Problem 166 - Objective Biology for NEET

Question

Which of the following are marine but migrate for spawning to fresh water. After spawning, within a few days, they die. Their larvae, after metamorphosis, return to ocean. Specify the correct one. (a) Petromyzon (Lamprey) (b) Myxine (Hagfish) (c) Scoliodon (d) Both (a) and (b)

Step-by-Step Solution

Answer

(a) Petromyzon (Lamprey)

Step 1: Identify the nature of species

Identify the nature and spawning habits of each fish species. Out of Lamprey, Hagfish and Scoliodon, only Lamprey (Petromyzon) and Hagfish (Myxine) species are known to be of marine nature and migrate to fresh water for spawning.

Step 2: Identify spawning behavior

Identify the species which die within a few days after spawning and whose larvae return to the ocean after metamorphosis. Only Petromyzon (Lamprey) exhibits this behavior.

Step 3: Confirmation

After having analyzed each individual option, the correct answer is (a) Petromyzon (Lamprey) as it fits all the given conditions.

Key Concepts

Lamprey

Lampreys are fascinating, eel-like parasites known for their jawless mouths filled with sharp, keratinized teeth. These interesting creatures primarily live in marine environments but have a unique spawning process. Lampreys exhibit an anadromous migration pattern, meaning that they travel from saltwater to freshwater to reproduce. This behavior is similar to that of salmon.

Once they have reached a suitable freshwater habitat, they build small nests on the riverbed where they spawn. A remarkable aspect of their life cycle is that adult lampreys die shortly after spawning, thus never returning to the ocean themselves. This contrasts with their larvae, known as ammocoetes, which undergo a long metamorphosis before making their way to the ocean. Months to years later, these transformed young lampreys return to saltwater, continuing the life cycle in a marine environment. This unique reproductive strategy contributes to the ecological balance in both marine and freshwater systems.

Hagfish

Hagfish, commonly referred to as 'slime eels', are intriguing marine creatures that bear no resemblance to traditional fish. These jawless vertebrates are known for their ability to produce copious amounts of slime as a defense mechanism against predators. Despite being marine inhabitants, hagfish differ from lampreys in their spawning habits.

Rather than migrating to freshwater environments to spawn, hagfish remain within the marine ecosystem throughout their life cycle. They have a fascinating reproduction process where the female lays eggs that are covered with a protective, leathery casing. Although hagfish do not show the same anadromous behavior as lampreys, they are important scavengers in the ocean, aiding in the recycling of nutrients and maintaining healthy marine ecosystems.

Spawning Behavior

Spawning behavior in marine animals can vary significantly, providing insights into their ecological strategies and adaptations. For certain species like the lamprey, spawning involves a dramatic journey from the ocean to freshwater rivers or streams. This anadromous behavior is characterized by precise timing to ensure that the environment is optimal for the survival of their offspring.

Like the lamprey, many anadromous fish spawn in freshwater after migrating from the sea. This migration is often synchronized with environmental cues such as water temperature and light availability. Once the eggs are laid and fertilized, the adult fish usually do not survive long afterward, as their physiological energy is heavily invested in this reproductive endeavor.

• After hatching, larval stages often occur in freshwater.
• Gradual metamorphosis allows the young to adapt before returning to the ocean.
• This strategy helps to reduce predation risks and provides rich nutrient environments for growth.

Each species, through their unique spawning behaviors, contributes to the ecological richness and stability of both freshwater and marine habitats.