PREDATORS / PREY
Sea stars are especially successful because as adults they have relatively few predators. Their rigid calcareous skeletons deter many carnivores, although ironically other sea stars rank among their most dangerous adversaries.
The glaucus-winged gull quite often eats intertidal sea stars. This gull swallows entire, surprisingly large PURPLE STARS by deftly bending two of the stiff arms together and stuffing them into its beak. The remaining three arms are then pushed against the ground to bend them together. Then the gull gradually swallows the whole works. Dainty table manners don’t count in the wild.
Subtidally, sea stars fall prey to crabs, including the Puget Sound king crab, the hairy-spined crab and the red rock crab. Perhaps the most bizarre dining behaviour yet documented is a video showing a 3 m (10 ft) sixgill shark swallowing a hefty SUNFLOWER STAR in one gulp. Flossing its teeth perhaps?
Although sea stars are well protected by their rigid skeletal meshwork of calcareous plates and spines, many also possess pedicellariae. These tiny pincers possess a formidable grip for their size and keep the surface of the star free of unwanted pests and ectoparasites. The pincers vary in dimension and shape from one species to another, but some sea stars possess especially tenacious pedicellariae.
The SUNFLOWER STAR, SAND STAR and VELCRO STAR can deliberately jettison arms that have been trapped by a predator. Most other stars are not capable of such a feat, but can recover from mutilation by rapidly regenerating damaged or lost arms as long as a portion of the central disc remains.
Sea stars also possess a number of chemical defences that provide protection from predators. Most contain saponins, extremely foul-tasting compounds. A few even have deadly tetrodotoxins, nerve-blockers that can immobilize or even kill a predator. The SLIME STAR produces huge amounts of slimy mucous when disturbed, very effective at deterring an attacker.
The majority of PNW sea stars are carnivores (and occasionally scavengers) while the rest are omnivores and ciliary-mucous (using mucous secretions to trap organic particles) feeders. Sea stars feed by either swallowing their prey or by enveloping the food with their bodies while extruding their cardiac stomachs to digest the meal externally. The latter method enables sea stars to consume large prey such as clams and scallops too large to be swallowed whole.
Most carnivorous stars are not overly fussy about what they eat, but one prey species or group often predominates. Some, such as the SPINY RED STAR and the MORNING SUN STAR star have specialized diets consisting of a particular group of prey. Several sea stars, such as the SUNFLOWER STAR, the ROSE STAR and the NORTHERN SUN STAR are known to eat other asteroids, but in the PNW the MORNING SUN STAR is one of the few that feeds mainly on other stars.
It will take some time, but eventually this glaucus-winged gull will manage to swallow an entire PURPLE STAR plucked from a rocky beach at low tide.
Dozens of tiny pedicellariae (pincers) surround the spines on the surface of this VELCRO STAR. They provide the star with significant defences against predators and can actually capture small fishes that the star itself can consume.
Some stars can deliberately cast off an arm when attacked in order to escape. Here a MORNING SUN STAR has just attacked a SUNFLOWER STAR and is about to eat the single arm left behind by the fleeing prey.
The SLIME STAR has a unique defence against predators. It produces copious volumes of mucous that very effectively deter attackers.
Most stars, such as this LEATHER STAR, are able to digest prey outside their bodies by everting their cardiac stomach and enveloping the prey in its baggy folds.
The MORNING SUN STAR specializes in attacking and eating other stars. It quickly gains the advantage by crawling on top of the prey (in this case a BLOOD STAR), then securing it with its flexible arms and everting its stomach to begin digestion.
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