# Body Size Awareness and Modular Self-Representation in Reedfish (Erpetoichthys calabaricus): Near-Field Passability Judgments

## TL;DR

Reedfish can judge if they can fit through an opening based on their body size, using tactile and hydrodynamic cues.

## Contribution

The study shows reedfish use modular self-representation to judge passability, extending evidence to a phylogenetically distant fish.

## Key findings

- Reedfish made pass/not-pass decisions using tactile and hydrodynamic cues at close range.
- Choices were random when all openings were passable but focused on the correct one when only one was passable.
- The findings support a modular model of self-representation in reedfish behavior.

## Abstract

Body size awareness is an animal’s ability to take its own dimensions into account when negotiating obstacles. We examined this ability in reedfish, a long, bottom-dwelling species. In a tank we installed a partition with three openings. In Experiment 1 all three openings were passable but differed in size. In Experiment 2 only one opening was passable, whereas two larger ones were not. Fish first approached any opening, but they attempted to pass almost exclusively through the truly passable one; after such attempts, they always entered the target compartment. When all openings were passable, choices were at chance: fish did not favor the largest opening but simply used one that was “big enough”. These findings indicate that the pass/not-pass decision is made at close range, likely using tactile and hydrodynamic cues from the head. The study extends evidence for body size awareness to a phylogenetically distant fish and may inform better husbandry and environmental enrichment for aquatic species, as well as bio-inspired navigation strategies in confined spaces.

Body size awareness—a component of bodily self-representation—allows animals to match their own dimensions to environmental constraints. This study tested whether reedfish (Erpetoichthys calabaricus), a benthic ray-finned species with limited vision, can evaluate aperture passability relative to their body size. Eight fish performed a “body-as-obstacle” task. After training, each individual completed 36 trials in Experiment 1 (three passable circular apertures of different diameters) and 72 trials in Experiment 2 (one small passable and two larger non-passable apertures). We scored first approach, first penetration attempt, and full passage; data were analyzed with generalized linear models. In Experiment 1, choices were random, unaffected by aperture size or position. In Experiment 2, first approaches were random, but first penetration attempts—and ensuing passages—were directed almost exclusively to the single passable aperture. These results indicate near-field formation of pass/not-pass judgments, likely via tactile and hydrodynamic sensing. The behavioral dissociation between exploratory (epistemic) and goal-directed (pragmatic) actions supports a modular model of self-representation, where distinct sensorimotor loops underlie information gathering and goal execution. Thus, reedfish demonstrate body-size awareness and contribute to comparative evidence that modular self-representation and embodied anticipation may extend deep into vertebrate evolution.