Marine Fish Characteristics

• Key traits of marine fishes include being cold-blooded, using gills for oxygen extraction, having swim bladders for buoyancy, possessing a closed circulatory system with a two-chambered heart, and being covered in scales.

Fish Scales

• Placoid scales are webbed-foot shaped and found in Chondrichthyes.
• Ganoid scales are diamond-shaped and located in some Osteichthyes.
• Ctenoid/cycloid scales are turtle-shell shaped and common among bony fish,

Reproductive Strategies

• Oviparous fish lay eggs that hatch outside the body.
• Viviparous fish give birth to live young.
• Ovoviviparous fish have eggs that develop inside the mother, with the young born live and relying on yolk.

Swim Bladder

• The swim bladder is an internal organ that controls buoyancy by adjusting gas levels.
• This allows fish to float or sink without expending energy, thus conserving energy and aiding in positioning.

Closed Circulatory System

• A closed circulatory system keeps blood contained in vessels, ensuring efficient oxygen and nutrient delivery
• The two-chambered heart (one atrium, one ventricle) is simple yet sufficient for pumping blood through gills and the body in an aquatic environment.

Class Agnatha

• Hagfish are scavengers in deep ocean waters.
• Lampreys are parasitic and found in freshwater but migrate to the sea.
• Agnathans have a cartilaginous skeleton and an elongated body for navigating tight spaces.
• Jawless, sucker-like mouths allow hagfish to tear flesh and lampreys to latch onto hosts for blood feeding.
• Hagfish decompose dead organisms, recycling nutrients, while lampreys are parasites, impacting fish populations and sometimes harming native species as invasives.
• Hagfish develop directly into juveniles without a larval stage
• Lamprey larvae develop in sediment and undergo metamorphosis before emerging.
• Unpaired fins (e.g., dorsal or caudal) are singular along the midline, providing stability during swimming rather than precise maneuvering.
• Keratinized plates are simple, tooth-like structures in hagfish used for tearing flesh during scavenging.
• Lampreys have more complex, grasping teeth in their sucker-like mouths for parasitic blood-feeding.

Class Chondrichthyes

• Sharks, rays, skates, and chimaeras are included with placoid scales (dermal denticles).
• Elasmobranchii includes sharks, rays, and skates. Holocephali includes chimaeras (ghost sharks).
• A cartilaginous skeleton reduces weight for mobility.
• Dermal denticles reduce drag and protect skin.
• Olfactory bulbs detect prey from afar.
• The lateral line senses water vibrations.
• Ampullae of Lorenzini detect electrical fields for hunting.
• Sharks are apex predators, regulating populations for biodiversity.
• Rays and skates cycle nutrients by disturbing sediment, aerating the ocean floor.
• Catadromous fish are born in saltwater and migrate to freshwater for maturation/reproduction, like the European eel.
• Dermal denticles reduce water drag for faster swimming, protect against parasites, and enhance durability.
• The ampullae of Lorenzini are electroreceptors that detect electrical fields from prey movements, helping sharks locate hidden or distant organisms in murky water.

Class Osteichthyes

• Osteichthyes have bony skeletons and include fish like tuna and swordfish.
• Ganoid scales are mentioned, though ctenoid/cycloid are also common.
• Actinopterygii (ray-finned) includes most bony fish with rayed fins.
• Sarcopterygii (lobe-finned) includes fish like coelacanths and lungfish with lobed pectoral fins.
• The operculum protects gills, countercurrent flow maximizes oxygen uptake, and a fusiform body shape with various fins reduces drag for agile swimming.
• Anadromous fish are born in freshwater and migrate to saltwater for maturation/reproduction, like salmon.
• Countercurrent flow moves blood in the opposite direction of water over the gills, maintaining a high oxygen gradient
• Fusiform shape (wide in the middle, tapered at the ends) reduces water resistance paired with fins, it improves agility and stability.

Reproductive Strategies

• Anadromous fish are born in freshwater and mature in saltwater, while catadromous fish are born in saltwater and mature in freshwater.
• Osmoregulation is the ability to regulate body fluids (water and salts), critical for fish transitioning between saltwater and freshwater.
• Fish biodiversity maintains ecological balance through interconnected species roles (e.g., predators regulate populations, decomposers recycle nutrients).
• Agnatha and Chondrichthyes have cartilaginous skeletons.
• Osteichthyes have bony skeletons, offering greater structural support and attachment points for muscles.
• Hagfish decompose dead matter, sharks regulate populations
• Rays aerate the ocean floor and cycle nutrients.
• Chromatophores are specialized cells that change color to match the environment.
• Countershading is a dark dorsal side and light ventral side which hides fish in the water column.
• Disruptive coloration uses stripes or spots blur the body outline, distorting size.
• Mimicry coloration mimics predators or dangerous organisms to deter threats.