Invertebrate Diversity in Marine Environments

Invertebrate Diversity in Marine Environments

• Invertebrates are animals without a backbone, while vertebrates have one.
• 97% of all animals are invertebrates.
• Marine environments host representatives from all major animal groups.
• Some animal groups are exclusively marine.

Invertebrate Characteristics

• Multicellular and diploid organisms with tissues, organs, or organ systems.
• Heterotrophic (obtain energy from other organisms).
• Require oxygen for aerobic respiration.
• Reproduce sexually, asexually, or both.
• Most are motile (move) at some point in their life cycle.
• Life cycles involve embryonic development.

Types of Symmetry

• Asymmetrical: No plane of symmetry (e.g., sponges).
• Radial symmetry: Central axis, many planes of symmetry (e.g., starfish).
• Bilateral symmetry: One plane of symmetry (e.g., sea turtles).

Phylum Porifera (Sponges)

• Aggregations of specialized cells, not true tissues.
• Simplest multicellular animals.
• Almost all are marine and sessile (permanently attached).
• Tiny pores (ostia) allow water to flow through to filter feed on plankton and dissolved organic matter.
• Water flow also removes waste and carries gametes.
• Choanocytes (collar cells) are essential for water current generation.
• Pinacocytes and porocytes form the outer surface, and choanocytes line feeding chambers.
• Water exits through the osculum.
• Can reproduce sexually or asexually, and some species are hermaphrodites.
• Spicules (siliceous or calcareous structures) or spongin fibers provide structural support.

Phylum Cnidaria (Stinging Animals)

• Radial symmetry.
• Mostly marine with two body forms:
  • Medusa: Free-floating, mouth and tentacles downward.
  • Polyp: Sessile, mouth and tentacles upward.
• Two tissue layers (epidermis and gastrodermis) surround a gelatinous mesoglea.
• Capture prey using nematocysts (stinging cells).
• Incomplete digestive system (sac-like with one opening).
• Nerve net coordinates movement.
• Reproduction includes sexual (medusa stage producing gametes) and asexual (polyp buds).

Classes of Cnidarians

• Class Anthozoa: Corals, anemones, sea whips, solitary or colonial polyps, lack a medusa stage. Many have symbiotic algae (zooxanthellae).
• Class Hydrozoa: Colonies of polyps, small medusae. Includes siphonophores (e.g., Portuguese man-of-war).
• Class Scyphozoa: Jellies (e.g., sea nettles, moon jellies, lion's mane jellyfish).
• Class Cubozoa: Box jellies with box-shaped bell and four tentacles. Neurotoxic nematocysts.

Phylum Mollusca (Soft-bodied Animals)

• Includes diverse groups like gastropods, bivalves, and cephalopods.
• Gastropods (e.g., snails) and some cephalopods have a radula for feeding.
• Bivalves (e.g., clams, oysters) have two shells and use gills for filter feeding.
• Cephalopods (e.g., squid, octopus) are highly intelligent, with advanced eyes and a closed circulatory system. They use a powerful beak and radula, and some have ink sacs and camouflage.

Phylum Arthropoda (Jointed-legged Animals)

• Largest phylum of animals. Marine arthropods are largely crustaceans.
• Segmented bodies, jointed appendages, and exoskeletons of chitin.
• Must molt to grow.

Subphylum Crustacea

• Crustaceans – 68,000 known species.
• Copepods, barnacles, amphipods (small crustaceans found widely, including in plankton).
• Decapods (e.g., shrimps, lobsters, crabs) – 10 legs, cephalothorax, and abdomen.

Class Merostomata (Horseshoe Crabs)

• “Living fossils.”
• Important in medical research (limulus amebocyte lysate test).

Phylum Echinodermata (Spiny-skinned Animals)

• Bilateral symmetry (early developmental stage), then radial symmetry.
• Internal endoskeleton of calcium carbonate ossicles.
• Water vascular system for respiration, feeding, locomotion, and waste removal.
• Includes sea stars (Asteroidea), sea urchins (Echinoidea), sea cucumbers (Holothuroidea).

General Study Guide for Marine Invertebrates