The Emperor shrimp is a very small, rarely imported shrimp, typically sporting bright orange and white primary colors. They can be found across the Indo-Pacific. Typically, we see this species associated with “Medusa worms” in captivity as their most popular symbiont, but in nature, they can form relationships with a wide array of different species, including sea cucumbers writ large, nudibranchs, cowries, “sea slugs” and starfish.
Often, multiple shrimp will be found per host. Shrimps usually split into pairs and even enter territorial disputes for the most desirable spot on their host. Another interesting fact is that the white chromatophores on the top of their carapace can shift color depending on their host.
It’s believed Emperor shrimp form these relationships primarily for transportation. The host will slowly move across the substrate while the shrimp hitches a ride, occasionally leaving their vehicle to graze. While it’s widely believed this relationship is commensal, with only the shrimp gaining benefit from it, some cursory evidence has suggested that the shrimp may aid in the control of parasites found on the ectoderm of their host.
While it’s still debated whether the relationship between these species is commensal or symbiotic, it is likely to be required for the long-term survival of the shrimp.
While little data exist about the emperor shrimp in academia, an interesting study reveals how their close relative, “Zenopontonia soror,” can identify and find a host; while this species is known only to be a symbiont of sea stars, it’s likely its biochemical mechanism for host identification are conserved due to how closely related the species are.
It was found that there is no evidence of specific receptors for identifying a host of any kind and that this species of shrimp relied on a combination of vision and chemoreceptors that could identify the “smell” of their original host. Additionally, it was determined that the shrimp preferred the host they were initially found on, and they could, with some accuracy, find this host again based on their vision, which suggests the role of environmental imprinting in determining host selection.
Essentially, if a shrimp larva settles on one host, it will grow up and adapt to life on that host, and, therefore, it will continue to seek it out in the future. Similar behavior has been observed in some species of anemone shrimp and female pinnotherid crabs, which are symbionts of various mollusks. This gives some credence to the idea that host imprinting may be a general mechanism utilized across commensal crustaceans.
Husbandry
After scouring the forums, it seems aquarists have typically had poor luck keeping this species in captivity without a host. Additionally, because they are widely sold already paired with medusa worms, this further complicates things due to the difficulty of keeping such echinoderms. Since medusa worms are believed to feed almost exclusively on detritus, they often slowly starve in captive systems without extremely large and robust sand beds.
Some old posts on Reef Central cited success in getting emperor shrimp to pair with species from the Holothuria genus, such as “tiger tail sea cucumbers,” which are much easier to keep in the industry. This could provide a much more accessible avenue for hobbyists to keep these beautiful shrimp with success.
Taxonomy
While often referred to as Periclimenes imperator in the industry, this name is not formally accepted. Initially, the species was characterized in 1922 by Kemp, who described them as Periclimenes rex. In 1967, AJ Bruce described the same species as Periclimenes imperator in his work “Notes on some Indo-Pacific Pontoniinae III-IX. Descriptions of some new genera and species from the western Indian Ocean and the South China Sea.”
Eventually, in 1975, Bruce updated the genus to Zenopontonia from Periclimenes in “Notes on some Indo-Pacific Pontoniinae, XXV. Further observations upon Periclimenes noverca Kemp, 1922, with the designation of a new genus Zenopontonia, and some remarks upon Periclimenes parasiticus Borradaile (Decapoda Natantia, Palaemonidae).”
Since the species had been initially described as Periclimenes rex in 1922 by Kemp, this made Bruce’s later 1967 description of them as Periclimenes imperator unaccepted due to it being a “junior subjective synonym” (meaning the species had already been described, thus a new name was not needed). Despite this mistake, Bruce did shift the genus of this species; therefore, the currently accepted name of what we know as “emperor shrimp” is “Zenopontonia rex,” keeping both the original species’ new and up-to-date genus names.
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Sources:
https://www.sciencedirect.com/science/article/abs/pii/S0022098119302771
https://www.marinespecies.org/aphia.php?p=taxdetails&id=871470#sources
https://www.marinespecies.org/aphia.php?p=taxdetails&id=206501
https://www.marinespecies.org/aphia.php?p=taxdetails&id=210576
Main image credit Nhobgood Nick Hobgood, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons
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