Facts About the Commensal Emperor Shrimp

Emperor Shrimp

One of the fascinating things about the underwater world is that no matter how closely you look at something, you can always be surprised by what is living down there.

A sea cucumber or urchin might seem like something worth ignoring by most divers, however our dive guides know that it needs a closer look, because who knows what is living on it!

When you start looking at the mundane things in closer detail, a whole new world will begin opening up to you. Today, we will be talking about the emperor shrimp – a small, yet fascinating species of shrimp, that might not be particularly rare, but is often overlooked by divers who are in a rush to get to the ‘more interesting things’.

What is an Emperor Shrimp?

Emperor Shrimp

Emperor Shrimp, scientifically known as Periclimenes imperator, are a small species of commensal shrimp that are part of the larger family Palaemonidae. They are regular sightings for us in Bunaken Marine Park, however they can also be found throughout much of the Indo-Pacific.

A fully grown Emperor Shrimp measures less than two centimetres from head to tail, but being small is necessary when you spend your life hitching a ride on someone else’s back (or belly, or gills…). They can be easily identified by their orange sides, white back which is flecked with tiny orange dots, and orange claws that are tipped with purple.

What does ‘Commensal’ Mean?

Emperor Shrimp

They are one of many different species of ‘Commensal Shrimp’, meaning that it forms a symbiotic relationship with at least one other species.

Commensalism is a symbiotic relationship where one party involved benefits from the relationship, while the other (or others) receive no benefits, nor do they come to any harm. An easy land example of this would be a spider building its web on a tree. While the spider is benefiting from having a place to build and hide, the tree is getting nothing from this arrangement, however it is also not suffering in any way.

What Commensal Relationships do Emperor Shrimps Form? 

Emperor Shrimp

An emperor shrimp is never spotted on its own.

It will always be hitch hiking on the back of a much larger, slow moving species of Holothurian (the scientific term for sea cucumber) or one of the larger species of Nudibranch, such as the Spanish Dancer (Hexibranchus sanguineus). It is more common to see a single emperor shrimp on a host, but if you keep an eye on the host you may spot a second shrimp hitching a ride. When there are two shrimps on a single host, they will often fight over who gets the best foraging turf – which is usually either around the mouth, or the anus.

The shrimp will live on the host’s surface, which provides it from sanctuary from the many dangers of the ocean. Should any potential predators appear, the shrimp will simply disappear underneath its host, or in other hard to reach places, such as between the branchial plumes of a dorid nudibranch.

It not only gets protection from its giant host. The emperor shrimp will also benefit as it does not need to hunt for food – instead, as the host moves and eats, it churns up sand, and the food is practically handed to the shrimp!

Although the emperor shrimp is considered a commensal shrimp, there is evidence to suggest that they will eat parasites and fungus from their hosts, which actually means that they form a mutualistic relationship (where all parties involved benefit from the relationship) with their host, rather than a commensal relationship.

What does an Emperor Shrimp Eat? 

Emperor shrimps are both carnivores and detritovores, which means they survive on a diet of other animals as well as feeding on decaying organic material.

When their host moves over the sea bed, they will churn up the sand, revealing a plethora of microscopic organisms (both dead and alive) for the emperor shrimp to feed on.

Interestingly, they have also been observed feasting on the eggs that are released when their host sea cucumber is spawning. This feeding is technically negatively impacting the host, as it reduces the likelihood of the host managing to reproduce, although the emperor shrimp is so small it couldn’t possibly cause a serious dent on the number of eggs released.  If you take this into account, there could be a valid argument to say that that the relationship is also slightly parasitic. Emperor shrimps are a great example of the grey area between the different symbiotic relationships.

Where can I Dive with Emperor Shrimps? 

Because their hosts do not have a fixed location (unlike anemone shrimp, for example), emperor shrimps can move around the dive sites, and even leave them completely.

While they can be spotted over sloping walls, such as Lekuan III, their hosts tend to live over sand and muddy flat bottoms, so you have a much better chance of spotting them while muck diving on the gentle slopes of the North Sulawesi mainland.

The majority of the time we spot them during night dives, as most of their hosts are nocturnal, including almost all sea cucumber species, as well as the Spanish Dancer nudibranch. That doesn’t mean we cannot find them during the day too, as they are also known to live on T-bar nudibranchs, which we tend to see more of during the day.

Photographing Emperor Shrimps

 Luckily for photographers, emperor shrimps are usually quite easy to photograph because their hosts are slow moving and not at all bothered by divers.

You may find the little shrimp tries to hide from you, but there are not so many hiding places on a sea cucumber, so if it does disappear, you just need to wait close to the host for a few minutes and it will come back out.

It is important that you never try to touch either the host or the emperor shrimp, as doing so may severely harm either one – sea cucumbers have very fragile skin, and gently poking a 1cm shrimp could easily kill it. Even if you cause it no harm, it is likely to be scared and will go deeper into hiding, which will ruin the encounter for you and the rest of your group.

Due to their size, you will need to have either a DSLR with a good macro lens attached (recommended either 60mm or 100mm), or if you are shooting with a compact, you will need a macro wet lens or a dioptre attached.

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