Sea stars, commonly known as starfish, are not actually fish but belong to the phylum Echinodermata, making them relatives of sea urchins and sea cucumbers. These invertebrates are widespread, inhabiting all of the world’s oceans, from shallow intertidal zones to the deep seafloor, with approximately 2,000 known species. Their presence across diverse marine environments points to their significant contributions to the health and balance of ocean ecosystems.
Key Ecological Contributions
Sea stars are recognized for their significant roles in marine environments, often acting as keystone species. This concept, describing a species with a disproportionately large impact on its ecosystem, was first identified through studies of the ochre sea star, Pisaster ochraceus, in the intertidal zones of the Pacific Northwest. This sea star primarily preys on mussels, preventing them from monopolizing rocky surfaces. By controlling mussel populations, Pisaster ochraceus creates open space, allowing a wider array of organisms, like barnacles and algae, to establish and thrive.
Beyond mussels, many sea star species are generalist predators, consuming a diverse range of marine invertebrates. Their diet can include barnacles, snails, chitons, and even other sea stars. This broad predation helps regulate the populations of numerous species, preventing any single one from becoming overly dominant. This regulation supports species richness and maintains ecological balance within their habitats.
A notable example of their impact on biodiversity is the sunflower sea star (Pycnopodia helianthoides), which preys on sea urchins. Sea urchins are voracious grazers of kelp, and unchecked populations can devastate kelp forests, creating “urchin barrens” devoid of life. By keeping urchin numbers in check, sunflower sea stars help sustain healthy kelp ecosystems, which provide vital habitat and food for countless marine organisms.
Many sea stars also act as scavengers, consuming decomposed organic matter and contributing to seafloor cleanliness. Some species function as ecological engineers by burrowing into sediments. This activity aerates the substrate and redistributes nutrients, benefiting benthic communities and nutrient cycling. In deeper ocean environments, certain sea stars are significant predators of sponges and corals, illustrating their diverse roles across marine ecosystems.
Indicators of Ecosystem Health
Sea stars serve as important bioindicators, meaning their presence, abundance, and health can reflect the overall condition of a marine environment. Their sensitivity to environmental changes makes them valuable monitors for ocean health. Changes in sea star populations or the prevalence of diseases among them can signal underlying ecosystem problems.
For instance, studies have shown that sea stars can accumulate heavy metals and polychlorinated biphenyls (PCBs) in their tissues, with concentrations mirroring pollutant levels in the surrounding sediment. Their responses to stressors, such as changes in pH, oxygen levels, or temperature, provide insights into climate change impacts. The widespread outbreaks of Sea Star Wasting Syndrome, potentially exacerbated by warmer ocean temperatures, underscore their role as indicators of broad environmental shifts.
Consequences of Sea Star Decline
When sea star populations diminish, the ecological balance of marine environments can shift dramatically, leading to cascading food web effects. As top predators, their absence results in unchecked prey proliferation. For example, the decline of Pisaster ochraceus leads to an overgrowth of mussels, which can then monopolize space on rocky intertidal shores. This expansion reduces available habitat for other species, diminishing overall biodiversity.
A striking example comes from the Sea Star Wasting Disease (SSWD) epidemic, which began in 2013 and caused mass mortalities along the North American Pacific coast. The loss of predatory sea stars, especially the sunflower sea star, led to a dramatic increase in sea urchin populations. These unchecked urchins overgrazed kelp forests, transforming once-vibrant underwater ecosystems into barren “urchin barrens.” Kelp forests are crucial habitats supporting a wide array of marine life and contributing to carbon sequestration.
The disruption from sea star decline extends beyond immediate predator-prey relationships. Food webs become imbalanced, and biodiversity loss can destabilize entire ecosystems. The severe ecological shifts observed during the SSWD outbreak illustrate the widespread negative impacts when these creatures are removed from their natural roles. Recovery from such disruption can be slow and challenging.
Major Threats to Sea Stars
Sea stars face multiple environmental challenges that threaten their populations globally. The most prominent threat is Sea Star Wasting Disease (SSWD), a severe affliction characterized by lesions, arm twisting, and eventual disintegration, often leading to death. Recent research has identified a bacterium, Vibrio pectenicida, as a cause of SSWD. This disease is exacerbated by rising ocean temperatures, as the bacterium thrives in warmer conditions.
Climate change contributes significantly to sea star vulnerability. Warmer waters can reduce oxygen availability, making sea stars susceptible to suffocation, and lead to increased bacterial growth that further depletes oxygen. Ocean acidification, resulting from increased carbon dioxide absorption, poses another threat. It can hinder the development and growth of sea star larvae and juveniles, impacting their ability to form calcified structures. Additionally, pollution, including heavy metals, microplastics, and nutrient runoff, can compromise sea star health and interfere with their development.