Burrowing behavior in mud and sand of morphologically divergent polychaete species (Annelida: Orbiniidae)

Muddy and sandy sediments have different physical properties. Muds are cohesive elastic solids, whereas granular beach sands are non-cohesive porous media. Infaunal organisms such as worms that burrow through sediments therefore face different mechanical challenges that potentially lead to a variety of burrowing strategies and morphologies. In this study we compared three morphologically distinct polychaete species representing different clades in the family Orbiniidae and related differences in their burrowing behaviors and morphologies to their natural environments (mud or sand). Worms burrowed in transparent analogs for muds and sands, and kinematic analysis showed differences both among species and between materials. Leitoscoloplos pugettensis lives in mud and burrows by fracture, using its pointed head to concentrate stress at the tip of the burrow. Naineris dendritica lives in sand and uses its broader head that fluctuates in width over a burrowing cycle to decrease backward slipping in sand, potentially preventing burrow collapse. Orbinia johnsoni lives in sand and uses internal body expansions to pack sand grains, another mechanism to prevent burrow collapse. By combining data from species and materials to obtain a broad range of burrowing velocities, we show that burrowing worms control their velocity by increasing or decreasing their burrowing frequency rather than by altering cycle distance as shown previously for crawling earthworms. This study demonstrates how fairly small evolutionary divergences in morphologies and behaviors facilitate locomotion in environments with different physical constraints.