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 sedentaria, 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. This has to be one of the creepiest associations in the marine world! 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.
Juvenile fishes, such as the Pacific butterfish (Peprilus simillimus) seen in these two photos, often lurk in the vicinity of large jellies. When potential danger approaches, they dive into the protective confines of the bell or among the tentacles and oral arms. Somehow the fish manage to avoid the nasty sting of the ever-moving tentacles. Some jellies may harbor an entourage of a dozen or more fish. The silvery butterfish appear pretty conspicuous, but within an always moving jellyfish, the hitchhikers seemingly disappear in the mass of oral arms. Juvenile butterfish and other hitchhiking fish dine on zooplankton that the jelly has collected, and probably nibble on gelatinous tissue when captured prey are scarce. Eventually the fish decide that it’s time to strike off on their own, and they begin an independent adult existence. It’s not clear whether the jellyfish host benefits from this association, but the advantages to the hitchhiking fish seem apparent.
Crabs, such as the slender crabs seen here (Cancer gracilis), often spend their formative months in association with a jellyfish before assuming a benthic existence. Chrysaora colorata seem particularly favored by these crustaceans. It is not unusual to see an older, battle-worn Chrysaora with 50 or more tiny crabs hitching a ride. Unfortunately for the jelly, the relationship is not totally benign. The crabs dine on food that the jellyfish has labored to collect, and probably have no qualms about nibbling on delicate gelatinous flesh. They even can enter the stomach of the jelly without apparent harm. After drifting for many miles, the juvenile crabs somehow determine that the time is ripe to jump free and begin the perilous journey to the ocean bottom. These in turn produce the planktonic zoea larvae that seek out gelatinous traveling hosts.
Pelagic barnacles (Family Lepadidae) will attach to just about anything floating in the open ocean. One species, Alepas pacifica, has taken things a step further and sets up shop on the bells of large jellies, such as egg-yolk jellies (seen here in the photo), purple-stripe jellies, and at least 5 other scyphozoan species. Typically the barnacles, which may occur singly or in clumps of up to 8 to 10 individuals, are attached at the top of the bell in the center. To lighten the load on their gelatinous host, the hard shell component characteristic of other barnacles is very thin and reduced. It’s hard to say whether the jelly is harmed by it’s crustacean hitchhikers, but once attached, there’s not much it can do. With certain scyphozoan species, it appears that the barnacles are parasitic, feeding on gonadal tissue of the jellyfish. Most large jellies however do seem to avoid the extra load – it’s relatively uncommon to see an egg-yolk jelly wearing a pelagic barnacle cap.
A large number of crustaceans known as amphipods (mainly those in the Family Hyperiidae) are associated with gelatinous animals. Hyperiid amphipods often have species-specific relationships, such that a particular species of amphipod may be found only on one or perhaps several related species of gelatinous zooplankton. Medusae, siphonophores, ctenophores, pteropods and salps all serve as homes for these crustaceans. The unfortunate comb jelly in the photo here (Hormiphora) is burdened by over a dozen of the pesky hitchhikers. Often an amphipod will excavate a protective pit in the tissue of the host, or may be embedded deeper inside the animal. Some living amphipods can even be found inside the stomachs of hydromedusae. It’s not clear if these amphipods typically consume host tissue and what other harm they may present. With a load like the comb jelly pictured here, it would appear that there must be some disadvantage to hosting a throng of amphipods. If disturbed excessively, hyperiid amphipods will swim away from the host and seek another gelatinous home.
An association that is certainly unfavorable to the jellyfish host is that between the larval sea anemone, Peachia quinquecapitata, and certain hydromedusae including Mitrocoma and Clytia. Young planktonic anemone larvae are ingested by the jellyfish, and then feed on the gonads and stomach of their hapless host. Eventually the anemones (two are seen in this photo) drop off and assume a more typical benthic lifestyle as adults.
When you’re a juvenile fish, cast into the dangerous waters of the open ocean, predators are an ever-present threat. Everywhere, it would seem, there is someone seeking a tasty meal. With shelter at a precious premium, anything goes. That’s where large jellies, like the egg-yolk jelly (Phacellophora), can come in quite handy. In addition to providing a place to hide, the jelly’s stinging ability is probably an effective deterrent for many would be predators. Here a tiny juvenile fish finds comfortable accommodations among the tentacles and oral arms of an egg-yolk jelly. Eventually it will abandon its gelatinous host and assume an independent existence.