Difficulty: Introductory to Intermediate | Prerequisites: Classification, Taxonomy, and Plankton study notes (Part 1 of this chapter)
This set of notes covers the six major groups of marine organisms you need to know for Chapter 4: echinoderms, crustaceans, bony fish, cartilaginous fish, macroalgae, and marine plants (seagrasses). For each group the chapter asks you to know the main structural features of a typical adult, understand the group's ecological role in marine food webs and habitats, and be able to discuss its economic importance to humans. It also introduces chordates as the phylum that unites bony and cartilaginous fish. You should already be comfortable with the taxonomic hierarchy and the distinction between producers and consumers from Part 1.
Six organism groups form the backbone of marine ecosystems. Echinoderms (starfish, sea urchins) and crustaceans (crabs, krill) are invertebrates with distinct body plans and huge ecological roles. Bony fish and cartilaginous fish (sharks, rays) are both chordates but differ in skeleton material, scale type and buoyancy mechanism. Macroalgae (kelp, seaweed) and marine plants (seagrasses) are the main large-scale producers in coastal waters, each with distinct structural adaptations.
Echinoderms
A phylum of marine invertebrates with pentaradial symmetry and tube feet. Think of them as the "spiny-skinned" animals: starfish, sea urchins, sea cucumbers, brittle stars and sea lilies.
Pentaradial symmetry
Five arms (or fans) radiating from a central body cavity. In simple terms, the body is arranged like a five-pointed star.
Tube feet
Tubular projections on the underside of echinoderm arms that assist in locomotion, feeding, respiration and sticking to the seafloor. They work via a hydraulic water-filled tube system.
Crustaceans
A group of marine invertebrates with a hard exoskeleton, ten jointed legs, and a nauplius larval stage. Includes crabs, lobsters, shrimp, krill, copepods and barnacles.
Nauplius
The distinctive larval stage of crustaceans. Think of it as the "baby form" that all crustaceans pass through.
Carapace
The part of the exoskeleton that protects the dorsal (top) side of the cephalothorax in crustaceans.
Bony fish (Osteichthyes)
Fish with a skeleton made of bone, belonging to the class Osteichthyes. About 96% of all fish species are bony fish.
Gills
The gaseous exchange surfaces of fish. Delicate, pink, membranous structures that provide a large surface area for exchanging oxygen and CO₂ between seawater and blood.
Operculum
A thin bony flap of skin covering and protecting the gills in bony fish. It opens and closes to allow water to flow over the gills for ventilation.
Lateral line
A canal running along the head and side of bony fish containing sense organs (electroreceptors and mechanoreceptors) that detect electrical field changes and vibrations in the water. Externally visible in bony fish but under the skin in cartilaginous fish.
Scales
Overlapping segments of bone covered by skin and mucus on the outside of bony fish. Two main types: cycloid (smooth edge) and ctenoid (toothed edge). Growth rings on scales can help determine a fish's age.
Fins
Structures that protrude from the body surface of fish, assisting in movement, balance, steering, reproduction and protection. Five main types in bony fish: pectoral, pelvic, anal, dorsal, caudal.
Swim bladder
A specialised buoyancy organ found in bony fish. By adding or releasing gas, it allows the fish to stay at a given depth without swimming continuously.
Cartilaginous fish (Chondrichthyes)
Fish with jaws and skeletons made entirely of cartilage, belonging to the class Chondrichthyes. Includes sharks, skates, rays and chimaeras.
Denticles
Tooth-like overlapping scales found on cartilaginous fish. They all point in one direction, giving the skin a sandpaper feel when rubbed the wrong way. Provide protection and improve swimming efficiency through streamlining.
Gill slits
External openings from the gills through which water taken in at the mouth passes back to the ocean. Sharks and rays have five to seven pairs; they also have a spiracle behind the eye.
Notochord
A flexible, rod-shaped organ extending the length of the body in chordates, allowing bending during muscle contractions. In bony and cartilaginous fish it is replaced by the vertebral column during development.
Holdfast
A strong, root-like structure that anchors macroalgae (such as kelp) to the seabed. It is for anchorage only and does not absorb minerals.
Stipe
The long, tough, vertical stalk of macroalgae, similar to the stem of a plant. Extends from the holdfast up to the blades.
Blades
Broad, leaf-like structures of macroalgae that hang in the water to absorb light and minerals.
Gas bladder (pneumatocyst)
A gas-filled structure found beneath the blades of some macroalgae species. Provides buoyancy, keeping the producer upright in the photic zone for photosynthesis.
Seagrasses (marine grasses)
Aquatic flowering plants found in estuarine and coastal environments, forming underwater meadows on the shallow continental shelf. They are not seaweeds.
Rhizomes
Horizontal underground structures that anchor seagrasses to the substrate and enable asexual reproduction.
A phylum of marine invertebrates including seastars (starfish), sea urchins, sea cucumbers, brittle stars and sea lilies. Over 7,000 species, found from the intertidal zone to benthic depths.
Name comes from Greek for "spiny skin": a thin layer of skin covers a hard calcium carbonate skeleton.
Planktonic larvae develop into adults with pentaradial symmetry: five arms radiating from a central body cavity containing a mouth and anus.
Move via a system of water-filled tubes that change hydraulic pressure in the tube feet on the underside of each arm.
Tube feet also help with feeding (opening bivalve shells), respiration (gas exchange) and adhering to the seafloor.
Feeding: seastars can evert their stomach through the mouth onto prey (e.g. oysters, clams), digest externally, then retract.
Ecological importance of echinoderms
Some are keystone species in coral reefs, kelp forests and sandy shores.
Coral reefs: The crown-of-thorns starfish (COTS, Acanthaster planci) feeds on fast-growing coral, promoting colonisation of slower species and increasing overall reef biodiversity. However, removing COTS predators can trigger COTS population explosions that destroy large areas of coral.
Kelp forests: Sea urchins feed on kelp holdfasts. Without predators like sea otters controlling urchin numbers, urchins can overgraze and create "kelp barrens." Conversely, if urchins are over-harvested, kelp overgrows.
Sandy shores: Sea cucumbers filter seawater, burrow into substrate, and provide oxygen to sand-dwelling organisms. Their planktonic larvae are food for plankton-eating fish. They also recycle nitrogenous waste.
Economic importance of echinoderms
Sea cucumbers are a delicacy in Chinese cuisine, with large-scale commercial harvesting (e.g. Galapagos Islands, where uncontrolled harvesting in the 1990s led to population collapse by 2005).
Sea urchins eaten in Japan, New Zealand, Peru, Spain and France. Calcium carbonate endoskeletons used to raise soil pH in agriculture.
Sea urchins used as model organisms in developmental biology research.
COTS outbreaks can damage ecotourism industries by destroying coral reefs.
Adapted to salt, brackish and fresh water. Includes crabs, crayfish, lobsters, shrimp, prawns, barnacles, copepods, amphipods, krill and fish lice.
All crustaceans pass through a distinctive larval form called a nauplius.
Exoskeleton: hard outer covering of calcium carbonate and chitin. Outer layer protects from predators and water loss; inner part supports muscles. Divided into a cephalothorax (fused head and thorax) and a segmented abdomen.
The dorsal side of the cephalothorax is protected by the carapace.
Head features: two pairs of antennae (one shorter pair called antennules), two compound eyes, three pairs of mouthparts (mandibles and two pairs of maxillae) for defence and feeding.
Legs: at least five pairs of jointed legs (hence "decapods" for the ten-legged groups). Unlike insects (three pairs) and arachnids (four pairs). Legs have two-part limbs, a distinguishing feature from other arthropods.
The cephalothorax has claws (chela) and walking legs (pereiopods). The abdomen contains swimmerets (pleopods) and ends in a fan-shaped tail (uropod and telson).
Ecological importance of crustaceans
Found everywhere from deep sea benthic habitats (amphipods) to surface pelagic waters (nauplius larvae, copepods).
Scavenging crustaceans recycle mineral nutrients as detritivores, accelerating the breakdown of dead and decaying organic matter.
Shrimps graze algae off seagrass blades, preventing algae from blocking sunlight and slowing seagrass photosynthesis.
Crustaceans are food for many marine organisms: marine birds feed on mud crabs in mangroves; copepods and krill are crucial trophic links.
Krill biomass in the Southern Ocean is approximately 110 billion kg. Antarctic krill are vital to Antarctic food chains, consumed by fish, birds, squid, whales, seals and penguins.
Economic importance of crustaceans
Prawns, shrimps, crabs, crayfish and lobsters are a major global food source.
Copepods and krill form the largest animal biomass on Earth and underpin the seafood industry.
Krill oil is rich in omega-3 fatty acids and antioxidants; sold as a dietary supplement.
Krill also used in aquaculture feed, sport fishing bait, and pharmaceuticals.
Harmful crustaceans: water fleas damage farmed molluscs; fish lice are parasites on farmed fish; pea-crabs parasitise oyster hosts.
96% of all fish have a bony skeleton and belong to the class Osteichthyes. About 27,000 known species.
External features:
Gills: pink membranous structures providing a large surface area for O₂/CO₂ exchange, supported by bony gill arches.
Operculum: bony flap covering the gills; opens and closes to move water across the gills for ventilation.
Lateral line: a canal along the head and body containing electroreceptors and mechanoreceptors. Helps with shoaling behaviour, navigation and prey detection. Externally visible in bony fish.
Scales: overlapping bone segments covered by skin and mucus. Cycloid (smooth edge) or ctenoid (toothed edge). Form growth rings, like tree rings, that help determine age.
Fins: five main types.
Pectoral fins: paired, behind the operculum. Turning, balance, stopping. Red Handfish use them to walk; Flying fish use them to leap.
Pelvic fins: paired, on the ventral surface. Stability; can be adapted for walking on substrate.
Anal fin: single, behind the anus/cloaca. Stabilises during swimming.
Dorsal fins: one to three, on the back surface. Steering, balance, protection from rolling. May have spines for predator defence. Hairy frogfish use a dorsal spine as a prey lure.
Caudal fin (tail): main source of propulsion. Five shapes linked to swimming speed: rounded (slow, e.g. butterfly fish), truncated (acceleration, e.g. salmon), forked (continuous, e.g. Peruvian anchoveta), lunate (fast sustained, e.g. tuna), heterocercal (fastest, mostly sharks, e.g. blue shark).
Internal features:
Jaw and skeleton of bone and cartilage.
Swim bladder: a buoyancy organ. Adds or releases gas to allow the fish to stay at a set depth without continuous swimming.
Ecological importance of bony fish
Store a significant proportion of marine organic nutrients in their tissue. When they migrate, feed and reproduce they link the nutrient cycles of different habitats.
Excrete nitrates, phosphates and other dissolved nutrients readily absorbed by primary producers, forming the nutrient and energy base of marine ecosystems.
Important trophic link: planktivorous fish connect plankton to higher-order consumers; piscivorous species link aquatic and terrestrial ecosystems (e.g. dead fish washed ashore redistribute nutrients).
Salmon are a classic example: they migrate upriver to spawn and die. Bears feast on salmon runs, then carry nutrient-rich faeces into surrounding forests. Salmon are considered keystone species because their abundance indicates the health of the whole wetland ecosystem.
Economic importance of bony fish
Fish meat is an important protein source containing five essential amino acids humans cannot synthesise. Liver oils (e.g. cod) are rich in vitamins A and D.
Non-edible parts: fish meal (animal feed, fertiliser), remaining oil (soap, varnish, paint, candles), skin (lanterns, glue), swim bladders (isinglass for wine and beer making), scales (jewellery).
Recreational fishing, sport fishing, scuba diving, ecotourism, breeding tropical fish for the pet trade, and scientific research.
Fish farming predicted to play an increasing role as wild stocks decline through overfishing.
Case study: Peruvian anchoveta
A small forage fish (up to 20 cm), filter feeders that strain water and zooplankton through bony gill rakers.
The biggest single-species fishery in the world, pivotal to the Peruvian economy.
Peak catches exceeded 10 million tonnes per year in the 1960s; 1971 catch was 13.1 million tonnes.
In 1972 the fishery collapsed due to overfishing and El Nino (a change in Pacific trade winds that deepens the thermocline, blocking nutrient upwelling into the sunlit zone, starving phytoplankton and the fish that depend on them).
Anchoveta numbers recovered by the mid-1990s (catch up to 12.5 million tonnes), but El Nino events continue to cause periodic crashes.
The fishery also affects guano production: fewer fish means less food for seabirds and therefore less guano (fertiliser).
Class Chondrichthyes: sharks, skates, rays and chimaeras. Approximately 1,250 species; sharks alone account for more than 500.
Skeleton: made entirely of cartilage (less calcium, softer and more flexible than bone).
Fins: most sharks have eight fins (two pectoral, two pelvic, one or two dorsal, one anal, one caudal). The caudal fin is larger on the dorsal side (heterocercal), providing more speed and acceleration. Rays get forward thrust from their large pectoral fins.
Skin: tough, covered in denticles (tooth-like overlapping scales), all pointing in one direction. Provides protection, reduces drag and improves swimming efficiency through streamlining.
Ventilation: five to seven pairs of gill slits plus a spiracle behind the eye. Two methods: "ram ventilation" (swimming forward with mouth open) and "pumped ventilation" (actively pumping water over the gills).
No swim bladder: cartilaginous fish cannot regulate buoyancy passively, so they must keep swimming forward to avoid sinking.
Lateral line: present but under the skin, not externally visible.
Ecological importance of cartilaginous fish
Occupy a wide variety of marine habitats, from estuarine inlets and lagoons to the deep benthos.
Mostly apex predators, though some are scavengers and filter feeders.
Sharks promote biodiversity by maintaining predator-prey balance. They remove weak and diseased individuals, keeping prey populations healthy.
When sharks are over-harvested, cascading effects occur: larger predatory fish (e.g. groupers) increase in abundance, feeding more heavily on herbivores (e.g. yellow tang), which reduces grazing on algae, leading to algae overgrowing coral.
Sharks also predate invasive species such as lionfish, helping protect native juvenile fish populations.
Example: the blue shark (Prionace glauca) is a wide-ranging apex predator. Up to 20 million blue sharks killed annually. Slow to reach sexual maturity and low reproductive rate make recovery from overfishing very difficult. Some populations have declined by 80%; IUCN classifies blue sharks as "near threatened."
Economic importance of cartilaginous fish
Shark fin soup is a culinary delicacy in China; fins are often taken by "finning" (cutting fins off live sharks and discarding the body).
Shark liver oil: historically a vitamin A source; now used in leather preservation, lubricants, cosmetics and medicinal products.
Denticle skin traditionally used in Japan for sword hilts; shark leather used for boots, belts and wallets.
Ecotourism: a reef shark caught by a fisherman in the Bahamas is worth about US$50; the same reef, used for dive tourism, can generate US$250,000 for the local community. A single whale shark in Belize can bring in US$2 million over its lifetime through dive tourism.
Both bony and cartilaginous fish belong to the phylum Chordata. All chordates share four features at some point in development:
Notochord: a flexible rod extending the body length, allowing bending during muscle contractions.
Dorsal neural tube: a tube-shaped organ along the body length. The anterior end becomes the brain; the posterior end becomes the spinal cord.
Pharyngeal slits: link the mouth cavity to the digestive system. In primitive chordates they filter water for food. In bony and cartilaginous fish they develop into gill arches for ventilation. In most marine mammals and birds they are present only during embryonic development.
Post-anal tail: mainly used for swimming; located to the rear of the fish.
Large marine producers including kelp and other seaweeds. Along with phytoplankton and marine plants, macroalgae are photoautotrophs: they make their own food using light energy from the sun.
Structure (the whole body is called a thallus, with three main parts):
Holdfast: root-like structure anchoring the alga to the seabed. Anchorage only, does not absorb minerals.
Stipe: long, tough vertical stalk, similar to a plant stem.
Blades: broad, leaf-like structures that hang in the water to absorb light and minerals.
Some species have gas bladders (pneumatocysts) beneath the blades for buoyancy, keeping the producer upright in the photic zone.
Many deep-water macroalgae contain accessory pigments (xanthophyll, fucoxanthin) to absorb wavelengths of light beyond red, which is absorbed in shallow water.
Ecological importance of macroalgae
Particularly important in intertidal (littoral) and subtidal (sublittoral) coastal habitats.
Giant kelp forests are one of the most important underwater ecosystems. Composed mainly of brown macroalgae, they require nutrient-rich, clear water between 8 °C and 16 °C. One genus, Macrocystis, can grow up to 0.5 m per day and reach 80 m in length.
Kelp forests proliferate on continental shelves, serving as crucial habitats for a diverse range of fauna and increasing nearshore productivity.
They generate vast species biodiversity by producing large quantities of detritus and are the base of many food chains.
Kelp forest food web: kelp is grazed by sea urchins, which are preyed on by sea otters. Sea otters are a keystone species: their loss leads to urchin population explosions, destruction of kelp forests ("kelp barrens"), reduced productivity and reduced biodiversity.
Sea otter threats: discarded nets, oil spills (clogs fur, leading to hypothermia), polluted waters (disease), historic fur hunting (near extinction in the 1800s), and reduced food from commercial fishing.
Economic importance of macroalgae
Harvested for thousands of years. Macroalgal mariculture is now a US$6 billion industry producing roughly 8 million tonnes per year.
Cooking: seaweed eaten in Japan, Korea, Iceland, Wales. In Japan, over 20 species used as food; the red alga Porphyra is dried to make nori for sushi. Rich in protein, vitamins, mineral salts (especially iodine), low in fat.
Food industry biochemicals:
Alginate: gelatinous substance from seaweed, used in ice cream, synthetic caviar, burns plasters.
Agar: used for vegetarian jellies and microbiology agar plates.
Carrageenan: used in chocolate milk, milk chocolate bars and other textured foods.
Cosmetics and herbal medicine: seaweed extracts in skin creams and remedies for arthritis, tuberculosis and common cold.
Fertiliser: rich nutrient source for farmland.
Aquaculture: processed into pellets to feed abalone.
Flowering plants found in estuarine and coastal environments. Three variants: floating, emergent (rooted, projecting above water) and submergent (rooted, below water). Seagrasses (submergent) are the main focus.
About 72 species of seagrass, forming large beds called meadows on the shallow continental shelf. Found in waters between 4 °C and 24 °C, so not found in subpolar regions and are under threat from global warming.
Root system: thick, well-developed, with horizontal rhizomes up to 25 cm deep in the substrate. Anchors the plant against water currents and wave action. Rhizomes also enable asexual reproduction.
Leaf structure: unusual features including an epidermis with chloroplasts (absent in most terrestrial plant epidermis), no stomata, a very thin waxy cuticle (allows direct mineral absorption from water), very few vascular bundles, and flexible leaves that resist breakage by currents. Contains aerenchyma ("air cells") in stems for aerobic respiration in submerged parts.
Reproduce both sexually (flowers that release pollen into the water) and asexually (via rhizomes).
Ecological importance of marine plants
Foundation of aquatic communities with high biodiversity. Sensitivity to water quality makes them important keystone species and indicators of coastal ecosystem health.
Dominant producers in estuarine habitats, providing a direct source of food energy for marine primary consumers. Key food source: turtle grass (Thalassia testudinum), vital for turtles, manatees and herbivorous fish.
Dead seagrass detritus provides organic energy for worms, sea cucumbers, crabs, echinoderms and sea anemones.
Many fish, seahorses and lizardfish live in seagrass year-round; seagrass beds serve as nesting and nursery habitats for vertebrate and invertebrate species.
Seagrass meadows occupy less than 0.2% of ocean area but hold about 15% of the ocean's total carbon. They act as oxygen producers, absorb wave motion, slow currents, reduce turbidity by trapping silt, and bind sediment with dense root systems.
Holistic conservation is needed: protecting only sea turtles (which graze seagrass) without also protecting sharks (which eat juvenile turtles) can lead to turtle overpopulation, overgrazing and seagrass collapse.
Economic importance of marine plants
Indirect value: nursery grounds for the next generation of commercially and recreationally important marine species. Coastal protection from erosion and storm damage.
Traditional uses: mangrove wood for fires and building; seagrasses for insulation, furniture, bandages, mattresses and fertiliser.
Ecotourism: scuba diving to observe manatees and other seagrass-dependent wildlife is a growing industry.
The kelp forest food web (sea otter, sea urchin, kelp) is one of the best-studied examples of a trophic cascade in ecology, and it comes up regularly in conservation discussions about reintroducing predators to restore habitats. The Peruvian anchoveta case study is a textbook example of how El Nino events and overfishing interact to collapse fisheries. Understanding the economic value of living sharks through ecotourism (compared with the one-time value of catching them) is increasingly used to argue for marine protected areas.
Students often confuse macroalgae (seaweeds) with marine plants (seagrasses). Seagrasses are flowering plants with roots, rhizomes and flowers. Macroalgae have holdfasts, stipes and blades, and are not true plants.
Students sometimes assume cartilaginous fish have swim bladders. They do not, which is why they must keep swimming to maintain buoyancy.
Students mix up the lateral line in bony and cartilaginous fish. In bony fish it is externally visible; in cartilaginous fish it runs under the skin.
Students often forget that echinoderms have pentaradial symmetry as adults but bilateral symmetry as larvae.
⚠️ Be able to list the main external features of bony fish (gills, operculum, lateral line, scales, five fin types) and describe the function of each.
⚠️ Know the key differences between bony and cartilaginous fish: skeleton material, scale type (scales vs denticles), buoyancy mechanism (swim bladder vs none), lateral line visibility, and gill covering (operculum vs gill slits).
⚠️ Be ready to describe the three parts of a macroalga (holdfast, stipe, blades) and explain the role of gas bladders.
⚠️ Understand the ecological role of sea otters as a keystone species in kelp forest ecosystems, including what happens when they are removed.
⚠️ Know the COTS (crown-of-thorns starfish) case study and how removing COTS predators can damage coral reefs.
⚠️ Be prepared to discuss how El Nino affects the Peruvian anchoveta fishery.
⚠️ Know the ecological and economic importance of at least one organism from each of the six groups.
True or false: Seagrasses are a type of macroalgae.
Fill in the blank: The three main structural parts of a macroalga are the holdfast, the ______, and the blades.
True or false: Cartilaginous fish have a swim bladder to regulate buoyancy.
Fill in the blank: The distinctive larval stage of all crustaceans is called a ______.
True or false: The lateral line is externally visible in both bony and cartilaginous fish.
Q: Compare the skeleton, scale type and buoyancy mechanism of bony fish and cartilaginous fish.
A: Bony fish have a skeleton of bone, overlapping cycloid or ctenoid scales covered in mucus, and a swim bladder for buoyancy control. Cartilaginous fish have a skeleton of cartilage, denticle (tooth-like) scales, and no swim bladder, meaning they must keep swimming to avoid sinking.
Q: Describe the structure of a macroalga and explain how each part is adapted to life in the ocean.
A: The holdfast anchors the alga to the seabed (anchorage only, no mineral absorption). The stipe is a tough vertical stalk providing support, similar to a plant stem. The blades are broad, leaf-like structures that hang in the water to maximise light and mineral absorption. Some species also have gas bladders (pneumatocysts) to provide buoyancy and keep the blades in the photic zone.
Q: Explain why sea otters are considered a keystone species in kelp forest ecosystems.
A: Sea otters prey on sea urchins, which graze on kelp holdfasts. Without sea otters, sea urchin populations explode, overgrazing kelp and creating barren areas. This reduces the productivity of the kelp forest, decreasing chemical energy and biodiversity for all other species in the food web.
Q: Outline the ecological importance of crustaceans, using named examples.
A: Scavenging crustaceans act as detritivores, recycling mineral nutrients by breaking down dead organic matter. Shrimps maintain seagrass health by grazing algae off blades. Copepods and krill are critical trophic links between primary producers and larger consumers. Antarctic krill (Euphausia superba) are consumed by fish, birds, whales, seals and penguins, supporting the entire Antarctic food chain.
Q: Explain how El Nino events affect the Peruvian anchoveta population.
A: During an El Nino year, trade winds that normally drive cold, nutrient-rich upwelling weaken, and the thermocline deepens. This blocks nutrients from reaching the sunlit zone, reducing phytoplankton growth and limiting primary productivity. With less food available, anchoveta populations decline sharply.
The COTS and kelp forest case studies connect to Chapter 5 (marine ecosystems), where trophic cascades and food webs are explored in more depth. The concept of El Nino links to Chapter 2 (oceanography) and Chapter 9 (human impacts). The economic uses of macroalgae (alginate, agar, carrageenan) reappear in biotechnology and food science contexts. Chordate features (notochord, pharyngeal slits) are foundational to vertebrate biology and evolutionary biology courses.
Echinoderms, starfish, sea urchins, sea cucumbers, pentaradial symmetry, tube feet, COTS, crown-of-thorns starfish, crustaceans, nauplius, carapace, cephalothorax, decapod, krill, copepods, bony fish, Osteichthyes, gills, operculum, lateral line, swim bladder, scales, fins, caudal fin, pectoral fin, cartilaginous fish, Chondrichthyes, sharks, rays, skates, denticles, gill slits, spiracle, chordates, notochord, pharyngeal slits, macroalgae, kelp, seaweed, holdfast, stipe, blades, gas bladder, pneumatocyst, marine plants, seagrass, rhizome, Peruvian anchoveta, El Nino, kelp forest, trophic cascade, sea otter keystone species, blue shark, shark finning