Shark Sensory ecology

1. Introduction & Context

• This lecture provides essential background for next week’s practical (shark dissection and brain morphology lab).

• Aim: Understand how sensory ecology and brain structure vary with:

  • Environment (light, depth, habitat type)

  • Behaviour (predation, navigation)

  • Phylogeny (evolutionary relatedness)

• Sharks have evolved an incredible range of sensory capabilities to adapt to diverse ecological niches — from shallow reefs to deep ocean trenches.

2. Sensory Ecology: Key Ideas

Definition

• Sensory ecology = how organisms acquire and use sensory information from their environment.

• It dictates:

  • Prey detection

  • Predator avoidance

  • Navigation

  • Communication and mating

• Evolution shapes sensory organs and brain regions to match the demands of each habitat.

Genetic vs Environmental Determinants

Two hypotheses:

1. Phylogenetic Determinism:
   Closely related species have similar sensory adaptations (genetic legacy).

2. Environmental Determinism:
   Sensory adaptations arise independently in response to ecological conditions.

→ In reality, both genetics and environment interact to shape sensory evolution.

3. The Shark Brain and Sensory Systems

Key Concept

• The brain morphology (shape, size, complexity) reflects which senses are most developed.
  Example: Sharks that rely heavily on vision have a larger optic lobe; those using electroreception have expanded hindbrains.

• Brain = central processor of:

  • Vision

  • Smell

  • Electroreception

  • Mechanoreception (lateral line)

  • Audition and balance

Structure Overview

• Forebrain: olfactory & sensory integration.

• Midbrain: vision (optic tectum).

• Hindbrain: balance, motor control, and electroreception (cerebellum, medulla).

4. Vision in Sharks

Depth and Light Gradients

• Light decreases with depth — photosynthetically active radiation attenuates rapidly.

• Shallow, tropical waters = bright; deep, temperate waters = dim.
  → Sharks must adapt visually to different light environments.

Adaptation 1: Eye Size

• Hypothesis: Larger eyes in low-light environments capture more light.

• Data:

  • Eye diameter (as % of body length) increases with depth.

  • Deep-water sharks have larger relative eye size than shallow species.

Adaptation 2: Other Ocular Modifications

Even if eye size remains constant, sensitivity may vary due to:

• Increased photoreceptor density

• More rod cells (low light vision)

• Enhanced visual pigments

• Wider pupils

• Improved retinal summation

→ Eye size alone ≠ complete measure of visual capability, but a useful proxy.

5. Electroreception

Definition

• The ability to detect electric fields in the environment.

• Used for:

  • Prey detection (e.g. fish heartbeats)

  • Navigation (Earth’s magnetic field)

  • Communication

  • Hunting in low-visibility conditions

Mechanism

• Ampullae of Lorenzini: Specialized electroreceptive pores filled with iron-rich glycoprotein gel.

  • Conducts electricity similar to seawater.

  • Located mainly on the head and snout.

  • Connected to sensory cells → transmit signals to brain.

Example (Anecdote)

• Sharks respond to heartbeats of divers — can track stressed individuals with rapid heart rates.

• During attacks, sharks roll eyes back to protect them → rely solely on electroreception for final strike.

Evolutionary Insights

• Early research suggested species-specific (phylogenetic) pore distributions.

• Later work found:

  • Intraspecific variation (differences between individuals of same species)

  • Convergent patterns between unrelated species in similar habitats.

• Conclusion: Both phylogeny and environment influence pore distribution.

6. Electroreceptive Pore Distribution

Measuring & Mapping

• Each shark species has hundreds of pores, mapped across dorsal (top) and ventral (bottom) head surfaces.

Findings

1. Deep-water sharks → Fewer pores (possibly because of simpler sensory needs, stable environments).

2. Coastal pelagic sharks → Most pores (require acute detection for fast-moving prey, navigation, and social behaviour).

Functional Distribution

• Most species have more ventral pores → key for benthic and ambush predators.

Critical Thinking

• Pore number ≠ sensitivity.

• Species with fewer pores might compensate with higher receptor sensitivity or better neural processing.

7. Standardising Morphological Data

When comparing sensory organs or brain size across species:

• Must standardise measurements to account for body size.

• For example:

  • Eye diameter → % of total length.

  • Brain weight → % of total body weight.

Example:
Larger sharks naturally have heavier brains; using proportional data prevents misleading comparisons.

8. The Shark Brain in Detail

Major Divisions

During the dissection:

• You’ll identify and weigh these regions.

• Data will help infer sensory emphasis (e.g., larger cerebellum = complex movement).

Standardising Brain Data

• Express total brain weight and component weights as % of body weight.

• Allows inter-species and inter-habitat comparison.

9. Habitat Categories and Expected Sensory Adaptations

Sharks are grouped by habitat type, which influences sensory priorities.

Trends Observed

• Eye Size: Deep-water species → largest relative eyes.

• Electroreceptive Pore Number: Coastal pelagic → highest count.

• Pore Distribution: Most species have more ventral pores (for benthic detection).

• Brain Complexity: Increases with behavioural complexity and habitat openness.

10. Brain and Behaviour Relationships

Cerebellum Complexity (Foliation)

• Folds = processing power.

• Species with complex, agile movement (e.g., thresher shark) have highly folded cerebellums.

• Benthic species = smoother cerebellums (less fine motor control needed).

→ Conclusion: Brain morphology directly reflects movement, behaviour, and habitat complexity.

11. Practical Application

In the Dissection Lab

You will:

1. Weigh your shark before decapitation (needed for standardisation).

2. Dissect and remove the brain carefully.

3. Divide it into regions (forebrain, midbrain, hindbrain).

4. Weigh and record each component.

5. Map electroreceptive pores (dorsal vs ventral).

6. Measure eye diameter as % total length.

All class data will be pooled → analysed collectively for your assignment.

12. Cerebral Types & Comparative Analysis

• Sharks can be grouped by “cerebral type” based on brain region proportions.

• Similar brain structures occur in species with similar lifestyles, even if not closely related.

• Reflects convergent evolution toward functionally efficient sensory systems.

13. Life History and Sensory Ecology

Behavioural Influence:

• Species chasing fast prey → need advanced vision, coordination, and larger cerebellums.

• Slow benthic hunters → rely more on electroreception and smell.

Habitat Influence:

• Dark or turbid environments → rely on non-visual cues.

• Open ocean → rely on vision and hydrodynamic sensitivity.

Phylogeny Influence:

• Related species may share similar sensory architectures, though habitat often overrides ancestry.

14. Summary: Major Themes

15. Key Readings Recommended

1. K.A. Yopak (2012):
   Brain organization in sharks and its relationship to sensory ecology.
   – Excellent overview of brain region functions and evolutionary trends.

2. Yopak & Montgomery (2008):
   Comparative brain morphology in elasmobranchs.
   – Links between behaviour, habitat, and sensory specialization.

3. Optional viewing:

   • National Geographic: Cora Yopak’s Great White Shark Brain Dissection.

16. Assignment Tips

• In your write-up:

  • Compare catshark data to literature and other shark species.

  • Discuss how results reflect habitat and lifestyle.

  • Critique data (e.g., measurement limits, potential errors).

  • Use proportional metrics (% body length, % body weight).

  • Consider both phylogenetic and ecological explanations.

17. Closing Notes (Lab Preparation)

• Bring lab coat (mandatory due to formalin use – carcinogenic).

• Work in pairs for efficiency and accuracy.

• Don’t panic about individual results – data will be compiled and shared for analysis.

• Virtual dissection video available on YouTube (recorded by Judy Wright – tribute resource).

Core Message