating age, longevity, and growth

Studies if individual fish reveal that they can shrink or grow seasonally

Survival is quite variable in different oceans: e.g., Australia (long-lived) vs Caribbean (short-lived)

 

Because available energy may be allocated to any of these three variables, fish ecologists often estimate fitness from measures of food intake

Fish foraging is ultimately a question of maximizing energy intake while minimizing energy expenditure and risk.

High quality (protein), low abundance vs low quality (algal), high abundance diets: short vs long guts for nutrient extraction.... a few good bites vs. many poor bites.....

e.g. predatory groupers may eat only every few days, while herbivorous parrotfish must feed all the time

Herbivory and the challenge of getting past the algal cell wall.

Parrotfishes grind cells open with a pharyngeal mill

Damselfishes digest cell walls with acid

Chubs use microbial digestion/fermentation like horses

Surgeonfishes grind cells within a muscular gizzard and have microbial symbionts

When fish can control access to resources, they may defend feeding territories... others may group to swamp out the territory defense of others

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