A Pederson's shrimp (Ancylomenes pedersoni) showing off its slender white antennae. Photo credit: Sarah Solie.

A Pederson's shrimp (Ancylomenes pedersoni) showing off its slender white antennae. Photo credit: Sarah Solie.

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Shrimp are delicious, ask many fish and seabirds: the scrumptious crustaceans are on their menus. But it's not a great idea to eat cohabitants that want to help you, no matter how tasty. Cleaner shrimp routinely hop inside the gills and mouths of fish to rid them of troublesome parasites. But how do cleaner shrimp avoid ending up on their clients’ dining plates? Instead of slinking around surreptitiously to avoid attracting attention, cleaner shrimp put on a show to advertise their services. Some whip and wave long slender white antennae, while others show off their white legs by dancing a jig. ‘The consistent factor among all of these species is white and motion’, says Eleanor Caves from University of California, Santa Barbara, USA, attributing their success in attracting clients to the conspicuousness of their white body parts. But how do cleaner shrimp make their whipping antennae so brilliantly white to catch the eye of passing fish? Caves, with Alexander Davis and Sönke Johnsen from Duke University, USA, decided to take a close look at the antennae of three distantly related cleaner shrimp to find out why they are so intensely white.

First, Caves gathered Pederson's shrimp (Ancylomenes pedersoni) from Caribbean coral reefs, while professional divers around Sri Lanka collected Pacific cleaner shrimp (Lysmata amboinensis) and fire shrimp (Lysmata debelius). At Duke University, Davis took tiny portions from the animals’ slender antennae for close examination in an electron microscope. Scrutinising the close-up images, Davis and Johnsen found layers of microscopic (300–400 nm diameter) spheres sandwiched between thin layers of the chitin exoskeleton that make up the antennae. And their calculations revealed that the spheres were perfectly sized to reflect all light colours (ranging from 400 to 700nm), producing a dazzling shade of white. Caves also suspects that the minute reflective white spheres are composed of isoxanthopterin, a material that forms other white body parts and makes the shrimp's eyes shine brightly when light reflects from them. ‘These species (or at least a couple of them) seem to have co-opted that “optical eyeshine” toolbox to make a different body part really reflective’, Caves explains. But what was truly remarkable is that Pederson's shrimp originate in the Caribbean, while Pacific cleaner shrimp hail from the other side of the planet. ‘These species are … very distantly related and live in different geographic regions, yet they both have the exact same structure underlying the bright white body parts that they wave around to advertise cleaning services’, says Caves. So, impressively, shrimp from opposite sides of the world have independently come up with the same strategy for standing out.

In contrast, fire shrimp have evolved an alternative tactic to produce brilliant white antennae. Instead of using layers of reflective spheres sandwiched in the exoskeleton, fire shrimp are equipped with thin (0.27 μm) exoskeleton layers that stack to form 6 μm thick structures, which Johnsen and Davis suspect should reflect a shimmering shade of white. And when they calculated how strongly the microscopic sphere sandwiches reflect light, they found they could make the antennae almost 20 times brighter than the flat reflective exoskeleton layers.

The team also took a close look at the white antennae of spotted cleaner shrimp (Periclimenes yucatanicus) – which only de-louse clients occasionally and don't need to be so eye catching – and found that their antennae completely lack the specialised structures used by the more dedicated cleaners. So, full-time cleaner shrimp have come up with different ways of producing dazzlingly white body-parts that catch the eye to make sure they don't end up being eaten by their clients.

Caves
,
E. M.
,
Davis
,
A. L.
and
Johnsen
,
S.
(
2024
).
Nanoscale ultrastructures increase the visual conspicuousness of signalling traits in obligate cleaner shrimps
.
J. Exp. Biol
.
227
,
jeb248064
.