Exocuticular hyaline layer of sea scorpions and horseshoe crabs suggests cuticular fluorescence is plesiomorphic in chelicerates

Abstract

The cuticle of scorpions (Chelicerata: Arachnida) fluoresces under long‐wave ultraviolet (UV) light due to the presence of beta‐carboline and 7‐hydroxy‐4‐methylcoumarin in the hyaline layer of the exocuticle. The adaptive significance of cuticular UV fluorescence in scorpions is debated. Although several other chelicerate orders (e.g. Opiliones and Solifugae) have been reported to fluoresce on exposure to UV light, the prevalence of cuticular UV fluorescence has not been confirmed beyond scorpions. A systematic study of living chelicerates revealed that UV fluorescence of the unsclerotized integument is ubiquitous across Chelicerata, whereas only scorpions and horseshoe crabs (Xiphosura) exhibit cuticular UV fluorescence. Scanning electron microscopy and histological sectioning confirmed the presence of a hyaline layer in taxa exhibiting cuticular fluorescence. The hyaline layer is absent in all other chelicerates except sea scorpions (Eurypterida) in which a taphonomically altered hyaline layer, that may have fluoresced under UV light, was observed in exceptionally preserved cuticle. Cuticular UV fluorescence appears to be associated with the presence of a hyaline layer, as has long been recognized in scorpions, and may be plesiomorphic among chelicerates. The presence of a hyaline layer in horseshoe crabs and sea scorpions suggests that several putative functions for cuticular UV fluorescence in scorpions can be discounted.

Biofluorescence in Terrestrial Animals, with Emphasis on Fireflies: A Review and Field Observation

“…Land-dwelling arthropods are found in all four subphyla: Chelicerata, Myriapoda, Hexapoda, and Crustacea (paraphyletic). A wide array of terrestrial arthropods has been reported fluorescing upon UV excitation [41][42][43][44][45][46][47][48][49]. No fluorescent land crustacean has yet been reported, though crab eye lenses are known to be fluorescent.…”

“…Terrestrial groups are exclusively in class Arachnida and consist of mites (order Acariformes), ticks (Parasitiformes), harvestmen (Opiliones), camel spiders (Solifugae), hooded tickspiders (Ricinulei), pseudoscorpions (Pseudoscorpiones), scorpions (Scorpiones), whip scorpions (Uropygi), tailless whip scorpions (Amblypygi), and spiders (Araneae). Fluorescent species have been recorded in at least eight out of the 10 orders (Figure 3), and more are awaiting future research [41][42][43][44][45].…”

“…Two fluorescent sources are known in chelicerates: excitation of cuticular fluorophores like beta-carboline and coumarin in hyaline layer, and integumentary/hemolymph fluorophores like tyrosol [5,43,50]. The former occurs only in extant scorpions and horseshoe crabs and extinct sea scorpions (Eurypterida), whereas the latter is prevalent in the other chelicerates which lack the hyaline layer.…”

“…The cuticular fluorescence is much stronger than hemolymph one and remains unfaded for years in museum specimens. The presence of the hyaline layer and accompanied UV fluorescence has been suggested to be a plesiomorphic (ancestral) trait in Chelicerata [43]. In this scenario, UV fluorescence either has no adaptive significance for scorpions or just serves as a function common to scorpions, horseshoe crabs, and eurypterids.…”

“…In this scenario, UV fluorescence either has no adaptive significance for scorpions or just serves as a function common to scorpions, horseshoe crabs, and eurypterids. Therefore, it might not be an adaptation of scorpions for land living [43,50]. Alternatively, UV fluorescence in scorpions has evolved a new adaptation deviated from its initial role in their marine ancestors and relatives.…”