Jellyfish Anatomy – Benthic Forms

Australia: The Land Where Time Began

Jellyfish Anatomy – Benthic Forms

Jellyfish are usually thought of as drifting organism there are in fact some forms of medusae and ctenophores that are benthic, with the adult sexual stage spending their entire life on the seafloor. These benthic forms of medusae are trumpet-shaped organisms, stauromedusae, which use adhesive pads at the base of a slender foot that is stalk-shaped to attach to rocks or algae. Benthic forms of ctenophore and platyctenes, which are creeping organisms resembling flat worms.

Most of the stauromedusae and platyctenes have free-swimming larvae, though their adult forms are not able to swim. Gershwin says although the stauromedusae and platyctenes evolved from ancestral forms that were jellyfish-like, the adults have a completely different appearance and don’t drift in the water column. The stauromedusae and platyctenes are the least known by far, and the jellyfish are generally not known well.

The Asexual, clonal stage of the familiar drifting medusae are other benthic forms. As tiny polyps they are stuck to rocks, shells or algae on the floor of the sea, the benthic zone.

Stauromedusae

These jellyfish are described by Gershwin as “exquisitely beautiful creatures” that are shaped like champagne flutes, usually having 8 arms radiating out in a star pattern. There is a tuft of short tentacles at the end of each and at the end of each tentacle is a small ball. These tentacles function in the capture of food and defence and they are packed with stinging cells. Many species of stauromedusae have small organs, known as anchors, between their arms; the function of anchors is not known, though it has been suggested they may function as sensory organs. Stauromedusae are radially symmetrical, as is the case with other medusae. Externally, most species appear to be octoradial (8-parted), though they may be tetraradial (4-parted) internally, as is the case with most of their counterparts that are more traditional drifting medusae. The various species of stauromedusae have a wide variety of colours and patterns which helps to camouflage them among the red and green algae where they are usually found.

According to Gershwin one of the most interesting features of stauromedusae is that “they are upside-down of upside-down – i.e., whereas normal medusae have all the structural features of polyps, but upside down, stauromedusae have flipped right back upward again. Therefore, though they appear to be “right side up,” this orientation came about late in their evolution. It is believed their ancestral forms were normal medusae, the ancestors of which were normal polyps.

Platyctenes

Platyctenes are another enigmatic form. And as with stauromedusae there adults are entirely benthic. They are commonly encountered by scuba divers but are rarely recognised for what they are. They are essentially an oval-shaped thin film of tissue that resemble flatworms that glides over sponges and algae and among sea urchins spines. Their tentacles, which resemble those of the related sea gooseberries in that they bear many lateral filaments arranged in a single direction, similar to the barbs along one side of a feather, are the feature that gives away their true ctenophore nature.

The newly hatched larval stage of platyctenes has an appearance that is very similar to that of a miniature sea gooseberry, even drifting into the water column in a similar manner. It grows out of its planktonic stage as it matures and settles to the sea floor. According to Gershwin finding a suitable host on which to grow is one of the biggest challenges faced by a young platyctene.

Stauromedusae are essentially medusae that have turned downside up, so that their mouth faces toward the surface of the ocean, while platyctenes are essentially sea gooseberries or sea walnuts that have been extremely flattened and turned upside down with their mouth facing the sediment or their host. Their bodies, that are almost amoeba-like, are extremely soft. Ciliated papillae, which are believed to function as respiratory structures, are prominent intermittently on the upper surface. A pair of “chimneys” appears occasionally, one at each of the 2 long ends of the body and long feathery tentacles emerge from them. Platyctenes have striking colours and patterns, as is the case with stauromedusae, which help with camouflage against the host species.

Sources & Further reading