Fish Biology

Fish shapes

• Cold-blooded aquatic animals with backbones, gills, and fins

• Most are torpedo-shaped (fusiform) for efficient travel through water, but they may be flattened and rounded (flounder), and vertical and angular (sea horses)

Fish Size

• “The tiny coral reef-dwelling fish called the pygmy goby has taken the record as the shortest-lived vertebrate. The pygmy goby lives an average of 59 days, pipping the previous record holder, an African fish which lives for just over two-and-a-half months.” (BBC News)

• Fish range in size:• the pygmy goby, which

reaches only 12mm long and 1.5g in weight; sexually mature at 6mm

• The whale shark grows to 18m and weighs over 20 tons

Fish Distribution

• Found throughout the world• Altitudes of more than

5000m to depths of about 10km

• Some inhabit hot springs, such as Cyprinodon, where water may reach 45°C

• Others are found in the Antarctic seas, such as Chaenocephalus, where it may be less than 0°C

• 107 species are distributed worldwide in tropical and subtropical waters, but many species have limited ranges

Fish Distribution

• There are 20 000 or more fish species• 60% live in marine waters• 40% are found in fresh water• Most of the world’s fish are continental -

either in freshwater on land, or in the sea close to the coast

• Coasts are good for fish as they are rich in nutrients such as nitrates and phosphates –from rivers, upwelling, aeration from the surf and tide, and the sunlight

Fish Anatomy

• Three classes:

• Agnatha (jawless fishes) that consists of hagfish and lampreys

• Chondrichthyes (cartilaginous –skeleton) that consists of sharks and rays

• Osteichthyes (bony-skeleton) – all other fish

Skeletons

• Skeletons of the three groups are very different

• Hagfish and lampreys have a notochord, a rod-like structure composed of unique tissue

Skeletons

• Sharks and rays have a notochord that is surrounded and constricted by the vertebrae, to form a backbone. The remainder of their skeleton is composed of cartilage, not bone (but it can be hardened by calcareous salts)

• Primitive bony fishes have vertebrae that are mostly cartilaginous, but advanced fish have bony vertebrae united to form a backbone, with no notochord

Fins

• Fins are either median or paired• Median fins are along the centerline of the

body, at the top, bottom, and the end• Dorsal fin (top) may be one or several fins

(one behind the other), and may include a fleshy fin (adipose) near the tail

• Bottom (anal) fin is located on the belly behind the anus

• End fin is called the tail, or caudal fin

Fins• Pectoral fins are at the front of the

body, behind the gill openings and usually provide maneuverability

• Pelvic fins (ventral fins) are along the bottom of the body but can vary in their placement (can be in the middle of the belly, below the pectorals, or in front of the pectorals.) Can provide maneuverability

Scales

• Scales are colorless• Coloring of the fish comes from the structures

beneath or associated with the scales• Not all species of fish have scales, or they can

be so tiny it would appear that they do not have any

• Scales may only be present on small areas of the body

• Arranged in imbricate (overlap like shingles) or mosaic (fitting closely together or barely separated)

Four Basic Scale Types:

• 1. Placoid scales (dermal denticles): – are on sharks and rays, and are tooth-like

in structure (enlarged ones are actual teeth in sharks)

– Scales do not increase in size, so new scales must be added as the shark grows

• 2. Cosmoid scales:– Coelacanth have

them– Also in lungfishes

(only single-layered)– Four-layered bony

scale

• 3. Ganoid scales:– Found on gars – Typically squarish– Single bony layer

• 4. Leptoid scales:– Derived from ganoid scales by the loss of

the ganoin layer– Single layer of bone– Found on the higher bony fishes and

occur in two forms: cycloid (circular) and ctenoid (toothed)

Fish Circulation

• Blood transports oxygen, nutrients, and wastes

• Single circuit: heart-gills-body-heart

• Fish heart is two-chambered, with an upper atrium and lower ventricle

• Other organisms may have three or four-chambered hearts

Fish Respiration

• Fish must extract oxygen from the water and transfer it to their bloodstream

• This is done by gills, lungs, specialized chambers, or skin

• More difficult than extracting oxygen from air• Some fish can extract as much as 80% of

the oxygen in the water passing over the gills whereas humans can only extract about 25% of the oxygen from the air taken into lungs

Gills are efficient because:

• Large surface area – gills have 10 to 60 times more than body surface area

• Short distance for oxygen to diffuse – blood in the gills is close to the water

• Countercurrent circulation – blood flows forwards while the water flows the other so that there is always less oxygen in the blood than in the water

• Water flows one direction only over the gills

Fish Body Temperature

• Cold-blooded – body temperature varies with external temperature

• Blood passing through the gills loses heat to the water so that a fish’s body temperature is usually within a degree of the water temperature

• There are some warm-blooded fish (tuna, mackeral sharks) – they have a countercurrent circulatory network that keeps their bodies 5 to 12 degrees warmer than the water

Fish Water Balance

• Blood of freshwater fish is more salty than the water in which they live

• Osmotic pressure causes water to diffuse into the fish’s body through the gills, mouth membranes and intestines

• Must eliminate excess water so they produce very dilute urine (ex. Lampreys may create an amount of urine up to 36% of their body weight per day)

• A gain in water means a loss in salt, so they absorb salt from the water with their gills

• Marine bony fishes have blood that is less salty than sea water, so they lose water and absorb salt

• So they drink seawater and produce little urine

• Excrete excess through gills or anus

• Hagfish have a salt concentration equal to that of seawater

• Sharks retain the excess salt to keep the sharks’ blood at a higher salt concentration than seawater

Fish Gas Bladder

• Reducing body weight is important to fish so they expend less energy to keep at a given depth

• Total body density equal to that of water would be weightless, not floating or sinking

• Fat is less dense than water, so some fish have up to 1/3rd of their weight in fat (such as deep sea sharks that have large livers that contain squalene)

• Many fishes have a gas-filled bladder • The amount of body volume that it takes up

will depend on whether the fish is freshwater or marine

• Freshwater is less dense and is less buoyant, so they need a larger gas bladder to keep from sinking

• -Gas bladders can be 7-11% of body volume in freshwater fish, and 4-6% in marine fish

• Gas levels within the bladder must be adjustable, or the fish would always remain at the same depth

• In some fish, air is gained through gulping fresh air, and expelling it through the mouth and gills; in others the gas is exchanged through the blood system

Fish Lateral Line System

• This is sensitive to differences in water pressure

• Can be due to changes in depth or waves• Consists of a neuromast, which is a bundle

of sensory and supporting cells that have hairs that are enclosed in a gelatin

• Neuromasts send out nerve impulses and when pressure causes the hairs to move, the nerve impulses will increase or decrease

• Neuromasts may be single, in small groups (pit organs), or in rows (lateral line system)

• Lateral line system runs along the sides of the body onto the head, where it divides into three branches – two to the snout and one to the lower jaw

• Another fish swimming will cause a pressure wave in the water that can be detected by the lateral lines of other fish

• When a fish is swimming towards something, the pressure waves will change around its body, which will tell the fish to swerve to avoid the object

Fish Reproduction

• Most are egg-layers• Some bear live young• Live-bearing fish are either ovoviviparous

(eggs hatch within the female) or viviparous (unborn are supplied with nutrients from the mother’s tissues)

• Live bearing and some egg-layers have internal fertilization and they have modified structures that will introduce sperm into the female’s body

• Three modes of reproduction:– Heterosexual (most popular)– Hermaphroditic– Parthenogenetic

Heterosexual

• In some live-bearing fishes the female may be able to store the sperm for up to 10 months and is used to fertilize new batches of eggs as they develop

• Some females may carry sperm from multiple males

Hermaphroditic

• Single fish could be both male and female, produce eggs and sperm (either at the same time or at different times) and will mate with another hermaphrodite

• External self-fertilization can occur, or internal self-fertilization

• Some fish will switch their sex as they age

Parthenogenetic

• Unfertilized eggs develop into embryos – known to one fish species, Poecilia formosa, of the Amazon River

• Development will continue without fertilization, but mating with a male is still required to stimulate egg development

Parental Care

• Some fish form huge schools of males and females, release their eggs and sperm, and then leave

• Other fish build nests and care for eggs and newly hatched young

• Others will carry the eggs with them in such places as their mouths, gill cavities or other cavities

“The most fascinating thing about the Arrowana is that it is a mouthbrooder,capable of carrying

hundreds of eggs in its mouth and even after they hatch, the little fish live in their mother's mouth till

the time they learn the ways of the world”

http://hubpages.com/hub/The-Mouthbrooder-Dragon-Fish-Arowana