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Starfish, both familiar and strangely alien, are fascinating creatures that inhabit diverse ocean ecosystems. Though their shape often mirrors their celestial namesake, these remarkable beings have plenty of unexpected traits. Imagine a creature without a brain but with eyes on its feet, or one without blood yet capable of digesting food externally. Get ready to explore animals that seem to break the conventional rules of biology.
10. Not Always in the Shape of a Star
While the term “starfish” conjures up the image of the classic five-point sea star commonly found in tide pools, sea stars come in many shapes. Sun stars, or sunflower stars, are a perfect example. These giants, found in the right conditions, have rounded bodies and many “rays” extending from them. In the Pacific Northwest, these sun stars can grow to over 1 meter (3 feet) wide and weigh up to 5 kilograms (11 pounds), with some species boasting more than 20 arms.
Sun stars are far more active than many other starfish species, capable of chasing down fast-moving prey. They possess the strength to tear open the shells of mollusks and crustaceans, and barnacles and large nudibranchs often become their next meal. These sunflower stars are skilled hunters, surprising their prey and capturing it before it can flee. While they are typically found in colder waters, sun stars come in a stunning range of tropical colors. In areas rich in food sources, they can even form large clusters.
9. No Blood, No Brains, No Problem
Sea stars are fascinating and unusual creatures, yet their biology is surprisingly simple in some ways. Despite having highly specialized stomachs and sophisticated ‘skin,’ starfish lack both brains and blood. Instead of a circulatory system, they rely on seawater to deliver nutrients, oxygen, and other essential fluids throughout their bodies. This seawater flows via a system known as the “water vascular system,” which consists of canals filled with seawater, circulating through the sea star’s body via a network of corpuscles and spiral tubes.
Seawater circulates through the starfish's body almost mechanically, with muscles and lymphatic glands moving and adjusting the water’s chemistry. Various sinuses, corpuscles, and tube systems cooperate to ensure the system functions optimally, all without any blood. The inner workings of the starfish body remain an ongoing mystery, and scientists have yet to fully understand how it functions. Studying the biology of starfish continues to be one of the most intriguing areas of research in the natural world.
8. Sticky Feet
Although we refer to the extensions extending from a starfish’s body as ‘arms,’ each of these limbs actually has feet. If you take a close look at the underside of a starfish, you’ll be amazed to discover up to 15,000 tiny tube feet per arm. These feet move in a coordinated, caterpillar-like fashion, propelling the starfish efficiently forward. In the intertidal zone, these tube feet anchor the starfish to rocks, preventing it from being swept away or crushed by the waves.
The tube feet secure the starfish's soft underside to the rock, while its hard, leathery exterior protects the upper side from heat, dryness, and sun exposure. Ongoing scientific research continues to uncover remarkable facts about the natural world, one of which is the starfish’s remarkable ability to produce a powerful foot glue. This adhesive can form a permanent bond with rocks or prey, while an adjacent tube releases a solvent to break the bond when the starfish is ready to move on.
7. Cannibalism
While many of us imagine sea stars as passive ornaments of the ocean, it turns out that these creatures are more often aggressive predators than simply grazers. What might be an even bigger shock is that cannibalism is a documented behavior among sea stars. After capturing their prey, sea stars will swallow it whole and invert their stomachs to start the digestion process. This cannibalistic act is often triggered by a lack of normal food sources.
Sea stars are generally tough to prey upon, thanks to their hard, leathery skin, but they are also well-prepared to hunt and consume their own kind. Some species will even target smaller members of their own species. A prime example of this is the chocolate chip sea star, a species popular among aquarists, who often learn the hard way that they should never house small individuals with larger ones.
6. Sea Stars Are Aggressive Predators With Two Stomachs
Sea stars may appear alluring, but they are fierce predators with a highly efficient dual-stomach system that enhances their hunting capabilities. One of their most unusual characteristics is the ability to eject their stomachs. Through the pressure of their water vascular system, the cardiac stomach can be pushed out of the body and into a shellfish. Once the sea star secures its prey by gluing its feet to the exterior of the shell and forcing it open, it will extend its stomach into the shell. This reversed stomach digests the prey inside the shell, turning it into a liquid meal. Supported by ligaments, the stomach then retracts back into the sea star, completing the digestion process.
The stomach’s second part, the pyloric stomach, is surprisingly intricate, featuring ceca that follow the sea star’s arm structure, facilitating nutrient absorption and distribution. Positioned above the cardiac stomach in the center of the body, the pyloric stomach processes and distributes nutrients via the pyloric ceca. These sophisticated digestive components form two rows resembling dendrites, extending along each arm, with each arm hosting two pyloric ceca. The sea star's digestive system is a remarkable example of complexity and evolutionary advancement, particularly when compared to its more primitive features.
5. The Crown-Of-Thorns Situation
No discussion of sea star ecology would be complete without addressing the environmental threat posed by the crown-of-thorns starfish. Found across the Indo-Pacific region, this species is distinctive for combining the typical sea star shape with the sharp, spiked armor reminiscent of sea urchins. With venomous spines covering their bodies, these creatures can grow up to nearly half a meter (1.6 ft) wide, posing risks to divers, swimmers, and coral reefs. Research has shown that increases in phytoplankton levels correlate with a tenfold rise in crown-of-thorns populations. Shifts in ocean temperature, currents, and a decrease in natural predators may also contribute to population explosions.
When crown-of-thorns starfish populations are low, they help maintain reef diversity by feeding on rapidly growing acropoid corals, which allows slower-growing corals to establish themselves. However, when their numbers surge, these echinoderms can severely damage coral reefs. A notable example of this is the harm caused to the Great Barrier Reef. Over a 30-year period, studies observed a 50 percent decline in coral cover on surveyed reefs, with half of that loss attributed to an overpopulation of crown-of-thorns starfish.
4. The Strange Cushion Stars
Sea stars are typically named for their star-shaped bodies, but some species defy this norm with entirely different appearances. One such example is the cushion star (Culcita novaeguinea), found in the Indo-Pacific region. While genetically a sea star, its appearance is far removed from the typical form, with nearly no arms and a rounded, blob-like shape. Often covered in tiny spines, these peculiar creatures resemble a pincushion. They can grow up to 25 centimeters (10 inches) across and come in a variety of colors.
Although sea stars are usually skilled predators, capable of using their powerful tube feet to hunt and open shellfish, cushion stars are much more passive. These animals primarily feed on detritus, plant matter, and occasionally coral. Cushion stars also engage in an unusual symbiotic relationship, serving as a home for small fish that live within their water-filled body cavity, while invertebrates on the exterior help clean their spiny surface.
3. Sea Stars Have the Ability to Change Gender at Will
In the animal kingdom, the simpler the creature, the more extraordinary its capabilities can be. Some sea stars have the remarkable ability to change their gender at will. They can begin life as one gender, switch to another, and even reverse the process later on. The reasons behind this gender fluidity are varied, including breeding needs and responses to environmental factors like water quality, temperature, and food availability. Gender differences in sea stars are subtle and not always easy to spot, although in certain species, males are smaller than females. Even flipping a sea star over may not always reveal its gender.
Unlike most species, where individuals are born with a fixed gender, cushion stars are always born male and later transition to female. Sea star sexuality is full of fascinating quirks. For instance, some species possess both male and female reproductive organs, allowing them to switch roles during mating. Additionally, sea stars don’t physically interact during mating; instead, they release sperm and eggs into the water where they mix. In some cases, sea stars also display a form of parental care, carrying their young on their backs until they are ready to venture independently across the ocean floor.
2. Sea Stars Have the Strangest Eyes
Despite lacking a typical blood circulation system, central nervous system, or face, one might assume sea stars don’t have eyes. However, they actually do have eyes, located at the tips of their arms, which help them navigate. These eyes function similarly to those of arthropods like insects and crustaceans, allowing sea stars to gather visual information from all directions. It was once a puzzling question how sea stars could see without a brain, but recent studies have shed light on this mysterious feature.
Recent research has revealed that sea stars use their eyes to navigate marine environments with surprising accuracy. In a study by Anders Garm from the University of Copenhagen and Dan-Eric Nilsson of Lund University, blue starfish were observed to move aimlessly until they came within 2 meters (6.5 ft) of a reef. Upon detecting the reef visually, the sea stars would head straight toward it. Although their light-detecting cells are slow and they are colorblind, steady images like reefs appear as dim splotches that guide the animals toward their target environment.
1. The Enigmatic Sea Star Wasting Disease
The alarming reports of mass sea star die-offs have brought widespread attention to these marine creatures' struggles. Known as sea star wasting disease, this condition causes severe decay, lesions, and ultimately fragmentation, leading to the death of the animals. A densovirus has been implicated in these die-offs, particularly the massive 2014 event along the Pacific Northwest coast. While die-offs are not new, the frequency and scale of these events raise serious concerns about the vulnerability of sea star populations to infections with varying causes. These incidents highlight the risks posed to keystone species and have sparked significant concern in the conservation community.
The massive loss of sunflower stars on Vancouver Island is one notable example, while other sea star species across the west coast, from Mexico to Alaska, have also been affected. The level of susceptibility varies between species. Key questions remain about the ecological consequences of these declines and how they might impact the health and biodiversity of marine ecosystems. Researchers are particularly interested in the role of environmental factors, such as pollution, which could increase the likelihood of infections and accelerate the deaths of sea stars. Ongoing research is being conducted, and the public has been encouraged by organizations like the Vancouver Aquarium to report their observations.
