The abundant yet understudied fossil record of ghost shrimps

“Ghost shrimp” usually refers to decapod crustacean species from the family Callianassidae and Ctenochelidae, although sometimes the term is also used for other crustacean groups, such as caprelloid amphipods or, mostly in aquarium trading, for palaemonid shrimps. Here, we use the first definition. In that respect, ghost shrimps are soft-bodied, fossorial (burrowing) decapods with a tail (or pleon) distinctly longer than the main body (or carapace; Fig. 1). They inhabit a variety of marine environments or environments under marine influence, for example, estuaries, marshes and mangroves. Although most species living today have been described from the intertidal environment, there are numerous species dwelling in deeper waters as well.

Fig.1. Ghost shrimp body plan. Glypturus acanthochirus: (A) view from above; (B) side-view; and (C) major cheliped. All scale bars 5.0mm wide. (Photos by Matúš Hyžný.)

Ghost shrimps exhibit a sophisticated behaviour involving digging complex permanent or semi-permanent burrow systems, and they are important bioturbators. Because they live in high densities (in some cases up to 120/m2 of burrow openings are known), they rework huge amounts of substrate and are considered true ecosystem engineers. Bioturbation enhances organic decomposition, nutrient cycling, redistribution of organic material and oxygenation of sediment (similar to earthworms on land). Numerous organisms benefit from these changes, including bivalves, worms and other crustaceans. Additionally, many animal species live directly within the ghost shrimp burrows as their associates.

Fig. 2. Types of ghost shrimp fossils. (A) Complete body fossil: Ctenocheles fritschi from the Late Cretaceous of the Czech Republic; (B) cheliped disassociation unit: Balsscallichirus florianus from the middle Miocene of Austria; (C–E) isolated elements: (C) chela of Eucalliax pseudorakosensis from the middle Miocene of Slovakia, (D) dactylus of Glypturus sikesi from the late Miocene of Florida (D), and (E) fixed finger of Balsscallichirus laepaensis from the late Miocene of Spain. All scale bars 5.0mm wide. (Photos by Pavel Dvořák (A), Matúš Hyžný (B, C and E) and Adiël Klompmaker (D).)

Not every fossil ghost shrimp is Callianassa

Fossil hunters specialising in decapod crustaceans often identify ghost shrimp specimens as Callianassa or Protocallianassa. However, since the 1980s, researchers on ghost shrimps living today have attempted to divide the genus Callianassa into several independent genera. Now, there are some fifty distinct ghost shrimp genera recognised, including several known exclusively from the fossil record.

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Matúš Hyžný (Slovakia) and Adiël A. Klompmaker (USA)

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