Sea squirt

 The sea squirt (or tunicate) is a curious creature.  It is an animal, but spends its adult life in one location, like a plant.  It is closely related to vertebrates, but has no backbone.   It is a hermaphrodite, each individual having both male and female sex organs.  Representatives of this subphylum are found in almost all ocean waters, but most individuals are too small and innocuous to garner notice by the layperson.  The sea squirt is roughly barrel-shaped and its entire body is invested with a tough coating called a tunic.  It has two openings: an in-current siphon and an out-current siphon.  The sea squirt sucks water through the in-current siphon, filters plankton and other nutrient material from the seawater, and squirts the remaining water out through the out-current siphon.  In some parts of Korea, Japan, and China, sea squirt is considered a delicacy.  In recent years, an aquaculture industry has developed, particularly in Korea, where various species of sea squirts are grown and harvested commercially.   A few species of sea squirts, though, have shown an opposing tendency.  These outliers have, when arriving in some new locations, proved able to expand over considerable areas and greatly reduce populations of native plants and animals.  In 2003, dense mats of sea squirts were first discovered clinging to gravel and other substrates of the Georges Bank off the coast of New England.  Similar colonies have recently been found along the west coast of North America.  Locals have started referring to sea squirts as rock vomit, because they usually attach to and sometimes envelope rocks on the sea floor.  While sea squirt larva travel great distances on ocean currents before developing into sedentary adults, there is evidence that the larva are also transported in ballast water and on hulls of ships.