Welcome to Chapter 3.6: Animal Life Cycles!
Hi everyone! This chapter is all about the incredible journeys marine animals take from birth to maturity. Understanding life cycles is crucial in Marine Science because it helps us protect species and manage ecosystems. If a species can't reproduce successfully, it can't survive!
We will focus on two amazing examples: the massive leatherback turtle and the tiny, colony-forming coral polyp.
Quick Prerequisite Recap (Reproduction, 3.2):
Before diving into life cycles, remember the two main types of reproduction:
• Asexual Reproduction: A single organism produces genetically exact copies of itself. This is fast and simple.
• Sexual Reproduction: Organisms produce male and female sex cells (gametes) that fuse. The offspring have characteristics from both parents, leading to genetic variation.
The Life Cycle of the Leatherback Turtle
The leatherback turtle (*Dermochelys coriacea*) is the largest sea turtle species, famous for its marathon migrations. Its life cycle involves incredibly long periods spent at sea, punctuated by brief, vital visits to the shore for nesting.
Key Facts About Leatherback Reproduction
• Females reproduce every 2–5 years.
• They show strong nesting site fidelity, meaning they return to the sandy shore where they originally hatched to lay their own eggs.
• Males live entirely at sea and never return to the shore after hatching.
Step-by-Step: The Leatherback Nesting Cycle
1. Returning to Shore:
The female turtle crawls up the beach, usually at night, and uses her large flippers to excavate a nest in the sand.
2. Egg Laying:
She lays a large clutch of eggs. Remarkably, she will repeat this process, laying several clutches of eggs at roughly 10-day intervals during a single nesting season. After laying, she buries the eggs and departs, leaving the offspring to develop on their own.
3. Temperature and Sex Determination (TSD):
This is a super important concept! The temperature inside the nest determines the sex of the hatchlings. This is called Temperature-Dependent Sex Determination.
• High incubation temperatures produce females.
• Low incubation temperatures produce males.
(Memory Aid: Think of the oven being hot for the girls baking cookies!)
4. Hatching and Sea Rush:
The eggs hatch after 55–60 days. The hatchlings emerge, often at night, and undertake the dangerous journey straight to the sea.
5. Growth and Maturity:
The young turtles feed and grow in the ocean, remaining at sea for decades. They will only reach sexual maturity after a very long time, typically 15–25 years.
The two most common exam points are Temperature-Dependent Sex Determination (high temp = female) and the fact that males live entirely at sea.
The Life Cycle of a Coral Polyp
Coral reefs are made of tiny animals called polyps. These organisms reproduce using both asexual and sexual strategies, which is key to how they form large, complex reef structures.
Two Ways to Make a Coral Polyp
1. Asexual Reproduction (Budding)
This is how a single polyp grows into a whole colony.
• A parent polyp produces a new polyp right next to itself.
• This process is called budding.
• Budding increases the size of the existing coral colony rapidly.
2. Sexual Reproduction (Spawning)
This is generally used to start new colonies in different locations and introduce genetic diversity.
Step-by-Step: Sexual Reproduction and Larval Stages
Phase A: Synchronised Spawning
• Corals release huge numbers of eggs and sperm into the water column. This event is often highly synchronised, happening simultaneously across an entire reef—sometimes called a 'mass spawning' event.
Phase B: Fertilisation and Planktonic Life
• The eggs and sperm fuse in the water (external fertilisation) to create fertilised eggs.
• These eggs float to the surface and develop into planktonic larvae (tiny, drifting organisms).
• Did you know? These larvae are part of the zooplankton! They feed and grow while they are moved around by ocean currents.
Phase C: Settlement and Growth
• The larvae are sensitive to light (they swim towards the light initially).
• Once they are ready to settle, they descend (sink) in the water column.
• They seek out and attach to a hard surface on the seabed.
• Once attached, they metamorphose and grow into new coral polyps, starting a brand new coral colony.
Corals use budding for colony growth and synchronised spawning to create planktonic larvae that drift on currents to find new settlement locations.
Final Thoughts
Understanding these life cycles highlights the vulnerabilities of marine animals. Leatherback turtles need clean, undisturbed beaches, and corals need clean, unpolluted water for their larvae to successfully settle. Your study of these cycles is a vital step in learning how to protect these amazing marine organisms!