Zoo geography

1. From Taxonomy to Zoogeography

So far, fish diversity has been described using:

  • Taxonomy (classification)

  • Phylogeny (evolutionary relationships)

This lecture introduces zoogeography, another way to describe diversity, focusing on geographic distribution patterns rather than ancestry alone.

Definition: Zoogeography

A branch of biogeography concerned with:

  • The geographic distribution of animals

  • Identifying regions characterised by specific faunal groups

  • Understanding the causes and significance of these distribution patterns

👉 In this module, “fish” refers to teleost (bony) fishes, unless stated otherwise.

2. Earth as a Marine Planet

Although called “Planet Earth”, it is more accurately Planet Water:

  • 71% of Earth’s surface is covered by water

  • Oceans are three-dimensional habitats

  • Mean ocean depth ≈ 4000 m

  • Maximum depth > 11,000 m (trenches)

Marine vs Terrestrial Living Space

  • Marine living volume is ~300× larger than terrestrial living space

  • The deep ocean is the largest habitat and ecosystem on Earth

3. Comparing Marine and Terrestrial Biodiversity

Species Diversity

  • Terrestrial systems have more species overall

    • Driven by:

      • Tropical biodiversity

      • Insects (millions of species)

  • Marine insects are extremely rare

Phyletic (Phylum-Level) Diversity

  • 33 animal phyla exist

  • 32 phyla occur in marine environments

  • 15 phyla are exclusively marine

👉 Marine environments dominate in phyletic diversity, even if terrestrial systems dominate in species counts.

4. Global Water Distribution

  • 97.5% of Earth’s water = saltwater (~1.4 billion km³)

  • 2.5% = freshwater

    • ~69% locked in glaciers & permanent snow

    • ~30% groundwater

    • Only 0.3% of freshwater is surface water (rivers, lakes, streams)

Despite this tiny fraction, freshwater supports ~41% of all teleost species.

5. Teleost Fish Diversity Overview

  • Total fish species ≈ 37,000–37,500

  • Teleosts ≈ 36,000 species

    • 58% marine

    • 41% freshwater

    • 1% diadromous (migrate between freshwater and marine)

Marine Teleosts

  • ~40% live in shallow tropical waters (mainly coral reefs)

  • ~5.6% in temperate shelf seas

  • ~11.4% in deep sea (pelagic + benthic)

Freshwater Teleosts

  • ≈ 14,400 species

  • Found in <0.01% of Earth’s total water volume

  • Indicates exceptionally high diversification rates

6. Freshwater Fish Categories

Primary Freshwater Fish

  • Strictly freshwater

  • Cannot tolerate brackish or saltwater

  • Dispersal highly restricted

Secondary Freshwater Fish

  • Prefer freshwater

  • Can tolerate short exposure to brackish/saltwater

Peripheral Freshwater Fish

  • Freshwater species belonging to predominantly marine lineages

  • Examples:

    • Freshwater cod (Gadiformes)

    • Freshwater flatfishes (South America)

    • Many Australian freshwater fishes

Diadromous Fish

  • Anadromous: spawn in freshwater, feed at sea (e.g. salmon)

  • Catadromous: spawn at sea, feed in freshwater (e.g. eels)

7. Marine Fish Diversity by Ecological Zones

Coastal Shelf Seas

  • ~79% of marine species

  • Most productive marine habitats

  • High light availability

  • Nutrient input from land runoff

  • Coral reefs = major biodiversity hotspots

Epipelagic Zone (0–200 m)

  • ~460 species (~1.3%)

  • Examples:

    • Herrings, anchovies, sardines

    • Tunas, mackerels

  • Many capable of long-distance migrations

  • Some species globally distributed

Deep Sea (>200 m)

  • Zones:

    • Mesopelagic (200–1000 m)

    • Bathypelagic (1000–4000 m)

    • Abyssopelagic (>4000 m)

  • ~4100 species described

  • Lanternfishes among most abundant and widespread vertebrates

8. Distribution Patterns: Global vs Allopatric Species

Globally Distributed Species

  • High dispersal ability

  • Example:

    • Albacore tuna

    • Yellowfin tuna

Allopatric Species

  • Closely related species separated geographically

  • Example:

    • Atlantic vs Pacific herring

    • Different Euthynnus tuna species

Convergent Evolution

  • Similar body forms evolve independently due to similar selection pressures

  • Common traits:

    • Fusiform bodies

    • Forked tails

    • Countershading

  • Adaptations for long-distance swimming

9. Barriers to Marine Dispersal

  • Continents

  • Open ocean expanses (for coastal species)

  • Underwater sills

Mediterranean Sea Example

  • Connected to Atlantic via Strait of Gibraltar (300 m deep)

  • Deep-water species cannot enter

  • Warmer deep-water temperatures (~14°C vs ~2–3°C)

  • Additional sill divides eastern and western Mediterranean

  • Result: low species diversity

10. Marine Zoogeographic Regions (Simplified Model)

Major Regions

  • Indo-West Pacific (most diverse)

  • Western Atlantic

  • Eastern Pacific

  • Eastern Atlantic

  • Mediterranean

  • Arctic

  • Antarctic

Indo-West Pacific

  • ~4000 species

  • Global biodiversity hotspot

  • Centre: Malay–Philippines Archipelago

  • High endemism

  • Species richness declines with distance from centre

Western Atlantic

  • ~1000 species

  • Highest diversity in Caribbean reefs

  • Amazon River outflow limits coral reef continuity

Eastern Pacific

  • Isolated from West Pacific by open ocean

  • Shared evolutionary history with Western Atlantic (pre-Panama Isthmus)

Eastern Atlantic

  • ~500 species

  • Low diversity due to:

    • High river discharge (sediment, freshwater)

    • Limited coral reef development

11. Panama Isthmus and Speciation

  • Closed ~3 million years ago

  • Previously allowed faunal exchange

  • Closure caused vicariant speciation

  • Produces closely related “geminate species” on either side

12. Polar Regions

Arctic

  • ~89 species

  • Low endemism

  • Dominated by temperate species at range limits

Antarctic

  • ~52 species

  • ~88% endemic

  • Notothenioids dominate

  • Antarctic = landmass surrounded by sea

  • Arctic = sea surrounded by land

13. Why Is Freshwater Fish Diversity So High?

Key Reasons

  1. High productivity

    • Shallow, well-lit systems

    • High nutrient input from land

  2. Geographic isolation

    • Rivers, lakes, drainage basins

    • Barriers restrict gene flow

    • Promotes allopatric speciation

14. Freshwater Zoogeographic Regions (Wallace)

Six major regions:

  • Nearctic

  • Neotropical

  • Palearctic

  • Afrotropical

  • Oriental

  • Australian

Neotropical Region

  • Most diverse freshwater fauna globally

8000 species

  • Amazon basin major hotspot

  • High rates of species discovery

  • Many peripheral freshwater species

Australian Region

  • Only two primary freshwater species

    • Australian lungfish

    • Saratoga

  • Dominated by peripheral freshwater species

  • Highly distinctive fauna

15. Archaic Freshwater Fish and Continental Drift

Archaic Groups

  • Lungfishes

  • Bichirs & reedfish

  • Paddlefishes

  • Gars

  • Bowfin

  • Osteoglossiformes (bony tongues)

Distribution Puzzle

  • Lungfish in Africa, South America, Australia

  • Paddlefish in North America & China

  • Gars found across multiple continents (fossil evidence)

Explanation

  • Vicariance due to continental drift

  • Some dispersal via freshwater connections during continental breakup

16. Pangaea and Plate Tectonics

  • Pangaea existed ~300 million years ago

  • Split into:

    • Laurasia (North America, Eurasia)

    • Gondwana (Africa, South America, Antarctica, India, Australia)

  • Continental drift explains modern fish distributions

17. Key Take-Home Messages

  • Zoogeography explains where fish live and why

  • Marine fish diversity structured by depth and geography

  • Freshwater fish diversity driven by productivity and isolation

  • Ancient fish distributions best explained by continental drift

  • Biodiversity patterns reflect both ecology and Earth history