The first amber caridean shrimp from Mexico reveals the ancient adaptation of the Palaemon to the mangrove estuary environment

Abstract

The aquatic and semiaquatic invertebrates in fossiliferous amber have been reported, including taxa in a wide range of the subphylum Crustacea of Arthropoda. However, no caridean shrimp has been discovered so far in the world. The shrimp Palaemon aestuarius sp. nov. (Palaemonidae) preserved in amber from Chiapas, Mexico during Early Miocene (ca. 22.8 Ma) represents the first and the oldest amber caridean species. This finding suggests that the genus Palaemon has occupied Mexico at least since Early Miocene. In addition, the coexistence of the shrimp, a beetle larva, and a piece of residual leaf in the same amber supports the previous explanations for the Mexican amber depositional environment, in the tide-influenced mangrove estuary region.

The tale of an endemic shrimp’s exceptional osmoregulation and the ancient Athalassic mangrove oasis

“…Their niche and site occurrences are based on specific salinity-induced physiological constraints associated with their osmoregulation pattern 28 . Indeed, these shrimps are considered bioindicators of salinity levels, with marked niche separation per congener according to the saline levels in freshwater, brackish water or seawater 29 , 32 – 34 .…”

“…The speciation of palaemonid species is recorded with different congeners inhabiting specific areas and niches according to salinity levels 29 , 32 – 34 , as discussed before. In this study, we recorded P. khori in dense biomasses dominating the niche with high salinity, and P. aff.…”

“…The regulation and toleration of particular salinity rates of some estuarine palaemonid shrimps are related to evolutionary adaptation for colonising specific environmental settings 25 , 29 – 31 . Palaemon are recorded inhabiting mangroves, estuaries, fresh water and brackish water and are constantly recorded in mangrove environments 29 , 32 , 33 . They are recognised as efficient osmoregulators, adapted to different salinity levels in estuaries 29 , 34 .…”

“…They are recognised as efficient osmoregulators, adapted to different salinity levels in estuaries 29 , 34 . These species often display niche specificity, and consistently exhibit speciation and niche separation based on salinity levels, with some congeners adapted to inhabit areas with lower salinity upstream in rivers, while others thrive in estuarine areas near the sea 29 , 32 – 34 .…”

Summary of the fossil record of megalopteran and megalopteran-like larvae, with a report of new specimens

“…Yet, it needs to be emphasised that aquatic organisms and even marine faunal components are not impossible to be preserved in amber. Numerous examples can be found, including aquatic larvae of various lineages of Insecta (Wichard & Weitschat 1996;Wichard 2005Wichard , 2017Bechly & Wichard 2008;Wichard et al 2009;Sroka et al 2018;Gustafson et al 2020;Schädel et al 2020), aquatic (including marine) crustaceans (Coleman & Myers 2001;Coleman 2004Coleman , 2006Jażdżewski & Kupryjanowicz 2010;Jażdżewski et al 2014;Sánchez-García et al 2015;Serrano-Sánchez et al 2015Huys et al 2016;Heard et al 2018;Du et al 2019;Schädel et al 2019Schädel et al , 2021a, but also other marine faunal elements (Girard et al 2008, Yu et al 2019. Hence, although aquatic fossils are still generally considered rare in amber (e.g.…”