by Laura Clark
Halosauropsis macrochir is fairly common species found along the Mid- Atlantic ridge in the North Atlantic. However little is known about its distribution, behaviour, growth and age. The halosaur was one of the species selected for investigations of life history strategy which is one of the priority research areas of MAR-ECO.
This study looked at how to age these fish by first analysing the ear bones, also known as otoliths. No attempt had been previously made to age this species. First the otoliths were cleaned, weighed and measured before several preparation techniques were investigated to establish which was the most suitable. Sectioning or cutting (taking slices) of the otolith after embedding in epoxy resin produced the best results. Once the sections were made, the rings presumed to represent annuli, were examined and counted at under a microscope. The otoliths showed ring patterns similar to those found in species for which validation of the annual deposition has been carried out. However, the patterns were not always clear and consistent, hence counting these rings and estimating age was challenging. and so the ages obtained were considered to be first estimates. Further work is needed to increase precision of the results.
Halosauropsis macrochir otolith section. Photo: Laura Clark
The distribution, of the species was also investigated, as this species inhabits various depths and locations along the entire ridge area from Iceland to the Azores. This analysis was conducted to see if larger fish were found deeper, as this is the case with many other deep-water species. However, the results show that this is not the case with Halosauropsis macrochir, this species was actually seen to decrease in size as with increasing depth.
There are many possible foraging strategies that a deep-water fish species may adopt and so the diet of this species was also analysed. Stomachs have been stored in ethanol (alcohol) since 2004, and so these were opened and the contents separated for identification. Food items were identified under a microscope to the lowest possible taxonomic level. Identification of highly digested contents was challenging and so certain objects such as fish eyes and shrimp legs were looked for to identify the contents. Experts on relevant taxa were also involved to try and identify whole food items further. Early results suggest that as the species grows their diet preference changes from food items such as shrimps, to a diet with a greater proportion of fish.
Shrimps found among Halosaurorpis macrochir stomach contents. Photo: Laura Clark.