Hydrozoa of the Mediterranean Sea are well known and a recent monograph covers 457 species. Mediterranean non-Siphonophoran Hydrozoa comprises 398 species, an increasing number due to continuous updates, representing about 10 % of the 3,702 currently valid species reported in a recent world assessment of hydrozoan diversity. Many new records are non indigenous species, previously described species that occurred elsewhere and whose arrival was presumably caused by human activities. However, many species reported in the past are not recorded in recent times. Realistic assessments of species pools require addition of new species, but also subtraction of species not found since a certain period. With the confidence of extinction index, cases of putative extinction can be raised. Out of the 398 known species, only 162 (41 %) have been reported in the last decade, while 53 (13 %) are not recorded in the literature since at least 41 years. According to the confidence of extinction index, 60 % of the 53 missing species are extinct, and 11 % are putatively extinct from the basin. From a biogeographical point of view, the missing species are: 34 % endemic, 19 % boreal, 15 % Mediterranean-Atlantic, 11 % Indo-Pacific, 11 % circumtropical, 4 % cosmopolitan, 2 % tropical-Atlantic, 4 % non-classifiable. Fluctuations in species composition into a certain area cause heavy variability in the expression of both structural and functional biodiversity. As consequence, the regional biodiversity should be analyzed through its temporal evolution, to detect changes and their possible causes. This approach has profound consequences on biodiversity assessments and also on the compilation of red lists.

中文翻译：

---

地中海非虹吸虫水生动物中缺少的物种。

地中海的水生动物是众所周知的，最近的专着涵盖了457种。地中海非水生动物的水生动物共有398种，由于不断更新而增加，在最近的世界水生动物多样性评估中报告的3,702种有效物种中约占10％。许多新记录是非土著物种，以前描述的物种发生在其他地方，其到来大概是由于人类活动造成的。但是，过去没有报道过过去报道的许多物种。现实的物种库评估需要添加新物种，但也要减去自某个时期以来未发现的物种。有了灭绝指数的置信度，可以提出假定的灭绝病例。在398种已知物种中，在过去的十年中，仅报道了162次（41％），而至少有41年以来未在文献中记录53次（13％）。根据灭绝指数的置信度，该流失的53种物种中有60％灭绝了，据推测该盆地灭绝了11％。从生物地理角度来看，缺失的物种为：34％特有，19％北方性，15％地中海-大西洋，11％印度-太平洋，11％环热带，4％大都会，2％热带-大西洋，4％非-可分类的。物种组成向某个区域的波动会导致结构性和功能性生物多样性的表达发生巨大变化。因此，应该通过区域生物多样性的时间演变来分析该区域生物多样性，以发现变化及其可能的原因。