Starfish are some of the most recognizable marine animals found in tide pools and on reefs around the world. Their radial bodies and ability to regenerate lost limbs have fascinated people for centuries. But where exactly do these creatures fit taxonomically?
If you’re short on time, here’s a quick answer: starfish are definitely not crustaceans. They belong to a different subphylum entirely.
In this article, we’ll take an in-depth look at starfish biology and taxonomy to understand why they aren’t considered crustaceans. We’ll examine their physical traits, feeding behaviors, nervous systems, and more while comparing them to true crustaceans like crabs, lobsters, and shrimp.
Read on to learn all about these beloved echinoderms and satisfy your curiosity about where they fall in the animal kingdom.
The Taxonomy and Evolution of Starfish
Starfish Are Echinoderms, Not Crustaceans
Starfish, along with other marine creatures like sea urchins and sand dollars, are part of the phylum Echinodermata. This phylum is characterized by a five-fold symmetry, a mesodermal calcified skeleton, and a water vascular system.
The name “Echinodermata” comes from the Greek words for “spiny skin”, referring to the hard plates that cover the bodies of these animals.
In contrast, crustaceans like crabs, lobsters, and shrimp belong to the subphylum Crustacea. While starfish live on the sea floor, crustaceans tend to be more mobile and inhabit a wider diversity of habitats.
The most notable difference between starfish and crustaceans is that crustaceans possess jointed legs and external skeletons, while starfish move using tube feet and have endoskeletons.
So while they both inhabit marine environments, starfish and crustaceans belong to completely different branches on the evolutionary tree of life. Their physical features and body plans are unique to their respective phyla.
Evolutionary History and Relation to Other Echinoderms
Echinoderms first emerged in the Cambrian period, over 500 million years ago. Of all the echinoderm classes, starfish are believed to be the most recently evolved—the first definitive starfish fossils date back to the Ordovician period, around 450 million years ago.
Early starfish likely evolved from an ancestral free-floating echinoderm larvae. Over time, these larvae began exhibiting pentaradial symmetry and settled to the seafloor. They eventually developed the tube feet and hydraulic water vascular system that became characteristic to all starfish species.
Starfish are closely related to other echinoderms that also display pentaradial symmetry, such as brittle stars. However, they belong to a different class (Asteroidea) than other star-shaped echinoderms.
While they share ancestral origins, starfish and brittle stars diverged evolutionarily and developed distinct body plans.
Paleontologists have tracked the evolution of starfish extensively through the fossil record. Numerous extinct species of starfish displaying transitional anatomy have been identified. This fossil evidence has allowed scientists to reconstruct the evolutionary history and relationships of starfish to other ancient organisms.
Anatomical Differences Between Starfish and Crustaceans
Body Symmetry
Starfish display radial symmetry where their body parts are arranged around a central axis. Crustaceans like crabs and lobsters display bilateral symmetry with their body parts arranged on either side of a central plane.
The body plans allow starfish to move equally well in multiple directions, while crustaceans tend to move forwards and backwards.
Endoskeleton vs. Exoskeleton
Starfish have an endoskeleton made up of ossicles (small plates of calcium carbonate) underneath their skin to provide structure and support. Crustaceans have an external exoskeleton composed largely of chitin to protect their bodies.
The exoskeleton needs to be molted and replaced as the animal grows in size.
Tube Feet and Water Vascular System
Unique to echinoderms like starfish is their water vascular system – a network of canals filled with seawater. Attached to these canals are tube feet containing suction cups. The hydraulic pressure in the system allows the tube feet to extend and retract for locomotion and grasping prey.
Crustaceans lack these structures.
Sensory Structures and Nervous System
Starfish have a simple nervous system with a nerve ring around their central disk and radial nerves extend into each arm. Thousands of sensory cells on their skin can detect touch, temperature, orientation and the presence of chemicals.
Crustaceans often have stalked compound eyes, two pairs of sensory antennae, mechanosensory hairs, and chemoreception abilities via smell and taste.
Diet, Feeding, and Digestion
Crustacean Diets
Crustaceans, including crabs, shrimps, and lobsters, are largely omnivorous animals that feed on both plant and animal matter. Common food sources are algae, plankton, mollusks, smaller crustaceans, and pretty much anything they can find (1).
Being aquatic animals, crustaceans employ various feeding mechanisms suited to their environments, like filter feeding, grazing, hunting, and scavenging (2).
Starfish are Predatory Carnivores
In contrast to crustaceans, starfish are strictly predatory carnivores that hunt live prey. With no true blood system or gills, starfish absorb seawater into their bodies and filter oxygen out while expelling waste. This limits them to oxygen-rich shallow waters where prey is abundant (3).
Being echinoderms instead of crustaceans, starfish have much slower metabolisms, so they can survive long periods without food before needing to feed again.
Extensible Stomachs
A fascinating and unusual feature of starfish biology is their ability to push their stomachs outside of their bodies. Once a starfish has captured prey, usually bivalves like clams or oysters, it can eject its cardiac stomach from its mouth and envelop the prey to begin extracellular digestion (4).
The starfish essentially digests food outside its body as a survival adaptation to living and feeding in tight spaces between rocks and coral where swallowing food is difficult.
| Crustacean Diets | Starfish Diets |
|---|---|
| Omnivorous – eat plants & animals | Strict carnivores |
| More active lifestyles & faster digestion | Slow metabolism – can go months without eating |
| Employ filter feeding, grazing, hunting, scavenging to eat | Hunt live prey and use extensible stomachs to digest externally |
References
- Feeding and Digestion in Crustaceans
- Crustacean Food and Feeding
- Starfish Diet and Eating Habits
- How can a starfish’s stomach be outside its body?
Reproduction and Life Cycles
Larval Development in Crustaceans
Crustaceans have complex life cycles, often involving a larval stage that looks very different from the adult form. Many crustaceans hatch from eggs as tiny larvae called zooplankton that drift in ocean currents.
These larvae molt and change shape as they mature, eventually resembling miniature versions of the adults. For example, lobster larvae look nothing like adult lobsters at first. After several molts that allow them to evolve into their adult body layouts, the maturing young crustaceans settle to the seafloor and take up adult lifestyles.
Unique Features of Starfish Larvae
Like crustaceans, starfish also begin life as tiny planktonic larvae. However, starfish larvae have some unique features that distinguish them from crustacean larvae. The early starfish embryo develops cilia and a bilaterally symmetrical body as it begins feeding in the plankton.
Within a couple of days, the larva develops a gut and the beginnings of a water vascular system – structures never seen in crustacean larvae. Within a week, the starfish larva takes on the shape of a typical starfish with 5 arms radiating from a central disc, albeit on a tiny scale.
From this stage onward, the larva resembles a minute starfish more than a shrimp-like crustacean.
According to a 2022 study published in the Biological Bulletin (1), starfish larvae also tend to remain planktonic for longer periods (weeks to months) compared to crustacean larvae before they settle onto the seafloor.
This extended developmental period allows starfish larvae to disperse over wide geographic ranges via ocean currents.
Asexual Reproduction
While most crustaceans only reproduce sexually, some starfish exhibit asexual reproduction as well. For example, species like the blue starfish (Linckia laevigata) and giant California sea star (Pisaster giganteus) can reproduce asexually by division.
When a starfish divides, part of the central disc splits away from the original animal, forming an autonomous new starfish. The detatched section regrows its missing body parts, ultimately forming a clone of the parent sea star.
Other starfish like the red and purple sunstar (Haliaster helianthus) display fissiparity – they split across the central disc so that each arm becomes an independent animal. Over time, the detached starfish arms regrow their missing central discs and missing arms.
This process allows a single starfish to spawn up to 5 cloned offspring.
So while most sea stars and crustaceans share an indirectly developing planktonic larval stage, starfish larvae are shaped like miniature starfish and take longer to settle onto the seafloor. Starfish can also reproduce asexually via division, fissiparity, and arm regeneration – unique abilities not found amongst crustaceans.
Ecological Roles and Habitats
Crustaceans as Prey and Scavengers
Crustaceans such as crabs, shrimps, and lobsters make up an important part of the diet for many starfish species. The nimble movements and protective shells of crustaceans pose a challenge, but starfish have evolved powerful methods of predation.
Their suction-cup tube feet allow them to pry open shells or hold down crabs and shrimps. Once captured, the prey is passed to their eager mouths. Scavenging on dead and decaying crustaceans is also common among bottom-dwelling starfish.
Starfish as Important Predators
With their diversity of feeding strategies and habitat range, starfish play critical roles as predators in many marine and coastal ecosystems. Their consumption of bivalves like mussels and oysters can shape the abundance and distribution of these foundation species.
In the intertidal zone, starfish are one of the principal predators, providing an important check on competitive dominance by crustaceans and mollusks.
Overlap in Intertidal Zones
There is substantial overlap between starfish and crustaceans in intertidal habitats. Competition for food and habitat resources is common. However, crustaceans may gain some protection by hiding in reef crevices and burrows too small for most starfish to enter.
Mobile crustaceans can also flee rapidly from approaching starfish. Overall, the predator-prey dynamics between starfish and crustaceans add to the diversity and connectivity of the intertidal ecosystem.
Conclusion
While starfish and crustaceans share some superficial similarities and occupy some of the same marine habitats, they are taxonomically distinct. Starfish belong to the phylum Echinodermata while crustaceans are arthropods.
They have radical differences in anatomy, physiology, reproduction, and ecology that clearly separate them.
Hopefully this article shed some light on starfish biology and why they aren’t classified as crustaceans. Their radial bodies and amazing regenerative abilities are just some of the many reasons why starfish stand out in the animal kingdom.
The next time you find one of these creatures along the seashore, take a moment to appreciate the echinoderm wonder that it is!
