Project description

Introduction
Nudibranchs (Gastropoda, Nudibranchia) are a group of marine molluscs found within the opisthobranchs. They are traditionally defined as gastropods lacking a shell and the operculum in adult stages. The name "nudibranch" means "naked gill", and refers to the bushy extremities on the back; the branchial plume of the dorids and the cerata of the aeolids. They are hermaphroditic as sperm and eggs mature simultaneously, but they cannot fertilize themselves and have to exchange sperm with other individuals (obligate reciprocal copulation). Information on general biology of nudibranchs is presented in Thompson (1976).

All nudibranchs are carnivorus, and can be divided into four different categories based on feeding preferences; sponge feeders, hydroid feeders, bryozoan feeders and a group feeding on a variety of diets like tunicates, barnacles and egg from various organisms (Todd 1981). A preview of the diet of different nudibranchiate species from the north Atlantic was given by Todd (1981) and Picton & Morrow (1994). An overview of nudibranch food habits on a worldwide basis is given in McDonald & Nybakken (1991, 1997, 1999).

The loss of the shell has led to a variety of cunning ways to defence from potential predators. Quite a number of nudibranchiate species are cryptic, hiding in the surroundings or on prey organism (Morrow et al. 1992), others make use of distasteful strategies. One of them is to use the same defensive mechanical defence as their diet species; many dorids use spicules from sponges in the outer part of their skin (Cattaneo-Vietti et al. 1993), and many aeolids retain the cnidocells from hydroids and store them in cnidosacks in their cerata (Greenwood & Mariscal 1984). The other strategy is to use toxins either derived from the diet or made by themselves (Avila et al. 1991, Avila & Paul 1997, Cimino et al. 1999), or to excrete acidic substances (Thompson 1976). Some species might also go to the extreme as to "sacrifice" parts of their body (autotomy) (Bickell Page 1989, Piel 1991), or just as well be of low nutritional value to a potential predator (Penney 2002).

The nudibranchiate fauna of the Norwegian coast can be divided in two main taxonomic groups, the dorids (Anthobrachia) and the aeolids (Cladobranchia).

The dorid nudibranchs have a mantle and a branchial plume on the dorsal side. The mantle is often covered with tubercles. Onchidoridids and Archidoridids are typical representatives of this kind of habitus. The Goniodoridids and Polycerids represent groups where the mantle is more or less secondarily reduced, but where the branchial plume and tubercles are still present.
The aeolid nudibranchs lack the mantle and the branchial plume. Instead they have cerata, tentacles of various form and shape along the dorsal side. The cerata of the Tritonids and Dendronotids are ramified, or they can be coneshaped as within the Dotids, or simple and fingershaped as within the Aeolidids.

Nudibranchs can be found in all the worlds' oceans, and in most marine habitats, and they vary in size from 4 mm to 60 cm (Thompson 1976). It is suggested that there are more than 3000 species worldwide (Willan & Coleman 1984).

Methods
Identification of nudibranchiate species is best done with live specimens. Identification should preferably be done in the field, where a minimum of specimens is brought back for fixation and further study. Preserved specimens are difficult to identify without examining the radula. This project aim to observe and identify nudibranchs in situ. Underwater photography will be employed as much as possible, to ensure that living and ecological characters can aid in the identification of different species. Underwater photography is conducted with Nikonos V under water cameras using extension tubes for macro photography (1:2, 1:1 and 2:1) and flashlights (SB-105).

Nudibranchs that are collected for further study, are narcotised in 7% MgCl2 diluted 1:1 in seawater before fixation. Specimens are then preserved in 4% formaldehyde neutralised with borax. After four weeks they are preserved in 70% ethanol. All collected and preserved material is deposited at the Museum of Natural History and Archaeology, NTNU.

For nudibranchs that are found from the intertidal to 40 meters depth, scuba diving is a suitable method for observation and collection. Not only is it possible to identify and record living species in situ, but also valuable ecological data on abundance, reproduction and diet is also collectable.

The nudibranchiate fauna of the Norwegian coast has a long and proud history beginning with the early works of Ascanius (1774), O.F. Müller (1776, 1778, 1781, 1789) and M. Sars (1829). Information on nudibranchs were usually a part of larger faunal surveys, and these continued to embed nudibranchs with Asbjörnsen (1854), Friele & Hansen (1876), G.O. Sars (1878) and Friele & Grieg (1901). It was the works of Krause (1897), Løyning (1922), Larsen (1925) and Odhner (1922, 1929) that started to put nudibranchs from Norwegian waters in focus.

The Trondheimsfjord has for many centuries also been in great interest to marine biologists (Sakshaug & Sneli 2000), where scientific studies of the fjord started with bishop Gunnerus in 1760. But it was not until the works of Storm (1878), Norman (1893), and last but not least Odhner (1926, 1939) and Dons (1933, 1934, 1942a,b,c) that nudibranchs emerged from the great depths and from beneath the littoral boulders in the Trondheimsfjord.

As we can see, there has been published several papers including and dealing with nudibranchiate species from the Norwegian coast and the Trondheimsfjord. This is mainly old literature from expeditions and travels, but none the less invaluable information since modern investigations are lacking. Høisæter (1986) and Høisæter et al. (1997) are the only recently accounts that deals with the distribution of nudibranchiate species along the Norwegian coast. In Brattegard & Holthe (1997) the coastline of Norway was delineated in to 26 faunistic sectors, accounting for the distribution within each sector of the marine macrobenthic fauna of Norway. A revised edition can be found at Directorate of Nature Management.

All recent papers compilations of older literature. This project aims to publish a synopsis of the nudibranchiate fauna of the Trondheimsfjord, including sector 12-15 (middle Norway) as defined in Brattegard & Holthe (1997), based on collected material and material deposited at VM, NTNU, together with a review of all accessible literature dealing with or including nudibranchiate species. The project also aims to publish an easy to use identification guide to the nudibranchiate species in our area, and bring this knowledge to divers and nature-interested persons in general.