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Invertebrate of the Week #17 – Stygiomedusa gigantea

A total of 114 times in 110 years. That’s how many times researchers have observed the elusive Stygiomedusa giganteaa giant deep sea jelly with a bell 1.4 meters wide and broad arms 10 meters long trailing behind like drapes.

Stygiomedusa gigantea
Stygiomedusa gigantea. Source: MBARI ROV footage.

To-date, almost nothing is known about this rarely-encountered animal aside from detailed accounts of its physical description and sporadic sighting notes. It appears to have a wide distribution; having been collected in Antarctica and encountered during ROV operations in Japan, the Gulf of Mexico, Gulf of California. It has been encountered, live (in situ), at depths between 750 meters down to 2187 meters in the Gulf of California and between 996m to 1747m in the Gulf of Mexico. Specimens were collected in Antarctica at depths between 723  and 6669 meters.

Stygiomedusa gigantea
Stygiomedusa gigantea. Source: MBARI ROV footage.

The broad, curtain-like arms are perhaps an adaptation which allows to jelly to better capture prey items when it encounters them. It is generally assumed that prey items are difficult to locate in the deep sea and predators have evolved extraordinary means of keeping a hold on prey when it is encountered. You can image the large arms swirling around a hapless animal and completely enveloping it in stinging drapes.

Stygiomedusa gigantea
Stygiomedusa gigantea. Source: MBARI ROV footage.

Remarkably, at least one animal might maintain a symbiotic association with the jelly. A fish, currently believed to be Thalassobathia pelagica, has been observed swimming near the bell of the jelly, though the exact nature of the relationship remains to be established. It’s possible that the relationship favors the fish more than the jelly, with the fish opportunistically feeding on scraps from the jelly’s feedings while also benefiting from a degree of protection-by-association.

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