The Caribbean enigma: the presence of unusual cryptic diversity in intertidal mites (Arachnida, Acari, Oribatida)

Abstract

The definition, as well as the existence of cryptic species, is still a subject of controversial debates. Some scientists claim that cryptic diversity is a real phenomenon that should be extensively studied while others argue that cryptic species do not exist as they are nothing more than an incompatibility of species concepts. We investigated the enigmatic case of two widely distributed Caribbean intertidal oribatid mites, Carinozetes bermudensis and Carinozetes mangrovi, consisting of five distinct genetic lineages. Morphological features allowing to clearly distinguish between these lineages are absent, and despite certain congruence with genetic data, comprehensive morphometric analyses also do not show clear separation. Species delimitation analyses based on COI sequence data, on the other hand, suggest five distinct genetic species. Despite the lack of diagnostic characters for these suggested species, the lineages can be classified at least into two morphological groups, the bermudensis and the mangrovi group which can only be distinguished by the arrangement of cuticular ventral carinae. Specimens within a group show nearly identical phenotypes, impeding morphological identification and hence rendering the found diversity cryptic. Stabilizing selection caused by the extreme conditions of the intertidal environment is suggested to be responsible for the found morphological stasis. The genetic lineages show more or less clear geographic patterns; in C. mangrovi, there is a northern, an Antillean, and a Pacific lineage, whereas in C. bermudensis, there is a Bermudian and a Caribbean lineage. In a few places, e.g., the Bahamas and Panama, distributions may overlap. Neither the found biogeographic pattern nor the observed ecological needs could explain the reason for the genetic diversification of Caribbean Carinozetes.

Testing for phylogenetic signal in claws suggests great influence of ecology on Caribbean intertidal arthropods (Acari, Oribatida)

“…Gene bank accession numbers are given in Table S1 . In congruence with the previous work of Pfingstl et al 19 , we could confirm the existence of five genetic lineages within the genus Carinozetes . Maximum likelihood (ML), Bayesian inference, and neighbor-joining analyses produced largely congruent phylogenies for all clades.…”

“…Morphological structures are often commensurate with phylogenetic relatedness because less morphological divergence is supposed to have occurred in closely related taxa compared with those that are more distantly related 20 . The Carinozetes species used in our study are closely related and were recently shown to consist of five morphologically identical but genetically distinct lineages with differing ecologies 19 , which allows us to observe the interplay of claw morphology, ecology and phylogenetic relatedness below species level.…”

“…We investigated herein the claw shapes of five different intertidal oribatid mite species from the Caribbean belonging to two different families and showing different substrate/habitat preferences. The fortuyniid Litoribates bonairensis and Litoribates floridae are distinct species which show very limited distributions and are classified as typical mangrove dwellers 19 ; the selenoribatid Thalassozetes barbara shows a trans-Caribbean distribution and exclusively inhabits rocky habitats; and Carinozetes bermudensis and Carinozetes mangrovi , show also a wide Caribbean distribution but consist of five distinct genetic lineages, three of these lineages occur on a variety of substrates, whereas the remaining two lineages exclusively dwell either on rocky substrate or in mangrove habitats 19 .…”

Unexpected diversity in the host-generalist oribatid miteParaleius leontonychus(Oribatida, Scheloribatidae) phoretic on Palearctic bark beetles

“…The fact that syntopically found specimens congruently clustered in the single gene trees derived from COI-2 and 18S sequence data (and also the D3-28S data, considering the poor resolution of this marker), additionally indicates that there is no ongoing gene flow between the respective groups. Genetic distances, based on the COI sequences, among these six main groups amounted to 12.7–19.6% (uncorrected p -distances), which is well in line with previously reported species-level divergences in other oribatid mites ( Heethoff et al, 2007 ; Pfingstl, Baumann & Lienhard, 2019 ; Schäffer et al, 2010 ; Schäffer, Kerschbaumer & Koblmüller, 2019 ) and suggests that this radiation is not of recent origin. Despite intensive investigations, it was not possible to differentiate these groups morphologically.…”

Patterns of intraspecific morphological variability in soil mites reflect their dispersal ability

“…More thorough studies conducted after molecular genetic analyses (for example, on fine-scale morphology, behavior, chemical properties) usually support the new species status (Skoracka et al 2015 ), which indicates that ‘cryptic’ species often only are an example of inadequate taxonomy. In mites, several studies demonstrated that genetically well separated species did not show differences in distinct morphological characters and only differed slightly in morphometric analyses (e.g., Knee et al 2012 ; Pfingstl et al 2019 ; Schäffer et al 2019 ). The morphometric differences revealed in S. carinthiacus thus point to the possible existence of a species complex, even more so as phenotypic variation analyzed by GMM can be as sensitive as molecular data (Karanovic et al 2016 ).…”