University of Montana Western Newport, Washington, United States
Abstract Body : Sea Lampreys (Petromyzon marinus) are anadromous jawless vertebrates that reproductively mature during their long-distance migration to freshwater spawning habitats. Since gametes are released directly into the coelom, which is surrounded by muscle, if peritoneal pressure increased during locomotion, then gametes would be lost before reaching the spawning site. Yet, gravid females arrive to spawning sites carrying hundreds of thousands of eggs. Hence, the anatomical structure of the coelomic wall must ensure ambient peritoneal pressure during locomotion. We, therefore, investigated the anatomical structure of the coelomic wall with microdissection and histology. The wall comprises multiple layers of collagen fiber bundles formed by connective tissue contributions from the perimysial fascia enveloping the muscular myomeres and from the connective tissue myosepta. Mid-laterally, the wall is formed by three layers: A dense superficial layer is formed by fibers from the perimysial fascia that are arranged helically and are oriented caudoventral to craniodorsal. A dense deep layer is formed by fibers from the myosepta that interdigitate perpendicularly with the fibers of the superficial layer before traveling deep to the latter and becoming helically oriented caudodorsal to cranioventral. Some of the myoseptal fibers also contribute to the formation of a thin innermost layer of connective tissue that supports the peritoneum. Dorsally, the layers of the wall diverge: The superficial layer runs vertically to join the periaxial fascia surrounding the notochord and the fat body, whereas the deep layer curves horizontally to continue the wall dorsally, while integrating with the tunica externae of the aorta and cardinal veins. Ventrally, the wall is formed by multiple interdigitating cross-helical layers made of fibers from both the perimysial fascia and the myosepta. Furthermore, external to the coelomic wall within the pericoelomic fascia are organized fiber bundles running dorsoventrally from the periaxial fascia to the midventral line. When muscle contraction occurs, the tension in the perimysial fascia and the myosepta pulls outward on the coelomic wall to counteract the radial expansion of the contracting myomeres. When swimming, the cross-helical arrangement of the superficial and deep layers enables the coelomic wall to change length laterally during undulation so that total coelomic volume remains the same. When spawning, lampreys expel gametes volitionally by contorting their bodies around one another to decrease the volume of the coelomic cavity, which, thereby, becomes pressurized through the resistance to expansion by the layers of the coelomic wall. Hence, the coelomic wall of lampreys acts as a paradigmatic hydrostatic organ.
