SERIES: Invertebrate of the Week

Invertebrate of the Week #2: Sea Sapphires (Genus: Sapphirina)

Last week, we highlighted a tarantula that appears blue due to structural color instead of pigments. This week, I thought I would continue with the structural color theme by featuring a marine invertebrate: sea sapphires of the genus Sapphirina.

Sea sapphires are small parasitic copepods that can be found in oceans around the world at depths up to 1000 feet (305 m). Of course, you have to spot them first. Sea sapphires are transparent and only reach a few millimeters in length. Females parasitize tubular, gelatinous creatures called salps in order to reproduce. They are distinguished from males by their elongated bodies the sacs of eggs attached near the rear of their abdomen.

Male sea sapphires, on the other hand, are much wider and shorter than females. They are also the sea sapphires most likely to be spotted by the casual observer. Unlike females, male sea sapphires have the ability to shimmer when exposed to light.

A sea sapphire (sapphirina species) imaged under high resolution
Male sea sapphire (Sapphirina sp.)
Image by Stefan Siebert
American Chemical Society video of a male sea sapphire changing color.

How Male Sea Sapphires Change Color

Males have microscopic parallel layers of guanine hexagonal plates (~1.5 µm wide) coated in cytoplasm just below the dorsal and ventral aspects of their body surface. In other words, they have ‘crystal skin’ extending along the top part of their body. When light strikes the layers of crystals, it can reflect and produce a vibrant iridescence.

The color is also tunable – it can be changed. A honeycomb of chitin-based plates structures interacts with the guanine plate stacks to produce a range of colors. Research by Kimura, et. al. (2020) suggests that this is achieved by the chitin honeycomb changing the interplate distance of the guanine plates.

Male sea sapphire structural framework responsible for allowing the sea sapphire to change color.
Figure from Kimura, et. al. (2020)
The framework structures in the coloring portions of S. nigromaculata. Three-dimensional images reconstructed from serial-sectioning SEM images of consecutive serial sections of a specimen that were collected in an automated manner using FIB-SEM (a,b); cross-sectional views (cg); schematic illustration of a photonic structure composed of guanine crystal arrays and cytoplasm with the organic framework (h); Raman spectra (i) for the chitin standard (I) and the dorsal body surface skin (II) and honeycomb framework (III) of S. nigromaculata. The assignment of Raman signals (A–E) is shown in Fig. S4 in the ESI.

Why Male Sea Sapphires Change Color

The exact purpose of the male sea sapphire’s color changes is unknown but research suggests it may have something to do with reproduction. This is based, in part, on the obvious sexual dimorphism (i.e. only males produce color) and the fact that the females have huge eyes compared to the males (implying that they may be extra sensitive to flashiness).

The males also tend to “flash” in short bursts, implying that the behavior has evolved to favor occasional conspicuousness rather than a more prolonged illumination that might draw unwanted attention (e.g. from predators).

If you’d like more information on sea sapphires, I recommend starting with this article by Deep Sea News which features a video of shimmering sea sapphires.

This article was updated from a previous version published in 2014

References and Further Reading

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