Mollusca Department, Bavarian State Collection of Zoology, Münchhausenstr,21, 81247, München, Germany.
BMC Evol Biol. 2012 Dec 18;12:245. doi: 10.1186/1471-2148-12-245.
Many marine meiofaunal species are reported to have wide distributions, which creates a paradox considering their hypothesized low dispersal abilities. Correlated with this paradox is an especially high taxonomic deficit for meiofauna, partly related to a lower taxonomic effort and partly to a high number of putative cryptic species. Molecular-based species delineation and barcoding approaches have been advocated for meiofaunal biodiversity assessments to speed up description processes and uncover cryptic lineages. However, these approaches show sensitivity to sampling coverage (taxonomic and geographic) and the success rate has never been explored on mesopsammic Mollusca.
We collected the meiofaunal sea-slug Pontohedyle (Acochlidia, Heterobranchia) from 28 localities worldwide. With a traditional morphological approach, all specimens fall into two morphospecies. However, with a multi-marker genetic approach, we reveal multiple lineages that are reciprocally monophyletic on single and concatenated gene trees in phylogenetic analyses. These lineages are largely concordant with geographical and oceanographic parameters, leading to our primary species hypothesis (PSH). In parallel, we apply four independent methods of molecular based species delineation: General Mixed Yule Coalescent model (GMYC), statistical parsimony, Bayesian Species Delineation (BPP) and Automatic Barcode Gap Discovery (ABGD). The secondary species hypothesis (SSH) is gained by relying only on uncontradicted results of the different approaches ('minimum consensus approach'), resulting in the discovery of a radiation of (at least) 12 mainly cryptic species, 9 of them new to science, some sympatric and some allopatric with respect to ocean boundaries. However, the meiofaunal paradox still persists in some Pontohedyle species identified here with wide coastal and trans-archipelago distributions.
Our study confirms extensive, morphologically cryptic diversity among meiofauna and accentuates the taxonomic deficit that characterizes meiofauna research. We observe for Pontohedyle slugs a high degree of morphological simplicity and uniformity, which we expect might be a general rule for meiofauna. To tackle cryptic diversity in little explored and hard-to-sample invertebrate taxa, at present, a combined approach seems most promising, such as multi-marker-barcoding (i.e., molecular systematics using mitochondrial and nuclear markers and the criterion of reciprocal monophyly) combined with a minimum consensus approach across independent methods of molecular species delineation to define candidate species.
许多海洋小型生物物种的分布范围很广，这与它们假定的低扩散能力形成了矛盾。这种矛盾与小型生物的分类学赤字特别高有关，部分原因是分类学上的努力较低，部分原因是假定的隐种数量较多。基于分子的物种划分和条形码方法已被用于小型生物多样性评估，以加快描述过程并发现隐种。然而，这些方法对采样覆盖范围（分类和地理）敏感，并且尚未在 mesopsammic Mollusca 上探索成功率。
我们从全球 28 个地点采集了小型海洋海蛞蝓 Pontohedyle（Acochlidia，Heterobranchia）。通过传统的形态学方法，所有标本都归入两种形态种。然而，通过多标记基因方法，我们在单基因和基因树的串联系统发育分析中揭示了多个互为单系的谱系。这些谱系在很大程度上与地理和海洋参数一致，导致我们的主要物种假设（PSH）。同时，我们应用了四种独立的基于分子的物种划分方法：广义混合尤尔合并模型（GMYC）、统计简约法、贝叶斯物种鉴定（BPP）和自动条形码间隙发现（ABGD）。通过仅依赖于不同方法的无争议结果（“最小共识方法”）获得次要物种假设（SSH），导致发现了至少 12 种主要隐种的辐射，其中 9 种是新的科学发现，有些与海洋边界共生，有些与海洋边界隔离。然而，在我们在这里识别的一些 Pontohedyle 物种中，小型生物的悖论仍然存在，它们具有广泛的沿海和跨岛分布。
我们的研究证实了小型生物中广泛存在的形态隐种多样性，并强调了小型生物研究的分类学赤字。我们观察到 Pontohedyle 蛞蝓具有高度的形态简单性和均匀性，我们预计这可能是小型生物的一般规则。为了解决未充分探索和难以采样的无脊椎动物类群中的隐种多样性，目前，多标记条形码（即使用线粒体和核标记的分子系统发育学和互为单系的标准）与独立的分子物种划分方法的最小共识方法相结合的综合方法似乎最有前途，以定义候选物种。
