2025 MA Lecture 8 - Cnidaria - scleractinians, coral reefs, soft corals

Online Quiz and Exam Information

• Quiz 3: Available at 5 p.m. on Wednesday, due by 11:59 p.m. on Thursday.

  • Topics Covered: Scyphozoa, Cubozoa, and Anthozoa (through January 24).

• Exam 1: Scheduled for Monday, Feb. 3.

  • Content: All lectures up to and including January 31, and assigned readings for the week.

• Office Hours: Coker 403, Wednesdays 4 – 5 p.m. Possible Sunday session via Zoom TBA.

• Reading Assignments: Read two coral reef papers (found on Canvas).

  • Textbook Pages: 124-126 (Alcyonaria), 135-138 (Phylum Ctenophora).

Cnidaria Overview

Phylum Cnidaria

• Class Anthozoa: Includes anemones, corals, and their relatives.

  • Note: Text refers to Anthozoa as a subphylum (ignore this).

Class Anthozoa Details

Subclass Zoantharia (Hexacorallia)

• Order Scleractinia: Stony corals, can be solitary or colonial.

  • Basic structure of a scleractinian polyp resembles that of sea anemone; however:

    • Scleractinians have a calcareous skeleton (hard internal CaCO3 skeleton) beneath soft tissue.

    • Anemones have bodies that are entirely soft.

Balanophyllia

• A solitary cup coral and the only intertidal scleractinian along the U.S. West Coast.

Colonial Corals

• New polyps are produced by budding.

• Each polyp's structure remains uniform; adjacent polyps connected by a tissue sheet called coenosarc.

• Example: Montastrea, a scleractinian coral from Jamaica.

Nutritive Needs of Scleractinian Corals

• Feeding Methods:

  1. Feeding on plankton.

  2. Internal symbionts:

     • Zooxanthellae (dinoflagellates).

     • Zoochlorellae (algae).

  • Together, these are often collectively referred to as zooxanthellae.

  • Corals depend on photosynthetic products produced by symbionts.

  • Many corals require light; cannot thrive in deep water without it.

Coral Reefs

General Characteristics

• Found in tropical and subtropical seas with temperatures over 20 degrees C.

• Typically found in shallow water, most thriving at depths of 30 m or less.

• Based on hermatypic (reef-building) scleractinian corals.

Types of Coral Reefs

1. Fringing Reefs:

   • Border continents and islands, close to shore.

   • Fore Reef: Outer reef facing the ocean with a rapid drop in water depth.

2. Barrier Reefs:

   • Located near coasts, but can be far offshore (e.g., Great Barrier Reef is 15-150 km offshore).

   • Separated from land by a channel or lagoon.

3. Atolls:

   • Circular reefs with a lagoon in the center, often formed from sinking volcanic islands.

   • Coral reef grows upwards, forming a ring around the lagoon.

Coral Atolls and Environment

• Environment varies notably across different atoll locations.

  • Lagoon: Generally sheltered with few waves; low current.

  • Outer Sides: Subject to strong wave action.

Coral Bleaching

Characteristics

• Healthy coral tissue contains zooxanthellae; bleached coral results from zooxanthellae being expelled due to stress.

Causes of Coral Bleaching

• Concerns include climate change related to:

  • High temperatures.

  • Increased UV radiation.

• Lab investigations show other contributing factors:

  • Excess shade.

  • Disease.

  • Pollutants.

  • Sedimentation.

  • Changes in salinity.

Threats to Coral Reefs

• Main Threats:

  • Climate Change.

  • Pollution (e.g., runoff).

  • Coral collection (by aquarium trade).

  • Accidental destruction by divers.

Class Anthozoa Summary

Subclass Alcyonaria (Octocorallia)

• Includes soft corals which are flexible and can bend; rigid scleractinians contrast this.

Subclass Overview

• Subclass Zoantharia (Hexacorallia): Includes anemones and stony corals.

  • Nearly all are colonial; most have small polyps.

  • Diagnostic Feature: Feeding polyps have 8 tentacles.

• Subclass Alcyonaria (Octocorallia): Comprising soft corals and relatives.

Order Gorgonacea

• Known as gorgonians or gorgonian corals (e.g., sea fans and sea whips).