Pontohedyle

The genus Pontohedyle shows a circumtropical distribution with a single derived species (Mediterranean/ Black Sea Pontohedyle milaschewitchii) inhabiting temperate waters.[1] In the absence of a fossil record for meiofaunal slugs, the only available estimate for divergence times derives from a molecular clock approach, calibrated with shelled heterobranch fossils.[1] Jörger et al. (2010)[3] estimated the origin of the genus Pontohedyle to the late Cretaceous, 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.[1]

Pontohedyle has never been found in colder waters despite a well-studied meiofauna and hydrographic conditions similar to the Mediterranean.[1] The distribution of Pontohedyle might be constrained by ancestry from warm-water adapted animals.[1] 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.[1]

Map of Pontohedyle distribution.

With its vermiform 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.[1]

Pontohedyle is morphologically well-defined genus of meiofaunal slugs.[1] Specimens of Pontohedyle are externally uniform and easily distinguishable from other acochlids by the lack of rhinophores and the bow-shaped oral tentacles.[1] Pontohedyle typically bear monaxone, rodlet-like spicules distributed randomly and frequently accumulated between the oral tentacles.[1] No diagnostic differences in external morphology or spicules could be detected between the collected populations apart from eyes externally visible or not.[1] Comparative SEM-examination of the available radulae revealed two types of the typically hook-shaped radula: a lateral tooth without a denticle (Pontohedyle verrucosa) or with a denticle (Pontohedyle milaschewitchii).[1]

Pontohedyle slugs have a well-adapted body plan that can be conserved for millions of years in a worldwide evolutionary success story.[1] 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.[1]

The first discovered Pontohedyle species is well described and abundant Pontohedyle milaschewitchii from the Black Sea and Mediterranean.[1] The second discovered species is poorly known Western Pacific Pontohedyle verrucosa from the Solomon Islands.[1] In absence of distinguishing morphological characters Jörger et al. (2007)[4] synonymized the tropical Western Atlantic Pontohedyle brasilensis with its temperate congener Pontohedyle milaschewitchii.[1] Subsequently (2012) authors tested molecular markers to detect possible cryptic species and they reestablished Pontohedyle brasilensis as a valid taxon and they uncovered at least nine (undescribed) candidate species.[1]

There are at least nine candidate species plus three confirmed valid species:[1]

• Pontohedyle brasilensis (Rankin, 1979)[1] - tropical, East Atlantic, subtidal, lateral radula tooth with denticle, eyes are sometimes externally visible.[1]
• Pontohedyle milaschewitchii (Kowalevsky, 1901)[1][5] - temperate, Mediterranean and Black Sea, subtidal, lateral radula tooth with denticle, eyes externally visible.[1]
• Pontohedyle verrucosa (Challis, 1970)[1][5] - tropical, Central Indo-Pacific and West-Pacific, intertidal, lateral radula tooth without denticle, eyes are sometimes externally visible.[1]
• Pontohedyle sp. 1[1] - tropical, Central Indo-Pacific, subtidal, lateral radula tooth without denticle, eyes externally visible.[1]
• Pontohedyle sp. 2[1] - tropical, East Atlantic, subtidal, lateral radula tooth with denticle, eyes externally visible.[1]
• Pontohedyle sp. 3[1] - tropical, Central-Pacific, subtidal, unknown radula, unknown eyes.[1]
• Pontohedyle sp. 4[1] - tropical, Red Sea, subtidal, lateral radula tooth with denticle, eyes externally visible.[1]
• Pontohedyle sp. 5[1] - tropical, Indian Ocean, subtidal, lateral radula tooth with denticle, eyes externally visible.[1]
• Pontohedyle sp. 6[1] - tropical, Central Indo-Pacific and Central-Pacific and West-Pacific and Indian Ocean, subtidal, lateral radula tooth with denticle, eyes externally visible.[1]
• Pontohedyle sp. 7[1] - tropical, West Atlantic, subtidal, lateral radula tooth with denticle, eyes not visible externally.[1]
• Pontohedyle sp. 8[1] - tropical, Central-Pacific, subtidal, unknown radula, unknown eyes.[1]
• Pontohedyle sp. 9[1] - tropical, East-Pacific, subtidal, unknown radula, eyes externally visible.[1]

A cladogram based on sequences of mitochondrial 28S ribosomal RNA, 16S ribosomal RNA and cytochrome-c oxidase I (COI) genes showing phylogenic relations of the genus Pontohedyle:[1]

Pontohedyle (and many other meiofauna taxa) are rare and can be expected to have small effective population sizes.[1]

This article incorporates CC-BY-2.0 text from the reference[1]

• Jörger K. M. & Schrödl M. (2013). "How to describe a cryptic species? Practical challenges of molecular taxonomy". Frontiers in Zoology 10: 59. doi:10.1186/1742-9994-10-59.