Date:28 July 2004
Author: Sven Klimpel (IFM-GEOMAR, Kiel, Germany), David Shale (photographer, England)
Today’s ROV dive was another successful dive to around 1000 m to observe and collect animals from the pelagic zone. The descent was marked by the abundance of amphipods. These small crustaceans are common in mid-water and are very active, many were attracted to the lights of the ROV. The main aim was to collect gelatinous animals for experiments carried out on board. In the upper layers between 50 and 100 m comb jellies or ctenophores are fairly common but difficult to see, as they are almost transparent. They are known as comb jellies because they have rows if cilia arranged in rows along the body and they resemble combs. When they beat to propel the jelly along they produce type of motion called metachronal rhythm. When seen in the artificial lights of the laboratory the comb plates refract the light producing colours in the same way water drops produce a rainbow after a rainstorm.
A very beautiful deep-sea jellyfish
We collected one specimen (Bollinopsis sp.) to photograph and film. Other specimens were collected using the suction device on the front of the ROV. This is a very skilful process as the pilot has to manoeuvre the ROV very slowly and carefully up to the jellyfish which is often no larger than 4 cm in diameter, position it in front of a 10 cm pipe and apply delicate suction to capture the animal undamaged! Success we collected a number of Aeginura grimaldii and one specimen of a deep sea jellyfish Atolla sp. Commonly seen in deep plankton hauls in a damaged state, this animal is so different in the wild, long streaming tentacles hanging down below it’s disc-like body, a truly beautiful sight
Later in the day we completed our twentieth haul with the big benthic trawl. The trawl was towed at depths ranging from 3100 to 3010 m. After nearly four hours in the water, the trawl was safely brought back, attracting nearly all participants to the trawl deck. The tow was technically unsuccessful, but still the bag contained a little catch; a typical bathypelagic fauna dominated by deep-sea smelts (Bathylagus euryops), spotted lanternfishes (Myctophum punctatum), sawtoothed eels (Serrivomer beanii), red-coloured deep-water prawns, as well as small beautiful deep-sea jellyfishes. However, rarer fishes were also caught including ……. see the picture. It has huge sharp teeth, very small eyes, and between the eyes is a light attached to a long rod. What is it, is it an alien? No, this very funny fish species is Melanocetus johnsoni a deep-sea anglerfish or humpback anglerfish. This bathypelagic species is a soft-bodied fish that lacks scales and pelvic fins and occurs worldwide in deep tropical and temperate waters. A spine of the dorsal fin acts as fishing rod, tipped with a fleshy bait which is luminous. Other fish species or crustaceans are attracted to this lure and get eaten. With their large mouths, the anglers can swallow prey larger than themselves. Another unusual adaptation in anglerfish is their sexual dimorphism. The males are very small in comparison with the females.
Is this an alien from the deep sea?
The grenadiers or rattails (Macrouridae) are not such spectacular fishes but a permanent member in our previous benthic catches and a very large and diverse family with some 34 genera and 383 species recognized. The family is found through an extensive depth range with some species found below 4000 m; however, most species can be found between 500 and 1500 m and are considered benthopelagic. They range in length from approximately 10 cm in the Graceful grenadier (Hymenocephalus gracilis) to more than 1 m in the Giant grenadier (Albatrossia pectoralis). An important commercial fishery exists for the larger species, such as the Giant grenadier and the Roundnose grenadier (Coryphaenoides rupestris). Many species are large and, in combination with their abundance, are potentially important apex predators in the deep-sea environment of the Mid-Atlantic Ridge. The family as a whole may represent up to 15 per cent of the deep-sea fish population.
For these reasons we have analysed available literature (world wide) about the parasites of the members of the familiy Macrouridae and we have found very interesting results. The parasite diversity is very high for almost all members of the family. This can be explained by their generalistic feeding behaviour. Crustaceans (including copepods, amphipods, mysids, euphausids), polychaetes, cephalopods and fish were recorded for most macrourid fish species. An additional large depth range between 500 and 4000 m, and possible vertical migrations during the night, enable the macrourids to accumulate a wide variety of parasites.
Percentage distribution of different parasite taxa in four macrourid groups
A comparison of the parasite fauna of the macrourids and other fish species of the Altantic shelf region revealed distinct differences. It seems that the macrourids have an entirely different parasite fauna, mainly characterised by deep-sea and oceanic parasite species. Similar differences between deep and inshore waters in nearby localities have been previously recorded by other authors. Macrourids in general show a greater similarity among themselves than with other teleosts. This might be explained by a wide host range of many deep-sea parasites, together with an overlap in distribution and migration patterns of their macrourid hosts.
In conclusion the diversity of the parasite fauna is a consequence of the large depth range of the hosts and of many different prey species that can serve as intermediate host. Therefore, similarities in diet and habitat of potential host species may result in similar parasite faunas. Other factors are host migration and a close phylogenetic relationship among the different macrourid hosts. However, only a few fish species from the Mid-Atlantic Ridge deep-sea ecosystem have been studied for parasites and stomach contents. These aspects clearly demonstrate the urgent need to investigate the parasite fauna of the deep-sea fishes, which we collected during the Mar-Eco cruise of the RV G.O. Sars. This task is a highly valuable contribution to the envisaged food web studies and to general biodiversity studies of the Mar-Eco project.