Echinoderms are a unique and fascinating group of marine animals that include starfish, sea urchins, sand dollars, and sea cucumbers. If you’re short on time, here’s a quick answer to your question: All echinoderms have an endoskeleton, a water vascular system, tube feet, and radial symmetry as adults.
In this comprehensive article, we will explore what defines this diverse phylum of spine-skinned animals. We’ll look in detail at the key anatomical, developmental, and physiological traits that unite them.
We’ll also overview the intriguing life histories of some of the best-known echinoderm species.
Common Anatomical Features of Echinoderms
Echinoderms, including sea stars, sea urchins, and sea cucumbers, share several distinctive anatomical features that set them apart from other animal phyla. These common structures and systems equip echinoderms for life in the ocean.
Endoskeleton
A unique feature of echinoderms is their endoskeleton composed of ossicles (small plates) made of calcium carbonate. These ossicles may be fused together, embedded in the skin, or loosely joined by ligament-like tissues, allowing flexibility.
The endoskeleton provides protection, points for muscle attachment, and structural support.
Water Vascular System
The water vascular system is the means by which echinoderms move and feed. This network of fluid-filled canals extends through the body and into tube feet. The system uses water pressure to operate suckers on the ends of the tube feet, enabling echinoderms to adhere strongly as they move across surfaces.
Their tube feet also pass food particles to structures near their mouths.
Tube Feet
The numerous tube feet extending through the body are key structures connected to the water vascular system. Each foot consists of an ampulla that regulates water pressure, a water canal, and the terminal sucker.
With thousands of tube feet employed at once, echinoderms can produce strong adhesion forces over 1,800 kPa, allowing them to withstand crashing waves and strong sea currents. Tube feet also facilitate locomotion, feeding, and gas exchange.
Radial Symmetry
Unlike most animals, echinoderms display radial symmetry as adults. Their internal organs are arranged in radiating patterns, with five or more planes which can divide the body into matching halves. This radial plan aligns with the directions that food grooves and sensory organs extend outward from the mouth.
The resulting symmetry equips them for movement and feeding equally well in all directions.
Echinoderm Larvae and Development
Echinoderms have some of the most complex and unique life cycles in the animal kingdom. They begin life as tiny larvae that look nothing like the adults they will eventually become. Let’s take a closer look at echinoderm larvae and their fascinating development.
Larval Forms
There are two main types of echinoderm larvae:
- Pluteus larvae – Found in sea stars, brittle stars, and sea urchins. These larvae are bilaterally symmetrical and have branching arm-like structures used for feeding.
- Auricularia larvae – Found in sea cucumbers. These larvae are oval-shaped and ciliated, allowing them to swim and feed.
The larvae look totally different from the adults! Sea star larvae don’t have arms, and sea urchin larvae don’t have spines. The larvae are specialized for dispersal and planktonic feeding. Their bilateral symmetry aids in swimming and capturing food.
Development and Metamorphosis
Larval development involves several key stages:
- Fertilization – Eggs are fertilized externally after the female and male release their gametes.
- Cleavage – The zygote undergoes rapid cell division to form a hollow ball of cells called a blastula.
- Gastrulation – The blastula folds inward on itself to form the gastrula, which has a primitive gut.
- Larval growth – The larva increases in size and feeds in the plankton, eventually developing juvenile structures.
- Metamorphosis – The larva undergoes a radical transformation, transitioning into the adult body plan.
Metamorphosis is especially amazing. Over just a few days, the bilateral larva reorganizes its body plan into a radially symmetrical adult while recycling some of its larval tissues. Scientists still don’t fully understand how this incredible transformation happens on a molecular level.
| Larval Stage | Adult Stage |
|---|---|
| Bilateral symmetry | Radial symmetry |
| Feeding arms | Tube feet |
| Swimming by cilia | Crawling by tube feet |
As you can see, echinoderm larvae look and function very differently than the adults. Yet their specialized bodies are perfectly adapted for surviving the plankton until they are ready to transform.
Ecological Significance
Echinoderm larvae play several key ecological roles:
- Dispersal – Larvae ride ocean currents far from their parents, spreading populations.
- Planktonic food source – Fish, whales, and other animals feed on the abundant larvae.
- Settlement cues – Larvae seek out habitat cues to know when to metamorphose and settle.
Larval settlement is critically important. Larvae must settle in suitable habitat in order to survive metamorphosis and thrive as adults. Population connectivity depends on currents transporting larvae between reefs and other habitats.
Respiration and Circulation
All echinoderms have a similar respiratory and circulatory system that distinguishes them from other animal phyla. Here are some key features of their respiration and circulation:
Water Vascular System
Echinoderms have a unique water vascular system that aids in respiration. This system consists of canals filled with water that branch throughout the animal’s body. Water is pumped through these canals by bulb-like organs called ampullae. The water vascular system has several functions:
- It helps exchange oxygen and carbon dioxide for respiration.
- It aids in locomotion and movement.
- It extends and retracts tube feet used for gripping surfaces.
This water vascular system is a major synapomorphy (shared derived characteristic) of echinoderms.
Gills for Gas Exchange
Most echinoderms, except crinoids, use gills to exchange gases for respiration. Oxygen is absorbed from water flowing over the gills while carbon dioxide is released.
Sea stars have paired gills along each arm. Sea urchins have specialized tube feet called pedicellariae that aid in gas exchange. Sand dollars have gills located on their lower surface. The exact gill structures vary among each echinoderm class, but their basic respiratory function remains the same.
Hemal System
Echinoderms have a hemal system rather than a closed circulatory system with true blood vessels. This hemal system consists of sinuses and spaces around the animals’s organs where fluid called hemolymph circulates.
The hemal system has no heart or capillaries. Hemolymph is propelled through sinuses mainly by cilia and muscular contractions. It helps distribute nutrients and oxygen while removing metabolic waste products.
This open hemal system is a primitive circulation method, yet it effectively meets the oxygen demands of echinoderms.
Radial Symmetry
Another feature of echinoderm respiration and circulation is five-part radial symmetry. Their internal organs, including respiratory structures, are arranged in a radial pattern.
This radial symmetry allows for more efficient diffusion and distribution of gases, nutrients and waste products compared to bilateral symmetry.
Feeding and Digestion
All echinoderms share some commonalities when it comes to their feeding and digestive processes. Most species are detritivores or filter feeders, meaning they consume tiny food particles suspended in water or found in seabed sediment and debris.
Feeding Structures and Behaviors
Echinoderms have a variety of specialized structures and behaviors to assist in capturing and ingesting food particles. For example:
- Sea stars use their tube feet to pry open bivalve shells and extrude their stomach to envelop and digest prey.
- Sea urchins have a unique jaw structure called “Aristotle’s lantern” used for scraping algae off surfaces.
- Sand dollars have a fuzzy coating of cilia to trap organic particles floating by.
- Sea cucumbers sweep nutrient-rich sediment into their mouths using sticky tentacles.
Digestive Systems
Once food is captured, it passes through a basic digestive system common to echinoderms:
- The mouth and pharynx begin mechanical digestion.
- The esophagus transports food to the stomach and intestine where digestive enzymes break it down further.
- Nutrients are absorbed and waste is excreted out the anus.
One study found the average gut passage time for food particles is about 5-6 hours in sea cucumbers (Massin, 1982). This allows them to process huge quantities of sediment daily.
Unique Digestive Adaptations
While their digestive systems follow a typical basic design, some echinoderms have special adaptations to their feeding and digestion strategies, including:
- External digestion – As noted earlier, sea stars can expel their stomachs to envelop and break down prey items too large to swallow.
- No anus – Several pelagic larval stages lack an anus altogether until maturation, getting nutrients from fat stored in their arms.
- Symbiotic bacteria – Chemosynthetic bacteria living in special organs of sea cucumbers, sand dollars, and sea urchins allow them to extract nutrients from inhospitable substrates like sulfur.
So while all echinoderms share some common digestive system components, structural specializations and ingenious feeding mechanisms give them access to a wide array food sources in the sea.
Key Species and Life Histories
Sea Stars
Sea stars, more commonly known as starfish, are some of the most iconic echinoderms. There are around 2,000 species of sea stars worldwide, exhibiting a wide variety of sizes, shapes, and colors. These marine invertebrates have a central disk with five or more radiating arms, which give them their star-shaped appearance.
Sea stars are slow-moving and use tube feet located on the underside of their arms to adhere to surfaces and move about. They feed on a variety of organisms like clams, oysters, and mussels by prying open their shells with their strong arms.
Some species can even push their stomach out through their mouth to engulf and digest prey!
Sea stars play key ecological roles in marine ecosystems. As predators, they help regulate the populations of their prey. Some species also act as keystone species, meaning their presence is crucial for maintaining biodiversity in their habitat.
For example, the decline of the sunflower sea star in the Pacific Northwest led to the overpopulation of its prey like mussels, causing drastic changes in the ecosystem.
Sea Urchins
The roughly 950 species of sea urchins are spherical or flattened animals encased in a hard shell called a test. Their bodies are covered in spines, which help protect them from predators. Sea urchins have a simple mouth located on the underside surrounded by five teeth for grazing on algae, marine plants, and sometimes invertebrates.
They move at a glacial pace, using hundreds of tube feet that protrude through holes in the test. These tube feet can also help anchor the sea urchin to surfaces.
Some sea urchin species play a vital role in regulating algae growth in kelp forests, preventing the kelp from being overgrown. Loss of these urchins can cause destructive algal blooms. Other species are considered a delicacy, with the edible part being the gonads or reproductive glands.
Overharvesting has led to population declines for these sea urchins.
Sand Dollars
Sand dollars are flattened, oval-shaped relatives of sea urchins that live buried in sandy or muddy seabeds partially exposed. Their shell is covered with fine spines, appearing almost furry. The approximately 200 living species of sand dollars feed on tiny organisms like algae and microbes that they sweep into their mouth using small, hair-like cilia.
While they move slowly, sand dollars will migrate seasonally to optimize feeding and regulate temperature. These harmless creatures sometimes wash up on beaches, where beachcombers delight in finding their skeletal remains. The five-petal flower shape of their skeleton makes them a popular souvenir.
Globally, some species are under threat from habitat damage from human activities like trawling fishing.
Sea Cucumbers
Sea cucumbers are soft-bodied echinoderms with elongated bodies that resemble fat cucumbers. Of the over 1,700 species, most feed on debris and plankton by consuming sediment and filtering out nutrients.
Some species can rapidly contract their body as a defense mechanism when disturbed, ejecting sticky filaments to ensnare predators.
Sea cucumbers are crucial to ocean ecosystems because they break down organic matter in sediments and recycle nutrients back into the environment. However, unsustainable overfishing driven by demand for use in traditional Asian medicines has led to dramatic population crashes for many species.
Conservation measures are urgently needed to prevent irreversible damage.
Crinoids
Crinoids, also known as sea lilies or feather stars, are relatives of sea stars that resemble colorful plumes. Their mouth and anus are located atop a stem-like structure with branching feathery arms. While some species are permanently attached to the seabed by a stalk, most are free-moving.
They are suspension feeders, capturing plankton and particles carried by ocean currents.
There are about 600 modern crinoid species, but they were far more abundant and diverse during the Paleozoic Era. Though not consumed by humans, many crinoids have fragile skeletons prone to damage from fishing gear and other human activity.
Their colorful, intricate bodies also make them prized additions to shell or fossil collections.
Conclusion
While echinoderms may appear very different on the outside, they share several definitive anatomical, developmental, and physiological characteristics. Their endoskeleton, water vascular system, tube feet, and radial symmetry unite them as members of the phylum Echinodermata.
From tiny brittle stars to gigantic sea stars, echinoderms continue to fascinate both scientists and beachgoers. We hope this overview has shed some light on what makes these spiny-skinned oddballs so unique in the marine world.
