# Diet Reconstruction Under Limited Prior Information: Dietary Contributions and Isotopic Niche of Metridium senile in the North Yellow Sea

## TL;DR

This study reveals that the sea anemone Metridium senile is a top predator in the North Yellow Sea, feeding mainly on small fish and invertebrates, which could impact local ecosystems.

## Contribution

The paper introduces a new framework for diet reconstruction when prior information is limited, using isotopic niche metrics and Bayesian mixing models.

## Key findings

- Metridium senile has a high trophic position (TP = 3.09 ± 0.25), indicating it is a top predator.
- Approximately 65% of its diet comes from small fish and invertebrates, not just suspended particles.
- Its opportunistic feeding behavior could suppress early fish recruitment and alter local food web dynamics.

## Abstract

The sea anemone (Metridium senile) has become much more common on the seafloor of the North Yellow Sea, but its true diet in nature has been unclear. We asked two questions: what role does this anemone play in the local food web, and how can we trace its diet when background information is limited? We compared the stable isotopes in anemone tissues with those of likely foods and used a clear set of tests to track where its nutrition comes from. The results show that this anemone is not just a passive filter of tiny particles. It feeds heavily on small fishes and shrimps, as well as other small seafloor animals, and sits high in the food chain—higher than some species usually thought of as predators. About two thirds of its long-term diet appears to come from small fishes and invertebrates, with the rest from suspended material. Ongoing feeding on young fishes could reduce the number that survive to adulthood and reshape local ecosystems. We suggest monitoring its size, density, and overlap with young fishes and piloting targeted removals with seafloor litter clean-ups in priority areas. Our framework can be applied to diet studies when food-source data are limited.

Biomass of the plumose anemone Metridium senile has surged in the benthic ecosystem of the North Yellow Sea in recent years. Understanding its diet and the proportional contributions of food sources is essential for assessing the ecological consequences of this expansion. The species is often characterized as a passive suspension feeder, yet laboratory feeding trials have documented shrimp consumption. Because prior dietary information from the region is scarce, conventional stable isotope approaches are poorly constrained. We developed an integrative framework coupling trophic position estimation, isotopic niche metrics, spatial point pattern analysis, and a Bayesian mixing model to improve diet attribution under limited prior information and to test whether M. senile preys on small-bodied and juvenile teleosts and invertebrates under natural conditions. Our analyses showed that: (i) M. senile occupied a high trophic position (TP = 3.09 ± 0.25), exceeding those estimated for putative predators in our dataset, implying weak top-down control; (ii) in isotopic niche analyses, M. senile showed high posterior probabilities of occurring within the niches of cephalopods and medium-sized fishes (78.30% and 63.04%, respectively), consistent with shared prey and inconsistent with a strictly suspension-feeding strategy; (iii) mixing space diagnostics informed by spatial point pattern analysis indicated that including small-sized fishes and shrimps as sources was necessary to reconcile the elevated TP; and (iv) the Bayesian mixing model estimated that small-bodied and juvenile teleosts and invertebrates supplied most long-term nutrition (posterior mean ≈ 0.65), with the remainder from suspension-derived sources, consistent with an opportunistic generalist rather than a strict suspension feeder. Sustained predation on small-bodied and juvenile teleosts and invertebrates could suppress early fish recruitment, impose top-down control on forage species, and alter the local food web structure. Management should monitor M. senile (size structure, population density, and co-occurrence with juveniles and forage biota) and consider targeted removals and seafloor litter cleanups in priority habitats. The framework is applicable to diet studies with limited prior information; adding δ34S, compound-specific amino-acid isotopes (CSIA-AA), and DNA-based dietary evidence should further sharpen source discrimination.