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92. Tryon’s Risbecia nudibranchs (Risbecia tryoni) with a pair of Imperator Shrimps


A pair of Emperor shrimps (Zenopontonia rex, previously known as Periclimenes rex and before that as Periclemenes imperator) is hitchhiking a ride on a couple of Tryon’s Risbecia nudibranchs (Risbecia tryoni).

These commensal shrimps feed on detritus that they pick off the bottom. The assumed reason behind this "hitchhiking" behavior might be that, thanks to the nudibranchs, they travel faster and cover more feeding grounds. Another reason can be that the shrimp, although brightly coloured themselves, benefit from the nudibranch’s aposematic colours to seek protection against eventual predators. Besides nudibranchs, they are also associated with sea cucumbers, starfish (although rare), and other big mollusks.

These beautiful shrimps come in different hues of red; the colour varies from pale pink to bright red. Females, who are slightly larger than males, carry their eggs under their abdomen. When the eggs hatch, they are released, and the larvae become planktonic.

The 3 cm/1.2-inch shrimp inhabit the tropical and warm waters of the Indo-Pacific, from Africa’s east coast and the Red Sea to the south of Japan and the waters of Australia’s northern coasts, including French New Caledonia, as well as the archipelagos of Indonesia and the Philippines. The shrimps, often found in pairs, venture within scuba diving limits to a maximum depth of 40m/130ft.

Emperor shrimps are not cleaner shrimps, and they don’t benefit their host in any way. This form of symbiosis, where one species benefits (the shrimps) and the other (the nudibranch in this underwater videoclip) is not significantly harmed or helped, is called commensalism. Because of this, the sausage-like small purple objects on these nudibranch’s gills, which are the egg sacs of a female copepod (a small parasitic crustacean), are left alone by the shrimps. The parasitic copepods burrow themselves into the nudibranch as juveniles. At a later stage, these eggs release planktonic larvae which search for a new victim.

Ironically, these small crustaceans often fall victim to parasites themselves. On many occasions, a big lump is seen on the side of their carapace. This lump holds a parasitic Bopyrid isopod (Bopyridae). These isopods are parasitic crustaceans and they infect various species of crustaceans.


To get the best out of this scene, I positioned myself as low as possible and pointed the camera upward in order to get the dark blue of the ocean as a background. During the editing, I boosted the red of the little shrimps and the purple edge of the nudibranchs’ mantle. The blue ocean background was slightly darkened to create a nice contrast with the bright colours of the nudibranchs and shrimp. Although the shrimps’ movements are inaudible to humans, I usually prefer to add a crawling sound in order to emphasise the crustaceans’ movements. On this occasion, I chose not to as in real life, the sound generated by the shrimp’s legs would be completely absorbed by the nudibranch’s soft and spongy texture. Adding a shrimp’s crawling sound would suggest that the nudibranch’s skin is a hard surface, which would make it very unrealistic.

More on this topic:

For another in-depth description about a nudibranch infected with parasites please go to our vlog post 29 or click on this link: https://www.beyondscuba.com/post/a-coi-nudibranch-infected-with-parasites

and for another in-depth description about a moray eel plagued with copepods go to post 130 or click this link: https://www.beyondscuba.com/post/plagued-with-sea-lice

and for more on parasitic Bopyrid isopods or click here or visit post 12:

and post number 79 for more information on the Imperial Shrimp’s other host, the Synaptid sea cucumber https://www.beyondscuba.com/post/a-sea-cucumber-of-the-synaptidae-family

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