Marine Science (9693) | Key groups of marine organisms

Marine Science (9693) AS Level Study Notes

Chapter 4.2: Key Groups of Marine Organisms

Hello future marine biologist! This chapter is all about meeting the residents of the ocean—the major groups of organisms that make up marine ecosystems. Understanding who they are and how they function is crucial, as they form the foundation of all marine food webs and cycles. Don't worry if the names seem complex; we’ll break down their key characteristics into easy-to-remember points!

1. Plankton: The Ocean's Drifters

The term plankton is generic, meaning it describes organisms based on their lifestyle, not their classification. Plankton are a diverse collection of generally microscopic organisms that have limited motility. Essentially, they cannot swim strongly enough to fight ocean currents, so they drift.

A. Phytoplankton (The Producers)

Think of phytoplankton as the microscopic plants of the sea.

• Role: They are producers. Like all producers, they absorb nutrients (like nitrates and phosphates) from the water and use energy to convert inorganic substances into organic substances (food).
• Nutrition: They obtain energy through photosynthesis. This means they are limited to the upper, sunlit layers of the ocean (the photic zone).
• Examples: Common examples are microscopic algae, such as diatoms and dinoflagellates.

B. Zooplankton (The Consumers)

Zooplankton are the microscopic animals of the sea, often feeding on phytoplankton.

• Role: They are consumers (usually primary consumers).
• Examples: This group includes the larval stages of many larger organisms (like fish or crabs), small crustaceans called copepods, and larger animals such as jellyfish.

Quick Takeaway: If it drifts, it's plankton. Phyto means light/plant (producer); Zoo means animal (consumer).

2. Key Invertebrate Groups

A. Echinoderms (Starfish and Sea Urchins)

Echinoderms are the group that includes sea stars (starfish), sea cucumbers, and sea urchins. Their name means 'spiny skin'.

Main Features (Syllabus Focus 4.2.4):

• Symmetry: Typical adults have pentaradial symmetry. This means their body parts are arranged in five sections around a central axis. (Think of a classic starfish shape).
• Movement: They use tube feet for locomotion, feeding, and gas exchange. These are tiny, fluid-filled extensions that operate using a water vascular system.

Ecological and Economic Importance (4.2.5):

Ecologically, they are important consumers. However, sometimes their impact is negative, such as the Crown of Thorns starfish, which is a significant predator of coral polyps, causing widespread reef erosion when their populations boom.

Did You Know? Sea cucumbers are sometimes commercially harvested for human consumption, making them economically important in some regions.

B. Crustaceans (Crabs, Shrimp, and Krill)

Crustaceans are essential arthropods, characterized by their hard outer skeletons.

Main Features (4.2.6):

• Exoskeleton: A rigid external shell, often featuring a carapace (a section that covers the head and thorax).
• Body Structure: They have a segmented abdomen and jointed legs.
• Sensing: They possess two pairs of antennae used for touch and chemical sensing.

Ecological and Economic Importance (4.2.7):

The most famous example is Antarctic krill. These tiny crustaceans form the largest biomass of any animal on Earth and are the primary food source for many whales, seals, and penguins in the Southern Ocean. Economically, groups like shrimp and crabs are vital fisheries globally.

Analogy: Think of a crustacean's body like a suit of armor with moving parts—that's the segmented body and carapace!

3. Key Vertebrate Groups (Phylum Chordata)

Before diving into fish, let’s talk about the major group they belong to: Phylum Chordata (4.2.12).

All organisms in the phylum Chordata, including bony fish, cartilaginous fish, and even humans, share four common features at some point in their development (even if only in the embryonic stage):

• A notochord (a flexible rod supporting the body).
• A dorsal neural tube (which develops into the spinal cord and brain).
• Pharyngeal slits (or gill slits).
• A post-anal tail (a tail extending beyond the anus).

A. Bony Fish (Osteichthyes)

This is the largest class of vertebrates in the ocean, including species like tuna, grouper, and cod.

Main Features (4.2.8):

• Skeleton: Made of true bony skeleton.
• Gills: Gills are protected by a hard flap called the operculum.
• Buoyancy: They possess a swim bladder (a gas-filled sac) for controlling buoyancy in the water column.
• Skin: Covered in overlapping scales.
• Sensory: They have an externally visible lateral line system for sensing pressure changes and vibrations.
• Fins: They typically have paired fins (pectoral and pelvic) and unpaired fins (caudal, anal, and dorsal).

Ecological and Economic Importance (4.2.9):

Bony fish are vital components of food webs, serving as major prey and predator species. Economically, they are crucial. For instance, the Peruvian anchoveta fishery is one of the world's largest, providing feed for aquaculture and livestock.

B. Cartilaginous Fish (Chondrichthyes)

This group includes sharks, rays, and skates.

Main Features (4.2.10):

• Skeleton: Made entirely of flexible cartilaginous skeleton (like the cartilage in your nose or ear).
• Gills: They have multiple, visible gill slits (they lack an operculum).
• Skin: Their skin is covered in placoid scales called denticles, which feel rough, like sandpaper.
• Sensory/Fins: They also possess a lateral line and similar fin types (pectoral, caudal, pelvic, anal, dorsal), but often have much larger, rigid pectoral fins compared to bony fish.

Ecological and Economic Importance (4.2.11):

Cartilaginous fish, like the blue shark, are often apex predators, playing a key ecological role in controlling populations lower down the food chain. Their removal can destabilize entire ecosystems. Economically, they are sometimes targeted for fins and meat, leading to conservation concerns.

Common Mistake Alert: Remember, both bony fish and cartilaginous fish are Chordates, but only bony fish have a bony skeleton and an operculum!

4. Marine Primary Producers: Plants and Algae

A. Macroalgae (Seaweeds, e.g., Kelp)

Macroalgae are large, multicellular algae (seaweed). They are not true plants; they lack the complex vascular systems of terrestrial plants.

Main Features (4.2.13):

Macroalgae structures are analogous (similar function, different origin) to plant structures:

• Holdfast: Anchors the alga to the seabed (not used for nutrient uptake like a true root).
• Stipe: The stalk-like structure (similar to a stem).
• Blades: The leaf-like flat structures where most photosynthesis occurs.
• Gas bladders: Air-filled sacs (sometimes present) that keep the blades buoyant and near the surface light.

Ecological and Economic Importance (4.2.14):

Macroalgae, particularly kelp, form massive, highly productive submerged "forests" which provide complex habitats and shelter for numerous fish and invertebrate species. Economically, seaweeds are harvested for food, fertilizers, and extracts (like agar and carrageenan) used in food and industry.

B. Marine Plants (Seagrasses)

Unlike macroalgae, seagrasses are true vascular plants that evolved from land plants and returned to the sea.

Main Features (4.2.15):

As true plants, they possess all the required structures:

• Rhizome and Roots: True roots and a connecting underground stem system (rhizome) that anchors them firmly and absorbs nutrients.
• Flowers and Leaves: They have specialized photosynthetic leaves and can produce flowers and seeds for sexual reproduction.

Ecological and Economic Importance (4.2.16):

Seagrass beds are immensely important. Ecologically, they stabilize the soft substrate (preventing erosion) and provide essential nursery areas for juvenile fish and shellfish. They are highly productive and are vital feeding grounds for organisms like manatees and sea turtles.

Quick Review Summary

Invertebrates:

• Plankton: Drifting (Phyto = producer; Zoo = consumer).
• Echinoderms: Pentaradial symmetry, tube feet. (E.g., Starfish, Crown of Thorns).
• Crustaceans: Carapace, segmented abdomen, two pairs of antennae. (E.g., Krill, Shrimp).

Vertebrates (Chordates):

• Shared Chordate Traits: Notochord, dorsal neural tube, pharyngeal slits, post-anal tail.
• Bony Fish: Bony skeleton, operculum, swim bladder. (E.g., Anchoveta).
• Cartilaginous Fish: Cartilaginous skeleton, visible gill slits, denticles. (E.g., Blue Shark).

Producers:

• Macroalgae (Kelp): Holdfast, stipe, blades, gas bladders (no true roots).
• Marine Plants (Seagrass): Rhizomes, true roots, flowers, leaves (true vascular plants).