# Intertidal Warfare: Synergistic Allelopathy Mediates Spatial Competition between Two Marine Calcareous‐Shelled Sessile Organisms

## TL;DR

A barnacle uses a chemical blend to inhibit mussel attachment, revealing a new form of interspecies competition in intertidal zones.

## Contribution

Discovery of a synergistic allelochemical blend used by a calcareous-shelled barnacle to inhibit mussel attachment.

## Key findings

- Balanus albicostatus releases palmitic acid and PGPC in a 1:1.92 ratio to inhibit Vignadula atrata attachment.
- The allelochemical blend reduces mussel byssus thread production and adhesion force.
- The chemical down-regulates genes and enzymes critical for mussel attachment.

## Abstract

Chemical warfare among marine sessile organisms remains poorly understood. Chemical defense in calcareous‐shelled organisms in particular has been largely neglected, yet this may be important in spatial dominance of crowded intertidal ecosystems. Using field survey data, spatial competition in intertidal zones between two calcareous‐shelled sessile species are discovered, the barnacle Balanus albicostatus and the mussel Vignadula atrata. Using chemical analysis and bioassays, it is found that B. albicostatus releases chemical cues with inhibitory activity against the attachment of V. atrata. This allelochemical is identified as a blend of palmitic acid (PA) and 1‐palmitoyl‐sn‐glycero‐3‐phosphocholine (PGPC) in a synergistic and unique ratio (1:1.92). This mixture of PA and PGPC synergistically reduced byssus thread production, adhesive plaque area and adhesion force of mussel foot proteins (MFPs) in V. atrata. Further analysis showed that this mixture down‐regulated expression of the genes associated with byssus formation and adhesion (PreCol‐NG, MFP2, MFP11, Tyr, BPP4, and PPO) and led to a lower activity of the enzyme polyphenol oxidase essential to mussel attachment, implying an underlying mechanism by which allelochemicals inhibit mussel attachment. This underlines the importance of allelopathy in interspecies competition between calcareous‐shelled sessile organisms and provides information which may be useful for developing novel biofouling control systems.

This study reveals a hidden chemical weapon among calcareous‐shelled sessile organisms in the intertidal zone: the barnacle Balanus albicostatus releases a synergistic blend of palmitic acid and 1‐palmitoyl‐sn‐glycero‐3‐phosphocholine as the allelochemical with the inhibitory activity against attachment of the mussel Vignadula atrata, which may mediate the spatial competition between these two marine species.