''Pontohedyle'' is a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

sea slug Sea slug is a common name for some marine invertebrates with varying levels of resemblance to terrestrial slugs. Most creatures known as sea slugs are gastropods, i.e. they are sea snails (marine gastropod mollusks) that over evolutionary t ...

acochlidian Acochlidiacea, common name acochlidians, are a taxonomic clade of very unusual sea snails and sea and freshwater slugs, aquatic gastropod mollusks within the large clade Heterobranchia. Acochlidia is a variant spelling. Description These ar ...

s, shell-less

marine Marine is an adjective meaning of or pertaining to the sea or ocean. Marine or marines may refer to: Ocean * Maritime (disambiguation) * Marine art * Marine biology * Marine debris * Marine habitats * Marine life * Marine pollution Military * ...

gastropod The gastropods (), commonly known as snails and slugs, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, from freshwater, and from land. T ...

mollusk Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is e ...

s in the family Microhedylidae. Sea slugs in this genus are highly simplified and uniform.

Distribution

The genus ''Pontohedyle'' shows a circumtropical distribution with a single derived species (Mediterranean/ Black Sea ''Pontohedyle milaschewitchii'') inhabiting

temperate water In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

s. In the absence of a fossil record for meiofaunal slugs, the only available estimate for divergence times derives from a

molecular clock The molecular clock is a figurative term for a technique that uses the mutation rate of biomolecules to deduce the time in prehistory when two or more life forms diverged. The biomolecular data used for such calculations are usually nucleoti ...

approach, calibrated with shelled

heterobranch Heterobranchia, the ''heterobranchs'' (meaning "different-gilled snails"), is a taxonomic clade of snails and slugs, which includes marine, aquatic and terrestrial gastropod mollusks. Heterobranchia is one of the main clades of gastropods. Cur ...

fossils. Jörger et al. (2010)Jörger K. M., Stöger I., Kano Y., Fukuda H., Knebelsberger T. & Schrödl M. (2010). "On the origin of Acochlidia and other enigmatic euthyneuran gastropods, with implications for the systematics of Heterobranchia". ''

BMC Evolutionary Biology ''BMC Ecology and Evolution'' (since January 2021), previously ''BMC Evolutionary Biology'' (2001–2020), is a peer-reviewed open access scientific journal covering all fields of evolutionary biology, including phylogenetics and palaeontology. It ...

'' 10: 323. . estimated the origin of the genus ''Pontohedyle'' to the

late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', the ...

, 84 mya (95% confidence interval ranging from 160–60 mya), providing a rough estimation of how much time was available for diversification and circum-global dispersal of ''Pontohedyle'' slugs. ''Pontohedyle'' has never been found in colder waters despite a well-studied meiofauna and hydrographic conditions similar to the Mediterranean. The distribution of ''Pontohedyle'' might be constrained by ancestry from warm-water adapted animals. Considering the estimated mid to late Mesozoic origin and the recent primarily tropical distribution pattern in ''Pontohedyle'', it is most likely that this meiofaunal slug clade originated in Tethyan waters.
Map of ''Pontohedyle'' distribution

Description

With its

vermiform Vermiform (ˈvərməˌfôrm) describes something shaped like a worm. The expression is often employed in biology and anatomy to describe usually soft body parts or animals that are more or less tubular or cylindrical. The word root is Latin, ''ve ...

body, a putatively multi-functional radula, ‘simplified’ organ systems and a special fast and imprecise mode of sperm transfer, ''Pontohedyle'' reflects a meiofaunal slug lineage highly adapted to its interstitial habitat. ''Pontohedyle'' is morphologically well-defined genus of meiofaunal slugs. Specimens of ''Pontohedyle'' are externally uniform and easily distinguishable from other acochlids by the lack of

rhinophores A rhinophore is one of a pair of chemosensory club-shaped, rod-shaped or ear-like structures which are the most prominent part of the external head anatomy in sea slugs, marine gastropod opisthobranch mollusks such as the nudibranchs, sea hares ...

and the bow-shaped oral

tentacle In zoology, a tentacle is a flexible, mobile, and elongated organ present in some species of animals, most of them invertebrates. In animal anatomy, tentacles usually occur in one or more pairs. Anatomically, the tentacles of animals work main ...

s. ''Pontohedyle'' typically bear monaxone, rodlet-like spicules distributed randomly and frequently accumulated between the oral tentacles. No diagnostic differences in external morphology or spicules could be detected between the collected populations apart from eyes externally visible or not. Comparative SEM-examination of the available

radula The radula (, ; plural radulae or radulas) is an anatomical structure used by molluscs for feeding, sometimes compared to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food ...

e revealed two types of the typically hook-shaped radula: a lateral tooth without a denticle (''Pontohedyle verrucosa'') or with a denticle (''Pontohedyle milaschewitchii''). ''Pontohedyle'' slugs have a well-adapted body plan that can be conserved for millions of years in a worldwide evolutionary success story. ''Pontohedyle'' presents a stunning example of extreme morphological stasis and uniformity over long evolutionary timeframes, probably constrained by their simplified bodyplan and by the requirements of the meiofaunal habitat.

Species

The first discovered ''Pontohedyle'' species is well described and abundant ''

Pontohedyle milaschewitchii ''Pontohedyle milaschewitchii'' is a species of sea slug, an acochlidian, a shell-less marine gastropod mollusk in the family Microhedylidae.Jörger K. M., Norenburg J. L., Wilson N. G. & Schrödl M. (2012). "Barcoding against a paradox? Combine ...

'' from the Black Sea and Mediterranean. The second discovered species is poorly known Western Pacific ''

Pontohedyle verrucosa ''Pontohedyle verrucosa'' is a species of sea slug, an acochlidian, a shell-less marine (ocean), marine gastropod mollusc in the family Microhedylidae.Jörger K. M., Norenburg J. L., Wilson N. G. & Schrödl M. (2012). "Barcoding against a paradox ...

'' from the

Solomon Islands Solomon Islands is an island country consisting of six major islands and over 900 smaller islands in Oceania, to the east of Papua New Guinea and north-west of Vanuatu. It has a land area of , and a population of approx. 700,000. Its capita ...

. In absence of distinguishing morphological characters Jörger et al. (2007)Jörger K. M., Neusser T. P. & Schrödl M. (2007). "Re-description of a female ''Pontohedyle brasilensis'' (Rankin, 1979), a junior synonym of the Mediterranean ''P. milaschewitchii'' (Kowalevsky, 1901) (Acochlidia, Gastropoda)". ''

Bonner Zoologische Beiträge The ''Bonn Zoological Bulletin'' (''BzB''), formerly ''Bonner zoologische Beiträge'', is a peer reviewed open access journal dealing with zoology Zoology ()The pronunciation of zoology as is usually regarded as nonstandard, though it is not ...

'' 55(3/4): 283-290
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synonymized the tropical Western Atlantic '' Pontohedyle brasilensis'' with its temperate congener ''Pontohedyle milaschewitchii''. Subsequently (2012) authors tested

molecular marker A molecular marker is a molecule, sampled from some source, that gives information about its source. For example, DNA is a molecular marker that gives information about the organism from which it was taken. For another example, some proteins can be ...

s to detect possible

cryptic species In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...

and they reestablished ''Pontohedyle brasilensis'' as a valid taxon and they uncovered at least nine (undescribed) candidate species. There are at least nine candidate species plus three confirmed valid species: * '' Pontohedyle brasilensis'' (Rankin, 1979) - tropical, East Atlantic, subtidal, lateral radula tooth with denticle, eyes are sometimes externally visible. * ''

'' (Kowalevsky, 1901)Schrödl M. & Neusser T. P. (2010). "Towards a phylogeny and evolution of Acochlidia (Mollusca: Gastropoda: Opisthobranchia)". ''

Zoological Journal of the Linnean Society The ''Zoological Journal of the Linnean Society'' is a monthly peer-reviewed scientific journal covering zoology published by Oxford University Press on behalf of the Linnean Society. The editor-in-chief is Maarten Christenhusz (Linnean Society). ...

'' 158: 124-154. . - temperate, Mediterranean and Black Sea, subtidal, lateral radula tooth with denticle, eyes externally visible. * ''

'' (Challis, 1970) - tropical, Central Indo-Pacific and West-Pacific, intertidal, lateral radula tooth without denticle, eyes are sometimes externally visible. * ''Pontohedyle'' sp. 1 - tropical, Central Indo-Pacific, subtidal, lateral radula tooth without denticle, eyes externally visible. * ''Pontohedyle'' sp. 2 - tropical, East Atlantic, subtidal, lateral radula tooth with denticle, eyes externally visible. * ''Pontohedyle'' sp. 3 - tropical, Central-Pacific, subtidal, unknown radula, unknown eyes. * ''Pontohedyle'' sp. 4 - tropical, Red Sea, subtidal, lateral radula tooth with denticle, eyes externally visible. * ''Pontohedyle'' sp. 5 - tropical, Indian Ocean, subtidal, lateral radula tooth with denticle, eyes externally visible. * ''Pontohedyle'' sp. 6 - tropical, Central Indo-Pacific and Central-Pacific and West-Pacific and Indian Ocean, subtidal, lateral radula tooth with denticle, eyes externally visible. * ''Pontohedyle'' sp. 7 - tropical, West Atlantic, subtidal, lateral radula tooth with denticle, eyes not visible externally. * ''Pontohedyle'' sp. 8 - tropical, Central-Pacific, subtidal, unknown radula, unknown eyes. * ''Pontohedyle'' sp. 9 - tropical, East-Pacific, subtidal, unknown radula, eyes externally visible.

Cladogram

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to d ...

based on sequences of mitochondrial

28S ribosomal RNA 28S ribosomal RNA is the structural ribosomal RNA (rRNA) for the large subunit (LSU) of eukaryotic cytoplasmic ribosomes, and thus one of the basic components of all eukaryotic cells. It has a size of 25S in plants and 28S in mammals, hence th ...

16S ribosomal RNA 16 S ribosomal RNA (or 16 S rRNA) is the RNA component of the 30S subunit of a prokaryotic ribosome (SSU rRNA). It binds to the Shine-Dalgarno sequence and provides most of the SSU structure. The genes coding for it are referred to as 16S rRNA ...

and cytochrome-c oxidase I (COI) genes showing phylogenic relations of the genus ''Pontohedyle'':

Ecology

''Pontohedyle'' (and many other meiofauna taxa) are rare and can be expected to have small

effective population size The effective population size (''N'e'') is a number that, in some simplified scenarios, corresponds to the number of breeding individuals in the population. More generally, ''N'e'' is the number of individuals that an idealised population wo ...

References

This article incorporates CC-BY-2.0 text from the referenceJörger K. M., Norenburg J. L., Wilson N. G. & Schrödl M. (2012). "Barcoding against a paradox? Combined molecular species delineations reveal multiple cryptic lineages in elusive meiofaunal sea slugs". ''

'' 12: 245. .

External links

* Jörger K. M. & Schrödl M. (2013). "How to describe a cryptic species? Practical challenges of molecular taxonomy". ''

Frontiers in Zoology ''Frontiers in Zoology'' is a peer-reviewed open access scientific journal covering all aspects of zoology. It was established in 2004 and is published by BioMed Central on behalf of the . The editors-in-chief are Jürgen Heinze (University of Re ...

'' 10: 59. . {{Taxonbar, from=Q7228329 Microhedylidae