Environmental Considerations

35 phyla of invertebrates Half are entirely marine Introduction

Physical properties of salt water, fresh water, and air determine the structure, physiology, and behavior of animals

Marine vs. Terrestrial Invertebrates Differ with regard to Gas exchange Fertilization Excretory systems

Gas Exchange: Marine Marine invertebrates exchange gases through their body surface, so their body walls are thin and water permeable. Respiratory structures may be external

Gas Exchange: Terrestrial Terrestrial invertebrates have to cope with dehydration, so their body wall is not permeable to water Respiratory structures are internal

Fertilization: Marine vs. Terrestrial Marine invertebrate fertilization is simple Terrestrial invertebrates require more complex reproductive systems

Excretion: Marine vs. Terrestrial Marine invertebrates constantly eliminate ammonia (a toxic waste product) Terrestrial organisms incorporate ammonia into less toxic compounds like urea, which is then excreted in a small amount of water. They require more complex excretory systems.

Benefits of Water Aquatic animals can take dissolved nutrients from surrounding water by diffusion or active uptake

Benefits of Water Terrestrial embryos must be supplied with nutrition, and be protected from desiccation.

Benefits of Water Being suspended in nutrient rich water allows primary producers to be small, suspended, single celled organisms (phytoplankton); roots are not mandatory

Benefits of Water Water is denser than air, so a rigid skeleton is not required in water. Yet, water supports delicate structures such as gill filaments, which would collapse in air.

Benefits of Water Animals can move with greater efficiency in water, expending less energy, or no energy at all. Since phytoplankton and zooplankton live in suspension, other aquatic animals can remain stationary and capture them as food as they flow past. This is called suspension feeding.

Benefits of Water Water supports sperm, eggs, and the embryo as it develops. External fertilization and external larval development is more common than internal. Embryos and larva of aquatic invertebrates often serve as the dispersal stages for sedentary adults. This is the opposite from terrestrial animals.

Benefits of Water Water temperature is stable when air temperature fluctuates. Metabolic rates are altered by temperature, wide fluctuations are stressful to most invertebrates. Terrestrial invertebrates require structures that allow them to adapt to temperature fluctuations.

Disadvantages of Water Habitats Light is extinguished over a much shorter distance in water than in air, so primary production is limited to the upper 50 m.

Disadvantages of Water Habitats The time required for a given molecule to diffuse across water is much greater than the air. An organism sitting motionless in water would have difficulty with gas exchange. The slightest movement of water enhances gas exchange significantly.

Disadvantages of Water Habitats Water has greater density and viscosity than air. Animals swimming through it have more frictional resistance than air, especially as temperatures drop. Water moves around bristly appendages; rake-shaped objects behave like solid paddles, so they cannot readily filter food particles from the water.

Disadvantages of Water Habitats Since water is a universal solvent, aquatic animals are in intimate contact with pollutants. Since many aquatic invertebrates are small, the surface area, across which pollutants can diffuse, is high relative to the animal’s volume. Larval stages are particularly sensitive.

Disadvantages of Small Fresh Water Habitats Smaller water bodies may be ephemeral (they dry up seasonally).

Fresh Water Habitats Marine invertebrates are approximately in osmotic equilibrium with the medium in which they live Freshwater invertebrates Marine invertebrates

Fresh Water Habitats Freshwater invertebrates are hyperosmotic compared to the surrounding medium. Since they are hyperosmotic to their surroundings, water diffuses inward. Complete impermeability of water is not possible because respiratory surfaces must remain permeable for gas exchange.

Fresh Water Habitats Freshwater organisms must constantly expel incoming water, so they also need special mechanisms for reclaiming salts from the urine before the urine leaves the body.

Fresh Water Habitats The pH of salt water is less variable than freshwater. Most freshwater environments lack buffering capacity, and the pH may change quickly with the addition of acid or base elements.