L7-Comprehensive Study Guide: Marine Fish Diversity and Biology and Ecology

Introduction to Marine Biology: Focus on Fish

• Instructor Information

  • Name: S (Danish origin, grew up in Denmark).

  • Background: Completed undergraduate and master's degrees in Denmark. Moved to Virginia, US, for a PhD focusing on trees and algae in estuarine communities. Moved to New Zealand in $2008$.

  • Course Role: Co-teacher with John; will be involved in field classes and labs.

  • Resources: A research web page is available via a link provided in lecture materials.

• Class Demographics and Context

  • The current class is noted as the largest Two One Two (likely BIOL 212) group in years.

  • The group includes many international students and students from environmental science and biology degrees.

  • The course follows a general textbook approach, progressing chapter by chapter. The current focus is Chapter Eight: Fish.

Evolutionary Background and Chrodal Characteristics

• The Evolutionary Tree

  • The curriculum has previously covered primary producers and "simple" (ancestral) life forms, moving through invertebrates.

  • Termonology Note: "Simple" refers to ancestral or old life forms that are often highly successful and have not needed to change over long evolutionary periods.

• The Chordates (Phylum Chordata)     All chordates possess four main characteristics at some point in their life cycle:

  1. Dorsal Hollow Nerve Cord: A nervous system structure (visualized as a yellow line in diagrams) used to send signals and respond to the environment.

  2. Pharyngeal Slits (Gill Slits): In fish, these develop into gills. In humans, they are present only in the early embryonic stage.

  3. Notochord: A rigid structural support that provides a base for muscle attachment and allows for larger body sizes and movement. In vertebrates, this develops into the backbone.

  4. Post-Anal Tail: Present in all chordates; in humans, it exists only during early embryonic development.

• Types of Chordates

  • Tunicates (Urochordata): Often look like sponges or blobs; possess chordate characteristics in the larval stage.

  • Lancelets (Cephalochordata): Simple, fish-like chordates.

  • Vertebrates: Chordates where the spinal cord is surrounded by either cartilage or bone. Groups include fish, amphibians, reptiles, birds, and mammals.

  • Note on Amphibians: They are excluded from this course because they are not marine organisms; they are restricted to freshwater or terrestrial environments.

Taxonomy and Defining "Fish"

• Defining a Fish

  • "Fish" is a paraphyletic definition, meaning it is a group of aquatic organisms that share certain characteristics but do not form a single, exclusive taxonomic group that includes all descendants of a common ancestor.

  • Minimum Requirements: Aquatic organisms possessing at least a skull (to protect the brain) and a skeleton (bony or cartilaginous).

• Dichotomous Breakdown of Fish Lineages

  • Skull Presence: If no skull, the organism is not a fish (e.g., tunicates).

  • Paired Fins: If no paired fins, the organism is a Hagfish.

  • Jaws: If no jaws, the organism is a Lamprey. If jaws exist, they move further down the tree.

  • Skeleton Material: Cartilage leads to sharks, rays, and skates. Bone leads to bony fish.

  • Fin Type: Ray-finned fish vs. Lobe-finned fish (e.g., Coelacanths). Lobe-finned fish are the evolutionary link to tetrapods (amphibians, reptiles, etc.).

• Diversity Statistics

  • Global Total: Approximately $33,000$ fish species.

  • Marine Total: Approximately $20,000$ species.

  • Discovery Rate: Approximately $150$ new species found every year, many from the deep sea.

  • New Zealand Context: Approximately $1,400$ fish species total. $300\text{--}400$ of these are found in the coastal zone. Offshore fish make up the majority.

  • Tropicalization: New Zealand periodically gains new species during heat waves or hot summers as tropical fish move south and occasionally establish themselves.

Groups of Fish: Jawless and Cartilaginous

• Agnatha (Jawless Fish)

  • Hagfish: Extremely successful and virtually unchanged for $300$ million years. They have a skull but no jaw, no spine, and no paired fins. They live in burrows and are famous for producing a biochemical slime for defense. They have a keen sense of smell and often eat dead or trapped fish (acting as a nuisance to fishermen).

  • Lampreys: Most are freshwater and thus rarely covered in marine biology.

• Chondrichthyes (Cartilaginous Fish)

  • Bones are made of cartilage, a softer and more flexible material than bone.

  • Broad Groups: Sharks, rays/skates, and ratfishes.

  • Ratfishes (Chimaeras): Important in New Zealand as a component of the "fish and chips" industry.

  • Shark Anatomy:

    • Heterocercal Tail: The upper lobe is larger than the lower lobe, providing uplift during swimming.

    • Pectoral Fins: Used for balance and uplift.

    • Placoid Scales: Also called "teeth scales," made of dentin. They give the skin a sandpaper-like texture.

    • Spiracle: A modified gill arch used to help with breathing.

    • Open Gill Slits: Usually $5\text{--}7$ pairs on the sides of the head.

  • Shark Diversity: Approximately $500$ species globally ($113$ in NZ). Range from the $25\,cm$ pygmy shark to the Whale Shark. The Greenland shark can live up to $400$ years. Most are pursuit predators (swimming at $50\text{--}60\,km/h$), but the largest are filter feeders.

• Rays and Skates

  • Flattened bodies with ventral gill slits ($5$ pairs on the underside). Most are benthic.

  • Rays: Possess venomous spines; many use ambush predation (e.g., stingrays).

  • Skates: Lack the venomous spine and produce egg capsules (mermaid's purses).

Groups of Fish: Bony Fish (Osteichthyes)

• Lobe-finned Fish (Sarcopterygii)

  • The Coelacanth: Discovered in South Africa in the $1930s$. Their fins have internal skeletal structures similar to digits/limbs, making them more closely related to amphibians/tetrapods than to ray-finned fish.

• Ray-finned Fish (Actinopterygii)

  • The most common group ($19,000$ species; $1,300$ in NZ).

  • Scales: Cycloid or ctenoid scales. These are smoother, overlapping, and reduce friction while providing a barrier against infection.

  • Operculum: A bony gill cover that protects gills and assists in pumping water (ventilation).

  • Tail: Usually homocercal (lobes are equal in size).

  • Swim Bladder: An internal gas-filled organ used to regulate buoyancy. It allows fish to be neutrally buoyant and save energy. Rapid ascent can cause the bladder to expand and push intestines out of the mouth (barotrauma).

General Fish Biology: Shape, Color, and Motion

• Body Shape and Ecology

  • Streamlined (Fusiform): Designed for speed in the water column (e.g., Tuna).

  • Flattened: Designed for life on the bottom (e.g., Flatfish).

  • Cryptic: Evolved for hiding in specific habitats (e.g., Seahorses in seaweed).

• Coloration

  • Chromatophores: Specialized cells containing pigments.

  • Structural Colors: Use mirror-like reflections.

  • Functions:

    1. Warning (Aposematism): Bright colors indicating toxicity (e.g., Lionfish).

    2. Cryptic/Camouflage: Blending into the background (e.g., Stonefish).

    3. Disruptive: Patterns like stripes that break up the outline or false eye spots that confuse predators about direction.

    4. Countershading: Dark on top (blends with seafloor from above) and white on the bottom (blends with light from below). Common in pelagic fish and sharks.

• Locomotion

  • Undulatory Movement: Side-to-side waves. Large waves involving the whole body (e.g., Eels) are less efficient for speed but good for tight spaces. Moving only the tail (e.g., Tuna) is better for high-speed travel.

  • Fin-based Movement: Used for maneuverability (Parrot fish, Boxfish). Boxfish can move in multiple directions, including backwards.

Sensory Systems and Internal Physiology

• Feeding and Digestion

  • Mouth Diversity: Long snouts (butterfly fish) for picking invertebrates; large mouths/gill rakers for filter feeding; sharp teeth for pursuit predation.

  • Herbivory: Rare in fish; require longer intestines and endosymbiotic bacteria to break down plant material. Parrot fish eat coralline algae and excrete it as white sand.

  • Internal Organs: Two-chambered heart. Bony fish have pyloric ceca (enzymatic extensions for digestion).

• Respiration: Counter-Current Mechanism

  • Blood flows in the opposite direction of water over the gills. This maintains a concentration gradient, allowing for maximum oxygen diffusion from the water ($\sim 8\,mg/L$) into the blood.

• Osmoregulation

  • Saltwater Fish: The environment ($\sim 35\,parts\,per\,thousand$ or $35\,g/L$) is saltier than the blood ($\sim 14\,ppm$). They lose water via osmosis and must drink seawater and excrete salt through kidneys and gills to avoid shrinking.

  • Freshwater Fish: Have higher salt concentration than the water; they must avoid "ballooning" with too much water.

• Advanced Senses

  • Otoliths: Ear stones used for balance and sound detection.

  • Lateral Line: Subsurface canals with neural mass cells that detect pressure changes and movements in the water (vital for schooling).

  • Ampullae of Lorenzini: (Sharks/Rays) Sense weak electric signals emitted by muscle movements.

  • Olfactory Sacs: Extremely sensitive smell (sharks can detect $1\,ppm$ of blood).

Behavior and Reproduction

• Migrations

  • Anadromous: Fish spend adulthood in the ocean and move to freshwater to spawn (e.g., Salmon).

  • Catadromous: Adult life in freshwater, move to the ocean to spawn (e.g., Eels and New Zealand Whitebait).

• Reproduction Modes

  • Broadcast Spawning: Most fish release eggs/sperm into the water. Requires synchronized timing/courtship. High mortality of larvae.

  • Internal Fertilization: Occurs in sharks (using claspers).

  • Protection Strategies: Mouth brooding, guarding nests, egg capsules (skates), and live birth (some sharks).

Questions & Discussion

• Question from student: Do sharks always have to move to breathe?

• Response: Generally, yes, sharks lack an operculum to pump water and rely on forward movement for ventilation. However, some (like nurse sharks) can use limited ventilation to breathe while resting on the seafloor.

• Question from student: Do rays have placoid scales?

• Response: Yes, as cartilaginous fish, rays and sharks both possess placoid scales, though they may be less obvious in some species compared to others.

• Discussion on colors: The instructor noted the sighting of a rare black lionfish in Sri Lanka and used the "Spitfire" airplane analogy to explain countershading in pelagic environments.