Application of species-specific primers to estimate the in situ diet of Bythotrephes [Cladocera, Onychopoda] in its native European range via molecular gut content analysis

Unexpected high species richness of Bythotrephes Leydig, 1860 (Branchiopoda: Cladocera: Cercopagididae) in subalpine Austrian lakes, with the description of new taxa

The presence of four taxa at the species and subspecies level was revealed based on morphological investigation of a small collection of the representatives of the genus Bythotrephes Leydig, 1860 from Austrian subalpine lakes. The presence of a new species, B. inexpectatus sp. nov. was revealed in two lakes and the presence of a new subspecies, B. longimanus austriacus ssp. nov., in six lakes. Specimens of the population from the Lake Halstättersee, distinguished by the presence of peculiar features, are provisionally assigned to the species B. brevimanus Lilljeborg, 1901 of which the main range is confined to Circumbaltic region. Few morphologically specific specimens from Wolfgangsee could not be accurately identified and therefore assigned to the taxon Bythotrephes sp. requiring further investigation. Previously described taxa from the region, B. styriacus Ischreyt, 1939 and B. longimanus var. carnica Ischreyt, 1939, were considered either required further taxonomic evaluation or having an unclear taxonomic status, respectively. In one lake, the co-occurrence of two species was recorded, which is a rather rare case for the representatives of the genus. The potential reasons for the high species richness of spiny waterfleas in Austrian subalpine lakes and their possible origin are briefly discussed.

Molecular diet analysis enables detection of diatom and cyanobacteria DNA in the gut of Macoma balthica

Detritivores are essential to nutrient cycling, but are often neglected in trophic networks, due to difficulties with determining their diet. DNA analysis of gut contents shows promise of trophic link discrimination, but many unknown factors limit its usefulness. For example, DNA can be rapidly broken down, especially by digestion processes, and DNA provides only a snapshot of the gut contents at a specific time. Few studies have been performed on the length of time that prey DNA can be detected in consumer guts, and none so far using benthic detritivores. Eutrophication, along with climate change, is altering the phytoplankton communities in aquatic ecosystems, on which benthic detritivores in aphotic soft sediments depend. Nutrient-poor cyanobacteria blooms are increasing in frequency, duration, and magnitude in many water bodies, while nutrient-rich diatom spring blooms are shrinking in duration and magnitude, creating potential changes in diet of benthic detritivores. We performed an experiment to identify the taxonomy and quantify the abundance of phytoplankton DNA fragments on bivalve gut contents, and how long these fragments can be detected after consumption in the Baltic Sea clam Macoma balthica. Two common species of phytoplankton (the cyanobacteria Nodularia spumigena or the diatom Skeletonema marinoi) were fed to M. balthica from two regions (from the northern and southern Stockholm archipelago). After removing the food source, M. balthica gut contents were sampled every 24 hours for seven days to determine the number of 23S rRNA phytoplankton DNA copies and when the phytoplankton DNA could no longer be detected by quantitative PCR. We found no differences in diatom 18S rRNA gene fragments of the clams by region, but the southern clams showed significantly more cyanobacteria 16S rRNA gene fragments in their guts than the northern clams. Interestingly, the cyanobacteria and diatom DNA fragments were still detectable by qPCR in the guts of M. balthica one week after removal from its food source. However, DNA metabarcoding of the 23S rRNA phytoplankton gene found in the clam guts showed that added food (i.e. N. spumigena and S. marinoi) did not make up a majority of the detected diet. Our results suggest that these detritivorous clams therefore do not react as quickly as previously thought to fresh organic matter inputs, with other phytoplankton than large diatoms and cyanobacteria constituting the majority of their diet. This experiment demonstrates the viability of using molecular methods to determine feeding of detritivores, but further studies investigating how prey DNA signals can change over time in benthic detritivores will be needed before this method can be widely applicable to both models of ecological functions and conservation policy.