Ultimate cause(s) of dwarfism in invertebrates: the case of driftwood talitrids

Question: What are the ultimate cause(s) of dwarfism in talitrids (Crustacea, Amphipoda, Talitridae)? Hypotheses: Dwarfism evolved: (1) to allow talitrids to negotiate small burrows in driftwood made by the primary colonizers of driftwood; (2) because driftwood was a poor quality food; or (3) to minimize oxygen uptake from seawater in small burrows. Experimental organisms: Platorchestia platensis - driftwood acclimated; Platorchestia platensis - wrack acclimated; and Orchestia gammarellus - wrack acclimated. Methods: (1) Behavioural, using maze experiments to test the ability of talitrids of a wide range of body sizes to exit from a circular hole of known diameter. (2, 3) Measurements of oxygen uptake rate from seawater by driftwood-and wrack-acclimated P. platensis. Results: (1) Maze escapes by talitrids were size sorted, such that adults of O. gammarellus were too large, whereas adults of the smaller P. platensis could complete most of their life cycle within small burrows made by primary colonizers of driftwood. (2) Oxygen uptake rate was significantly lower in driftwood-acclimated than in wrack-acclimated P. platensis, consistent with driftwood being a poor quality food. (3) Model predictions of the dissolved oxygen in seawater in burrows showed that both small and large talitrids would be asphyxiated in all available driftwood burrow sizes if they were closed systems. Conclusions: Results support both hypotheses (1) and (2), but not (3), as possible causes of dwarfism in talitrids.