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Hitchhikers on Gelatinous Zooplankton
For creatures of the open sea realm, there are precious few protective sites. Many gelatinous animals serve as convenient traveling homes or resting places for a variety of other creatures. Certain types of larval fish and crustaceans are the primary users of this resource. Careful observation of gelatinous zooplankton will often reveal the presence of one or more hitchhikers.
Hitchhiking serves a number of purposes, including protection, a source of food, and distribution. Some larval or juvenile animals use their gelatinous host as a platform for development to adulthood. Other species may spend their entire lives on a jelly after settling down. Juvenile fishes, such as the medusafish (Icichthys lockingtoni), Pacific butterfish (Peprilus simillimus), and walleye pollock (Theragra chalcogramma) often lurk in the vicinity of large jellies. When danger approaches, they dive into the protective confines of the bell or among the tentacles. On the West Coast, purple-stripe jellies, sea nettles, moon jellies, lion's mane and egg-yolk jellies frequently harbor piscine joyriders. Medusafish are even occasionally seen inside large salps. In some cases the relationship is commensal, in which case the jellyfish is not apparently effected by the association. Some fishes, however, may be ectoparasitic or even predatory on their host jelly. It's not entirely clear how the fish avoid becoming a meal for the jellyfish. One possible mechanism is avoiding contact with the tentacles. It does seem hard to believe that a fish could somehow avoid touching the ever-moving tentacles while diving deep beneath the bell when danger approaches. Others include immunity to the nematocyst stings and production of mucus that reduces firing of nematocysts.
Crabs, such as the slender crab (Cancer gracilis), often associate with jellyfish before assuming a benthic existence. Pelagia colorata seem particularly favored by these crabs. Many hitchhikers grab food that the host has collected, but they may also consume host tissue. For this reason the association can be somewhat deleterious to the gelatinous host. An association that is certainly unfavorable to the host is that between the larval sea anemone, Peachia quinquecapitata, and certain hydromedusae.
A large number of amphipods in the family Hyperiidae are associated with many gelatinous animals. Medusae, siphonophores, ctenophores, pteropods and salps all serve as homes for these crustaceans. Often an amphipod will excavate a protective pit in the tissue of the host, or may be embedded deeper inside the animal. Females of one hyperiid amphipod, Phronima, actually take over the tests of certain pelagic tunicates and swim while covered in their modified protective “barrel.” Phronima broods eggs within the barrel, and the hatchlings then consume their home before searching for more salp victims. Certain salps are also used by males of the epipelagic octopus, Ocythoe tuberculata. The octopus uses jet propulsion to swim, even while inside its protective gelatinous home.
All photographs in the JelliesZone © David Wrobel and may not be used or copied without permission!