Who cares about the Hydrozoa of the Mediterranean Sea? An essay on the zoogeography of inconspicuous groups

The Messininan crisis, 5 MY before present, marked the recent history of the Mediterranean Sea. The opening of Gibraltar, and the inflow of Atlantic water, brought in the ancestors of the species that inhabit the Mediterranean today. The opening of the Suez Canal allowed the entrance of Indo-Pacific species. Ships and aquacuhure transported species from all over the world. At present, the Mediterranean Sea is a melting pot of a blend of biota that probably has no equivalent in any other part of the world. The tropicalisation of the Mediterranean and Lessepsian migration are conductive to the establishment of tropical species into the basin, with some outstanding examples like that of Caulerpa species, and of the scyphomedusa Rhopilema nomadica. At present, the arrival of new conspicuous species is well monitored and researchers are ready to publish new records of recognisable species. These usually belong to popular groups such as fish. molluscs and decapod crustaceans, or to groups that cannot pass unnoticed due to a marked attitude to form outbreaks, like dinoflagellates or scyphozoan jellyfish and ctenophores. What about the other groups? The bulk of biodiversity, in terms of species numbers, is made of poorly known and inconspicuous species that, usually, can be noticed only by specialised taxonomists. Taxonomy is disappearing front most scientific communities, so that our appreciation of biodiversity is being biased towards conspicuous groups. The Hydrozoa are taken as an example of inconspicuous group whose knowledge has greatly progressed in the last decades due to the presence of some specialists in the Mediterranean area. The number of species recorded from the Mediterranean almost doubled in thirty years and the number of new records is still increasing. The ecological role of these animals, especially those represented also by a medusa stage, can be great due to their general ability to feed upon fish eggs and larvae and/or on the plankton that fish larvae feed upon, so acting as potential predators and/or competitors of commercial species. They might even be keystone predators, depressing potentially monopolising fish species, so leaving space for less competitive species. Neglecting this component of biodiversity might lead to ecological misunderstandings that, in their turn, might lead to misleading interpretations of the causes affecting the yield of fisheries. The main question arising from the example of the Hydrozoa is: are conspicuous groups sufficient to appreciate marine biodiversity and understand its functioning? The answer deriving from the example of the Hydrozoa is: No!