Functional traits explain trophic allometries of cephalopods

Abstract

1. Individual body size strongly influences the trophic role of marine organisms and the structure and function of marine ecosystems. Quantifying trophic positionindividual body size relationships (trophic allometries) underpins the development of size-structured ecosystem models to predict abundance and the transfer of energy through ecosystems. Trophic allometries are well studied for fishes but remain relatively unexplored for cephalopods. 2. Cephalopods are important components of coastal, oceanic and deep-sea ecosystems, and they play a key role in the transfer of biomass from low trophic positions to higher predators. It is therefore important to resolve cephalopod trophic allometries to accurately represent them within size-structured ecosystem models. 3. We assessed the trophic positions of cephalopods in an oceanic pelagic (0-500 m) community (sampled by trawling in a cold-core eddy in the western Tasman Sea), comprising 22 species from 12 families, using bulk tissue stable isotope analysis and amino acid compound-specific stable isotope analysis. We assessed whether ontogenetic trophic position shifts were evident at the species-level and tested for the best predictor of community-level trophic allometry among body size, taxonomy and functional grouping (informed by fin and mantle morphology). 4. Individuals in this cephalopod community spanned two trophic positions and fell into three functional groups on an activity level gradient: low, medium and high. The relationship between trophic position and ontogeny varied among species, with the most marked differences evident between species from different functional groups. Activity-level-based functional group and individual body size are best explained by cephalopod trophic positions (marginal R 2 = 0.43). 5. Our results suggest that the morphological traits used to infer activity level, such as fin-to-mantle length ratio, fin musculature and mantle musculature are strong predictors of cephalopod trophic allometries. Contrary to established theory, not all cephalopods are voracious predators. Low activity level cephalopods have a distinct feeding mode, with low trophic positions and little-to-no ontogenetic | 2693

Sexual dimorphism and biology of the large deepwater mastigoteuthid squid Idioteuthis cordiformis (Cephalopoda, Mastigoteuthidae) off Tasmania, Australia

“…High activity cephalopods identified by Murphy et al . (2020) had thick muscular mantles, larger firmer fins, and a fin length that is 50% or greater than the mantle length. The huge fins of I. cordiformis tend to dominate the organism and are well over 50% of the mantle length, but the mantle is not thick and muscular, which limits the application of categories suggested by Murphy et al .…”

“…Recent work by Murphy et al . (2020) used morphological traits to classify cephalopods into three functional groups based on low, medium and high activity levels as well as feeding mode (from passive feeders to active hunters). These factors along with body size were used to infer trophic position.…”

Discrete Spawning Aggregations of the Loliginid Squid Doryteuthis gahi Reveal Life-History Interactions of a Dwarf Morphotype at the Center of Its Distribution Range

“…Similar to these findings, recent research exploring trophic allometries in cephalopods have also suggested that there are some species or Families (i.e., Cranchiidae) that do not exhibit ontogenetic trophic position shifts across different size classes, differing from the general description of cephalopod as voracious predators. These low activity level cephalopods were characterized by having low trophic positions, feeding largely upon herbivorous zooplankton and do not shifting to higher levels of piscivory until larger sizes (see Murphy et al, 2020).…”

Food Web Trophic Structure at Marine Ranch Sites off the East Coast of Korea

“…Despite important roles as major foraging predators with prey preferences and avoidances but no individual specialization (Marsteller, 2018) and as prey in benthic marine communities, and the possible impact of fishing on ecosystem functioning (Sauer et al, 2019), systematic studies on the GPO trophic niche (trophic pathways and positions) in coastal marine food webs are still lacking compared with squids (Hobson and Cherel, 2006;Merten et al, 2017;Murphy et al, 2020).…”

“…More importantly, their diet compositions may seldom capture topological descriptions of food webs to characterize their trophic niche (Kang et al, 2020). Alternatively, carbon and nitrogen stable isotope analyses (referred to here as δ 13 C and δ 15 N values, respectively) have been applied successfully to the identification of diets, feeding habits, and trophic ecology of cephalopods (Cherel et al, 2009;Merten et al, 2017;Murphy et al, 2020). The dietary resources of a species within a marine food web can be resolved by the δ 13 C values of its tissues, as the latter contains information on the source (benthic or pelagic) of prey items (Kang et al, 2008;Park et al, 2020).…”