Starfish, often known as sea stars, are marine invertebrates belonging to the phylum Echinodermata. They are recognized by their radial symmetry, typically possessing five or more arms radiating from a central body. These animals also feature tube feet, operated by a hydraulic system, which aid in movement and feeding.
Starfish inhabit diverse marine environments across all the world’s oceans, from shallow intertidal waters to abyssal depths. Beyond their distinctive appearance, these creatures perform important functions within marine ecosystems.
Starfish as Key Predators
Starfish are predators of sessile or slow-moving invertebrates, influencing the structure of marine communities. They commonly prey on mussels, clams, oysters, barnacles, and sea urchins. Their feeding method is unique; many species evert their cardiac stomach through their mouth to digest prey externally. This allows them to consume organisms much larger than their oral opening, as digestive enzymes are secreted onto the prey before the partially digested material is reabsorbed.
Some starfish species are recognized as keystone species, meaning their presence has a disproportionately large effect on their environment relative to their biomass. The ochre sea star, Pisaster ochraceus, found along the Pacific coast of North America, exemplifies this role. Robert Paine’s foundational research demonstrated that Pisaster ochraceus predation on mussels, such as Mytilus californianus, prevents these mussels from dominating rocky intertidal zones. By controlling the mussel population, starfish create space for other species, thereby maintaining the overall biodiversity of the ecosystem.
The removal of such keystone predators can lead to ecological shifts, known as trophic cascades. In the absence of starfish, dominant prey species like mussels can rapidly increase in number, outcompeting and displacing other organisms. This can result in a reduction in species richness and a less diverse community. The crown-of-thorns starfish (Acanthaster planci), while sometimes causing outbreaks, also influences biodiversity by preying on fast-growing corals, which can allow for greater coral diversity when populations are balanced.
Starfish as Scavengers and Recyclers
Many starfish species contribute to marine ecosystems as scavengers, consuming dead or decaying organic matter. This scavenging activity provides a cleaning service to the marine environment.
By consuming organic waste, starfish help prevent the buildup of decaying material. Their feeding processes break down this matter, facilitating the recycling of nutrients back into the ecosystem. This nutrient cycling supports the broader marine food web by making resources available to other organisms. Examples of species known for their scavenging habits include granulated sea stars and the blue sea star (Linckia laevigata).
Starfish as Bioindicators
Starfish serve as indicators of marine environmental health due to their sensitivity to changes in their surroundings. Their well-being can reflect alterations in water quality, temperature, salinity, and pollution levels. Monitoring starfish populations, their health, and behaviors, such as sea star wasting syndrome, provides scientists with early warnings about broader ecological issues.
A decline in starfish populations or an increase in disease instances can signal environmental problems. For example, sea star wasting syndrome, which has caused mass mortalities in recent years, is linked to environmental stressors such as warming ocean temperatures and pollution. Ocean acidification can also impact starfish by affecting their overall health and resilience, increasing their vulnerability to pathogens. Observing these changes in starfish can prompt timely conservation efforts and management strategies to address issues like climate change impacts, ocean acidification, or contamination.