Morphological and molecular analysis of the meiofaunal cnidarian Halammohydra Remane, 1927 (Hydrozoa)

Abstract

Marine sediments inhabit many microscopic organisms with perfect adaptations to their environment, such as the small body size, adhesive structures or the lack of a planktonic larva. These so-called meiofauna are thought to not be able to distribute over a large scale of distance because of this, however some species are amphi-oceanic and even cosmopolitan. Many reinvestigations found morphological or molecular differences, but not for every species. There are several hypotheses brought forward to explain this “meiofauna-paradox”, but investigations of all groups are needed to find the underlying processes. Most studies are done on large groups, such as nematodes or copepods. Smaller groups with special habitat preferences are lacking. Especially the few meiofaunal cnidarians are understudied. The largest group of them is the highly modified medusa Halammohydra Remane, 1927 with nine species described so far, but the species identification is rather difficult. Together with Otohydra Swedmark & Teissier, 1958 they build the taxon Actinulida. This relationship is doubtful because of the many morphological differences and has to be investigated molecular. Additionally, detailed information are needed of specific structures, such as the reproductive system of both and the adhesive organ of Halammohydra to find species specific structures as well as structures for the classification within the cnidarian tree. This project investigates specimens of Halammohydra from different locations mainly in Europe and a few specimens of Otohydra from Roscoff in France. They were extracted from the sediment with the anesthesia-decantation method and fixed for molecular and ultrastructural investigations. The detailed investigation of the cell structure of H. vermiformis and Otohydra sp. with semi and ultra-thin sections helps to understand the general organization and adds information to the knowledge about both, especially concerning the gonadal compartment/ gonad of both and the aboral cone with adhesive organ of Halammohydra. For both it is proposed that the spermatozoa and oocytes (Halammohydra)/ juveniles (Otohydra) are released into the water via a rupture of the epidermis, due to the structural changes in the tissues. Additionally, detailed information about the female gonadal compartment of Halammohydra are documented for the first time. Every specimen of Halammohydra was documented with a camera to find species-specific characters for identification, which were used in combination with single gene sequencing (16S, 18S and CO1) and species delimitation tests (ABGD, GMYC and bPTP). This integrative approach helps with verification of describes species, adds character information as well as finds new species. Additionally, it fills the database with sequences, which are needed for future studies. Phylogenetic analyses (Bayesian Interference and Maximum Likelihood) delimit 16 clusters of which seven are assigned to four known species (H. vermiformis, H. octopodides, H. coronata and H. adherens), four are describes as new species (H. teissieri n. sp., H. swedmarki n. sp., H. kerblae n. sp. and H. joergerae n. sp.) and five remain unidentified (“Helgoland/ Sylt”, “Azores”, “Roscoff”, “Tenerife 1” and “Tenerife 2”). These results show a higher diversity and distribution as previously expected. In addition, the morphological documentation shows different variabilities of characters within the species and overlapping between different species. Molecular analysis of Otohydra specimens reveals no close relation to Halammohydra, thus invalidate the taxon Actinulida. They are positioned within Rhopalonematidae (Trachymedusae), as it is the case for Halammohydra, but clusters close to a species of Ptychogastriidae. If Otohydra belongs to Rhopalonematidae or Ptychogastriidae is not resolved yet, due to the slightly lower support value and the morphological differences to Ptychogastriidae. To answer this, the recently found meiofaunal cnidarian Marsipohydra pacifica Sanamyan & Sanamyan, 2012 might help, since it is positioned close to Otohydra and has some characters of Ptychogastriidae. Molecular information are needed. Nonetheless, these results show another independent transition to the meiofauna way of life within Cnidaria.