Prey size spectra and predator to prey size ratios of southern ocean salps

Abstract

While the importance of salp grazing has become increasingly apparent in shaping their local ecosystems, little is known about the size ranges of their prey in the field. This study investigated the feeding habits of 7 different species of salp of both solitary and aggregate life stages representing a variety of sizes across subtropical and subantarctic waters east of New Zealand. Scanning electron microscopy was used to examine the gut contents of 58 salps, which were then compared to water column communities sampled via epifluorescence microscopy, FlowCam, and flow cytometry. While most of the gut size spectra and taxonomy resembled ambient waters, substantial differences were found amongst some co-occurring species, such as increased retention of submicron bacteria by Thalia democratica. We find that even for such salps capable of feeding on bacteria efficiently, the majority of gut biomass was still made up by nanoplankton and small microplankton. Larger particles were much rarer in the guts than in the water column, potentially suggesting an upper size-threshold in addition to the lower size-threshold that has been the focus of much previous work. Salp carbon-weighted predator to prey size ratios were variable, with the majority falling between 1000:1 and 10000:1 depending largely on the size of the salp. Taken together our results indicate that despite being able to feed on submicron particles, picoplankton are relatively unimportant to the energetics of most salps in this region compared to nanoplankton such as small dinoflagellates and diatoms.

Selective and differential feeding on marine prokaryotes by mucous mesh feeders

“…Comparing the ratios of prey taxa to each other in the guts and seawater revealed changes in the relative abundances of prey, suggesting selective feeding (Figure 5A,B). Although size is an important property governing particle capture by salps (Fender et al, 2022), mounting evidence indicates that not only can mucous mesh grazers capture particles smaller than mesh openings at appreciable rates (Conley et al, 2018), but particles of similar size are not collected at the same rates (Dadon-Pilosof et al, 2019). In the present study, Prochlorococcus ($0.55 μm; Casey et al, 2019) was proportionally much less abundant in the guts than other prey types of similar size (Figure 5B, top).…”

“…Their removal by large mucous mesh grazers alters the picture of how planktonic archaea link to marine food webs. This work, combined with results from experiments on cultivated grazers, artificial prey, measurements of mesh pore sizes, and studies from coastal environments, suggests a role for mucous grazers in marine microbial mortality and evolution (Dadon-Pilosof et al, 2019;Fender et al, 2022;Stukel et al, 2021;Sutherland et al, 2010). As mucous mesh grazers are ubiquitous, and bloom rapidly in response to change, this trophic interaction likely has a global role in shaping nutrient cycling, microbial community ecology, evolution, and cross-scale ecosystem structure.…”

“…These macrozooplankton use adhesive mucous nets to capture food particles orders of magnitude smaller than themselves. Strong evidence exists for feeding of mucous mesh grazers on eukaryotic microbes, especially phytoplankton, through 18S rRNA gene sequencing of gut tissues (Frischer et al, 2021;Pauli, Metfies, et al, 2021;Thibodeau et al, 2022), microscopy (Fender et al, 2022;Stukel et al, 2021;Thompson et al, 2021), flow cytometry (Dadon-Pilosof et al, 2019;Stukel et al, 2021;Thompson et al, 2021), and laboratory-or culture-based experiments with model eukaryotic prey (Selander & Tiselius, 2003;Troedsson et al, 2007). The ubiquity of these filter feeders in the oceans (Bednaršek et al, 2012;Lucas et al, 2014) and their ability to restructure pelagic food webs when they bloom (Alldredge & Madin, 1982;Brodeur et al, 2018) suggest important global impacts by mucous grazers on primary production, phytoplankton communities, and carbon export and cycling (Luo et al, 2020(Luo et al, , 2022.…”