Question

Lungs ventilated with water occur in some animab-most notably sea cucumbers - but are rare. Why would water lungs be unlikely to be favored by natural selection? Give as many reasone as passible.

Short answer

Water lungs would be unlikely to be favored by natural selection due to their limitations, which include the low oxygen concentration in water, the higher energy cost of extracting oxygen from water, the limitation to aquatic environments, and the increased risk of contamination.

Step-by-step solution

Understanding Natural Selection

Natural selection is a process where traits that enhance survival and reproduction become more common in successive generations of a population. It acts on the phenotype, or the observable characteristics of an organism.

Analyzing the Functionality of Water Lungs in Sea Cucumbers

Sea cucumbers are a marine species that breathe by extracting oxygen from seawater using a specialized respiratory system known as 'water lungs'. This anatomical adaptation allows them to survive in their specific marine environment.

Identifying the Limitations of Water Lungs

Despite being functional for sea cucumbers, water lungs have their limitations when considering a broader range of species and environments. 1. Oxygen concentration: Air contains significantly more oxygen than water. For many species, breathing via water lungs would not supply enough oxygen to support metabolic processes. 2. Energy cost: Extracting oxygen from water requires more energy due to the viscosity and density of water compared to air. 3. Environmental limitation: This adaptation restricts the organism to aquatic environments, limiting the ability to exploit terrestrial habitats. 4. Risk of contamination: Contaminants and pathogens may be more easily introduced into the body through water.

Key concepts

Water Lungs

Water lungs are an intriguing aspect of marine biology, particularly seen in creatures like sea cucumbers. Unlike typical lungs which work with air, 'water lungs' allow an organism to extract oxygen directly from water. However, this adaptation is relatively rare and has distinct limitations. In the realm of natural selection, these lungs are not generally favored. The main reason is the difference in oxygen availability. Water contains significantly less dissolved oxygen than air, making it inefficient for sustaining high-energy or terrestrial life forms. Furthermore, the energy expenditure for extracting oxygen from water is markedly higher due to the water's viscosity and density.

This makes it a less efficient physiological adaptation compared to air-breathing systems. Additionally, water lungs tie species to aquatic environments, removing the versatility and mobility advantages provided by respiratory systems that function in air.

Sea Cucumbers

Sea cucumbers are remarkable marine organisms found throughout the world's oceans. They possess a unique respiratory system called 'water lungs,' or more technically, the respiratory tree. These are branching tubes in their body that draw in seawater to extract oxygen, which is then absorbed into their tissues. This system reflects an extraordinary adaptation to their underwater habitat.

These creatures play a critical role in the marine ecosystem by recycling nutrients and helping in the breakdown of organic matter. Their ability to breathe through water lungs allows them to thrive in ocean environments where oxygen supply is usually lower. Sea cucumbers demonstrate how specific adaptations enable survival in specialized ecological niches.

Oxygen Concentration

Oxygen concentration is a crucial factor in the respiration process for all living organisms. Air has about 21% oxygen, a much higher level than in water, which usually contains only a fraction of a percent. This disparity is why breathing air is more efficient.

For organisms with water lungs like sea cucumbers, the challenge lies in extracting adequate oxygen from a medium with inherently lower availability. This limits the speed and intensity of their metabolic processes compared to air breathers. Water's density and viscosity also mean that the mechanical act of drawing water across respiratory surfaces requires significantly more energy than moving air, further compounding the oxygen procurement challenge for water lung bearers.

Marine Adaptation

Marine adaptation refers to the various changes organisms undergo to thrive in ocean environments. In the case of species with water lungs, adaptations are primarily about making the most of available resources and overcoming the limitations posed by aquatic life.

Key adaptations include specialized respiratory systems to extract oxygen from water, osmoregulation to handle the salinity of marine environments, and various physiological and behavioral traits to maximize energy efficiency. Such adaptations allow marine organisms to occupy diverse ecological niches, but they also anchor them in water-bound habitats, offering little flexibility to transition to terrestrial life. Despite these constraints, marine adaptations enable species to thrive in some of the earth's most dynamic and competitive environments.