Prey taxonomy rather than size determines salp diets

Abstract

Salps are gelatinous planktonic suspension feeders that filter large volumes of water in the food‐dilute open ocean. Their life cycle allows periodic exponential growth and population blooms. Dense swarms of salps have a high grazing impact that can deplete the photic zone of phytoplankton and export huge quantities of organic matter to the deep sea. Previous studies described their feeding manner as mostly nonselective, with larger particles retained at higher efficiencies than small particles. To examine salp diets, we used direct in situ sampling (InEx method) of undisturbed solitary Salpa maxima. Aggregates (“chains”) of Salpa fusiformis and Thalia democratica were studied using in situ incubations. Our findings suggest that in situ feeding rates are higher than previously reported and that cell removal is size independent with ∼ 1 μm picoeukaryotes preferentially removed over both larger eukaryotes and smaller bacteria. The prey : predator size ratios we measured (1 : 104–1 : 105) are an order of magnitude smaller than previously reported values and to the best of our knowledge, are the smallest values reported so far for any planktonic suspension feeders. Despite differences among the three species studied, they had similar prey preferences with no correlation between salp body length and prey size. Our findings shed new light on prey : predator relationships in planktonic systems—in particular, that factors other than size influence filtration efficiency—and suggest that in situ techniques should be devised and applied for the study of suspension feeding in the ocean.

Hydrodynamics of sponge pumps and evolution of the sponge body plan

“…LNA is largely associated with SAR11, the smallest and most abundant bacterial clade in the ocean ( Mary et al, 2006 ). These bacteria may slip through the mucus filter of tunicates ( Dadon-Pilosof et al, 2017 ; Dadon‐Pilosof et al, 2019 ) and evade filtration in many sponges ( Ribes et al, 2012 ).…”

Size‐specific grazing and competitive interactions between large salps and protistan grazers

“…Nishikawa and Tsuda (2001) quantified size‐fractionated chlorophyll concentrations in salp grazing incubation experiments and concluded that salp grazing rates were highest on 2–20 μ m prey. Dadon‐Pilosof et al (2019) found substantially higher S. maxima , Salpa fusiformis , and Thalia democratica grazing rates on picoeukaryotes and nanoeukaryotes than on Synechococcus , Prochlorococcus , and heterotrophic bacteria. Synechococcus has also been found in the fecal pellets of salps (Pfannkuche and Lochte 1993), although this does not necessarily imply efficient feeding on cyanobacteria‐sized cells, because mesozooplankton can also consume Synechococcus contained in aggregates (Stukel et al 2013).…”

Phytoplankton community structuring and succession in a competition-neutral resource landscape

“…In other words, the broader prey size range of grazers feeding on large phytoplankton yields a steady-state prey biomass of lower average concentration at each size within that range than that for grazers of smaller phytoplankton. It is worth noting here that some grazers feed wholesale across the phytoplankton size domain (e.g., gelatinous tunicates feeding with mucous webs, 38,39 ) rather than in proportion to average prey size. Feeding by these organisms will positively tilt the phytoplankton SDS to a value between −4 and −3 in a 24 and (squares) unpublished data from the NAAMES program.…”