Buoyancy and Hydrostatic Balance in The West Indian Coelacanth Latimeria chalumnae

Abstract

Buoyancy and balance are important parameters for static or slow moving low‐metabolic aquatic organisms. The extant coelacanths, a single genus (Latimeria) consisting of two species living at moderate depths (100 – 400 m) in the Western Indian Ocean and off Indonesia and representing an ancient group of lobe finned fishes dating back to the early Devonian (410 Myr), have among the lowest metabolic rates of any living vertebrate, and thus would be predicted to expend precious little energy to maintain buoyancy and balance. Previous observations on living coelacanths support the hypothesis that the coelcanth is neutrally buoyant and is in close to perfect hydrostatic balance in any body posture. Likewise, point‐by‐point measurements of tissue composition have shown a generally high lipid content of coelacanth tissue supporting neutral buoyancy without a gas filled bladder as found in other sarcopterygians (lungs) and in many actinopterygians (swim bladder). However, precise measurements of buoyancy and hydrostatic balance at different depths have never been made in the coelacanth. Here we show using non‐invasive imaging that buoyancy of the coelacanth closely matches its depth distribution and a close‐to‐perfect balance can be used by the animal to change body posture with little effort. We found that the lipid filled “fatty organ” of the coelacanth is well adapted to support neutral buoyancy and simple maneuvers of fins can cause a considerable shift in torque around the pitch axis. This allows the coelacanth to assume and remain in any body posture like horizontal orientation, headstand or tail stand with little physical effort. Our results demonstrate a close match between tissue composition in the extant coelacanth, depth range and behavior (e.g. effortless vertical drift hunting with the electrosensitive rostral organ in close contact with the substrate). We anticipate our non‐invasive technique of mapping tissue components to be a starting point for more sophisticated models of buoyancy and hydrostatic balance in aquatic vertebrates in general. Specifically, we suggest the technique can be used on the few late term and juvenile coelacanth specimens in existence to predict the most likely habitat depth of the juvenile life stage which is currently unknown.