Ecophysiology Lecture Notes: Crustaceans

Introduction to Crustaceans
- Last day of the Ecophysiology course, focusing on crustaceans.
- Major topics include evolutionary origins, physiology, molting, and circulation.

Phylum Arthropoda:
- Contains approximately 80% of recorded extant animal species.
- First recorded fossils of crustaceans date back 500 million years.
Common Ancestor:
- Last common ancestor shared between chordates (fishes, mammals) and crustaceans existed around 1 billion years ago.
Focus on Decapod Crustaceans, especially the class Malacostraca.

Key Species:
- Include commercially important species like prawns, crayfish, crabs, lobsters, and mantis shrimp.
Anatomy:
- Body is generally segmented into three regions:
- Cephalon: Head
- Thorax: Mid-body region (often fused with the head to form cephalothorax)
- Abdomen: Pleon, colored orange in diagrams.
Each segment has a pair of appendages.
Exoskeleton:
- Composed of chitin and protein, mineralized with calcium.
- Muscles attach to the inside of the exoskeleton (difference from chordates).

Layers:
- Epi-, Exo-, and Endocuticle:
- Epicuticle: Outer layer made of protein, chitin, and lipids.
- Exocuticle and Endocuticle: Composed of organic matrix, primarily chitin, protein, and calcium.
- Membranous layer lies near living cells.
Calcification:
- Involves calcium-binding proteins within chitin matrix, providing high tensile strength.

Molting Mechanism:
- Crucial for crustacean growth, occurring in a stepwise fashion.
- Involves shedding the old carapace and softening the new shell by taking in water.
Stages of Molting Cycle:
- Post-Molt: New, soft exoskeleton is formed. Vulnerable to predation.
- Intermolt: Hard cuticle achieved, with significant tissue growth.
- Pre-Molt: Preparation for molting; new cuticle forms, feeding decreases.
Hormonal Control:
- Regulated by the endocrine system via X organ (molt-inhibiting hormone) and Y organ (ecdysteroids).
- Triggered by a combination of internal (appendage loss, development states) and external (temperature, light cycles, stress) factors.

Enzymes such as chitinases, clitobiases, and proteases play critical roles.
Calcium Homeostasis:
- Vital for molting; freshwater crustaceans form gastroliths to store calcium due to environmental variability.

Circulatory Type:
- Most crustaceans have an open to semi-closed circulatory system.
Heart Structure:
- Single-chambered heart with three pairs of ostia to receive hemolymph and seven arteries for blood distribution.
- Valves control the flow of hemolymph.
Circulation Time:
- Takes approximately 40 seconds for hemolymph to circulate in larger decapod species.

Hemolymph Composition:
- Contains hemocyanin, a copper-based respiratory pigment that is blue.
- Differs from hemoglobin (iron-based, red) in structure and function.
Efficiency Comparison:
- Hemocyanin is less efficient than hemoglobin but is adaptable to various environmental conditions.
- More suitable for the oxygen demands of decapods in fluctuating environments.

Responses to Environmental Changes:
- Cardiac output adjusts to oxygen and temperature demands; increase in heart rate correlates with temperature increases to maintain oxygen supply.
Temperature Dependency:
- Heart rate increases with temperature (tachycardia) until a threshold is reached, after which performance declines.

Decapod crustaceans have a semi-closed circulatory system with hemolymph containing hemocyanin for oxygen transport.
Molting is a crucial physiological process regulated by hormones and influenced by both internal and external conditions.
The anatomical adaptations and circulatory characteristics support their survival and functionality in diverse marine environments.