Marine Science (9693) | Life cycles

🐟 Chapter 8.1: Marine Life Cycles - The Foundation of Future Fisheries

Welcome to the first chapter in our A Level section on Fisheries! Understanding how marine animals reproduce and grow is absolutely fundamental to making fisheries sustainable. If we don't know the critical stages of an organism's life—when they breed, where their babies live, and how fast they grow—we can’t manage fishing properly. This chapter breaks down the different strategies marine species use to survive from egg to adult.

Ready to dive in? Let's go!

Understanding Life Cycle Terminology (8.1.1)

Many marine organisms have complex lives involving dramatic changes in body structure and location. You need to know these key definitions:

1. Sessile vs. Non-Sessile Organisms

• Sessile: Organisms that are fixed in one place and cannot move.
  Example: Mussels, oysters, corals, barnacles.
• Non-Sessile: Organisms that are free-moving or motile.
  Example: Fish (tuna, shark), marine mammals (whales), crabs.

Quick Check: Sessile organisms still need to spread their offspring! This is why their early life stages (larvae) are mobile.

2. Larval Stage and Metamorphosis

A larval stage is an immature phase that looks completely different from the adult form and often occupies a different ecological niche or habitat.

Metamorphosis is the biological process where an animal physically develops after hatching or birth, involving an abrupt and noticeable change in the animal's body structure. Think of it like a dramatic makeover!

• Analogy: A larva is like a caterpillar, and the adult form is the butterfly. The dramatic change between them is metamorphosis.

For marine species, metamorphosis typically changes the organism from a small, planktonic form into its final, adult form that settles on the seabed or becomes a free-swimming predator.

Simple vs. Complex Life Cycles (8.1.2)

Marine life cycles can be broadly categorised based on whether they include a larval stage and metamorphosis.

1. Complex Life Cycles

Complex life cycles include a distinct larval stage that undergoes metamorphosis before becoming an adult.

• Presence of Larval Stage: Yes, often planktonic and adapted for dispersal or rapid feeding.
• Metamorphosis: Necessary to transition from larva to juvenile/adult.
• Example: Crustaceans (like crabs or lobsters).
  • Lobsters begin life as tiny, planktonic larvae (e.g., phyllosoma).
  • They drift in the water column, feeding on plankton.
  • They undergo several moults and dramatic morphological changes (metamorphosis) before eventually settling on the seabed as tiny, benthic juveniles.

Don't worry if the specific larval names seem complicated; the key is recognising the existence of this distinct, separate stage.

2. Simple Life Cycles

Simple life cycles lack a distinct larval stage and typically involve direct development. Juveniles look like small versions of the adults.

• Presence of Larval Stage: No (or a very brief, non-feeding stage).
• Metamorphosis: Generally absent.
• Example: Marine Mammals (like whales, dolphins, seals).
  • Whales give birth to live young that are essentially miniature adults.
  • Development is direct, involving growth rather than structural change.
  • They also rely on high parental care (mammalian trait).

The Importance of Life Cycle Stages (8.1.3)

Each stage of a life cycle—egg, larva, juvenile, adult—is adapted to maximise survival, feeding, or dispersal. The importance varies greatly between sessile and non-sessile organisms.

A. Importance for Sessile Organisms (e.g., Oysters, Corals)

Since adults are fixed, the mobile larval stage is critical for:

1. Dispersal: Larvae drift on currents (planktonic), allowing the species to colonise new, distant habitats. This prevents local overpopulation and inbreeding.
2. Avoiding Competition: By moving away from the crowded adult population, larvae reduce competition for space and food.
3. Gene Flow: Dispersal ensures genetic mixing between different populations, maintaining high genetic diversity.

Think of the larval stage as getting a bus ticket to a new neighbourhood before settling down permanently.

B. Importance for Non-Sessile Organisms (e.g., Bony Fish)

While adults are mobile, many non-sessile organisms still benefit from a distinct larval stage, which is usually planktonic:

1. Exploiting Different Food Sources: Larvae often eat different, smaller food (phytoplankton or zooplankton) than the adults, reducing competition between generations.
2. Growth: The planktonic environment allows for rapid growth while avoiding the high energy cost of swimming long distances.
3. Recruitment: The larval stage must eventually transform and successfully recruit (move into) the adult habitat (e.g., coral reef or open ocean). This transition point is often where the highest mortality occurs.

Fertilisation Strategies and Parental Investment (8.1.4)

Reproduction methods are divided into two main groups, which are strongly linked to how much energy the parent puts into raising the young (offspring investment).

1. External Fertilisation (Tuna Example)

External fertilisation involves the male and female releasing their gametes (sperm and eggs) into the surrounding water where fertilisation occurs.

Species Example: Tuna (A bony fish)

• Strategy: Mass spawning (releasing millions of gametes simultaneously).
• Advantages:
  1. High Numbers: Produces a vast number of offspring, ensuring some survive the high rate of predation.
  2. Wide Dispersal: Gametes and resulting larvae are spread widely by currents.
• Disadvantages:
  1. Waste: Most gametes are lost, eaten, or fail to fertilise.
  2. Environmental Dependency: Requires precise timing and ideal water conditions (temperature, salinity).
  3. Low Offspring Investment: Zero parental care beyond gamete release.

2. Internal Fertilisation (Sharks and Whales Examples)

Internal fertilisation occurs inside the body of the female. This is typical of marine mammals and cartilaginous fish.

Species Example 1: Sharks (Cartilaginous fish)

• Strategy: Internal fertilisation, usually producing fewer, larger young.
• Advantages:
  1. High Fertilisation Success: Gametes are protected, increasing the chance of fertilisation.
  2. Increased Survival: Young are often born or hatched at a larger, more developed stage.
• Disadvantages:
  1. Fewer Offspring: Due to the higher energy cost per individual.
  2. Energy Cost to Mother: Requires significant energy investment during gestation.

Species Example 2: Whales (Marine Mammals)

Whales exhibit the highest investment in offspring care.

• Strategy: Internal fertilisation, extended gestation, and extensive maternal care.
• Advantages:
  1. Very High Survival Rate: Young are protected and nurtured, leading to a much higher chance of reaching adulthood.
  2. Maternal Training: Young learn complex behaviours and migration routes from the mother.
• Disadvantages:
  1. Extremely Low Fecundity: Whales produce only one calf every few years.
  2. Long Generation Time: Slow population recovery if stocks are depleted (a critical issue for fisheries management).

⚠ Fisheries Relevance: Offspring Investment

The level of parental investment directly impacts how vulnerable a species is to overfishing:

• Species with low investment (like tuna, high fecundity) can recover quickly, but require strict management during spawning seasons.
• Species with high investment (like whales and sharks, low fecundity) take decades to recover from population crashes, making sustainable exploitation incredibly challenging.