The purple sea urchin (Strongylocentrotus purpuratus) is a fascinating marine invertebrate found along the Pacific coast of North America. Known for its vivid purple color, spiny exterior, and important role in kelp forest ecosystems, the purple sea urchin has garnered attention from both ecologists and biomedical scientists. This article delves into the dietary habits of this unique creature, exploring its food preferences, feeding behaviors, and the impact of diet on its growth and reproductive capacity.
General Dietary Habits
The purple sea urchin is a grazing species with a diet that primarily consists of algae. As omnivores, they consume a variety of marine algae, including bull kelp and giant kelp. They are also known to feed on sponges, coral polyps, and dead animal matter, including dead urchins. Their mouth, located on the underside of their body, contains a complex structure called Aristotle's lantern, which is made up of five hard plates that function like a beak, allowing them to scrape algae off rocks and other surfaces.
Habitat and Food Availability
Purple sea urchins typically inhabit intertidal and subtidal zones, preferring shallow waters on hard substrates such as jetties and pilings. They can tolerate temperatures ranging from 41° to 74°F (5° to 23.5°C). Their habitat ranges from Alaska to Baja California Sur in Mexico, where they play a crucial role in maintaining the balance of kelp forest ecosystems.
Feeding Preferences and Selectivity
Strongylocentrotid sea urchins, the dominant herbivores in California kelp forests, strongly prefer giant kelp (Macrocystis pyrifera), but are highly catholic in their ability to consume other species. Sea urchins exhibit strong food preferences in the presence of a mixture of algae. When offered a mixed diet, purple sea urchins consume Macrocystis at the highest rate, over twice as fast as any other species. However, when algae were offered alone, the consumption rate of Chondracanthus and the mixed diet were similar to that of Macrocystis.
Experimental Studies on Diet and Growth
To understand the effects of different algal diets on the purple sea urchin, researchers have conducted controlled laboratory experiments. These studies have examined the impact of single and mixed species diets on consumption, growth, and gonad weight in Strongylocentrotus purpuratus.
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Experimental Setup
In one such experiment, urchins were fed single species diets consisting of one of four common species of macroalgae: the kelps Macrocystis pyrifera and Pterygophora californica, and the red algae Chondracanthus corymbiferus and Rhodymenia californica. A mixed diet containing all four species was also tested. The experiment was conducted over a 13-week period in a controlled laboratory setting.
Urchins used in the experiment were collected from a shallow boulder reef offshore of the University of California, Santa Barbara. They were then transported to the laboratory, placed in aquaria with running seawater, and starved for one week prior to the start of the experiment.
To measure jaw growth, each urchin was injected with the fluorescent marker tetracycline. This allowed researchers to track the growth of the jaw material present at the start of the experiment.
Results and Observations
The results of the experiment showed that diet type had a significant effect on all performance measures. Urchins fed Chondracanthus, Macrocystis, and mixed diets exhibited the highest test growth, jaw growth, wet weight gain, and gonad weight, with no significant differences between these three diets. Urchins fed Pterygophora exhibited significantly lower test growth compared to those fed Chondracanthus and Macrocystis diets, but had jaw growth and gonad weight that were not statistically different from either of them. Urchins fed Rhodymenia exhibited the lowest values of all growth metrics and gonad weight.
The four species of algae used in the experiment differed in nutritional value as determined by their C:N ratios. However, there was no relationship between an alga’s C:N ratio and food quality as measured by urchin performance, with the exception of a correlation between gonad weight and algal C:N ratio.
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Implications for Urchin Populations
The differences in urchin feeding behavior and growth observed between these diet types suggest the relative availability of the algae tested here could affect urchin populations and their interactions with the algal assemblage. Consumer growth and reproductive capacity are direct functions of diet. The biomass of Macrocystis fluctuates greatly in space and time, and the extent to which urchins can use alternate species of algae or a mixed diet of multiple algal species to maintain fitness when giant kelp is unavailable is important.
Cool Adaptations and Behaviors Related to Diet
Purple sea urchins have several cool adaptations and behaviors related to their diet.
Spines for Defense
Purple sea urchins have adapted large spines that can be as long as ½ the total width of their body, thus making some individuals 3-4 inches across. These spines are effective in deterring predators that might view this pin cushion-like animal as an unattractive food item.
Spine Movement
When shadows pass over an urchin, they have been observed to point their spines in the direction of the shadow, as if it were a potential predator. This behavior helps them to protect themselves from potential threats.
Tube Feet for Movement and Righting
The spines on the oral side (bottom) of their bodies are spatula-shaped spines that mostly aid in movement across sandy or rocky bottoms. If flipped upside down, urchins can right themselves through specialized movements of these spines. Purple sea urchins move using their spines but also "tube feet," which are actually part of their vascular systems and act as a hydraulic mechanism.
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Role in Kelp Forest Ecosystems
Sea urchins are dominant grazers in many benthic marine systems around the world and can exert a strong top-down influence on community structure. In kelp forests along the west coast of North America, strongylocentrotid sea urchins can have a large effect on the standing biomass of the giant kelp Macrocystis pyrifera and understory algal species.
Impact on Kelp Biomass
The standing biomass of giant kelp, a preferred food of sea urchins, fluctuates greatly in response to a range of physical and biological processes. When its abundance is low sea urchins are known to shift their diet to consume the remaining algal assemblage.
Keystone Species
The delicate balance between sea urchins and kelp forests is often taught in marine science and ecology classrooms as a classic example of what are known as keystone species in which protecting certain species leads to the survival of numerous other species in the ecosystem. When sea urchin predator populations suffer, such as when sea stars died off in 2013 due to sea star wasting disease, urchin populations can grow unchecked and contribute to widespread loss of kelp forests.
Conservation and Threats
Purple sea urchins may not be the cute and cuddly animals we are used to seeing in conservation advertising, but protection of these and other invertebrates is essential to the health and future of important ecosystems. Ocean acidification, habitat destruction, coastal development and physical damage from human activities, and warming oceans are all threats to sea urchin populations, which puts at risk the health of kelp forest ecosystems, including sea otter and sea star populations.