Matthew Palmer and Dr. Lee F. Braithwaite, Zoology
Throughout the phylum Mollusca, gills are used to accomplish a variety of tasks and are thus highly specialized organs. The chitons, class Polyplacophora, are a primitive group of molluscs whose gills have the unique condition of being aligned on either side of the foot in series of up to 60 gills per side. Despite this unique arrangement, chiton gills have been the subject of relatively few studies. The objective of this project was to elucidate the mechanism by which chitons extrude particles from their gill surfaces. The two approaches employed to this end were, first, observation of live chitons transporting experimentally introduced particles across the gill epithelium, and second, examination of the gills by electron microscopy. Four species of chitons were studied: Tonicella lineata, Mopalia muscosa, Mopalia lignosa, and Nuttallina californica.
Observation of particle transport by cilia was possible on live chitons by anesthetizing the chitons in MgCl2 to relax all muscles but preserve ciliary activity. Chitons were then inverted in a bowl of seawater. Fine-grade carborundum particles were injected into the mantle cavity using a pasteur pipet, and a dissecting microscope was used to observe the pattern by which the particles were extruded.
To prepare the gills for scanning electron microscopy, whole chitons were fixed by immersion in 3% glutaraldehyde in buffer for at least 2 hours. The gills were then removed, washed in buffer, dehydrated in ethanol, and critical-point dried in CO2. Finally, gills were mounted on aluminum stubs and sputtered with gold. A JEOL 6100 scanning electron microscope was used to examine the gills, and photographs were taken using TMAX film.
The observations of live chitons under a dissecting microscope showed that introduced particles are entrapped in mucus and passed toward the foot along the septa between adjacent gills. This is contrary to a paper by Russell-Hunter (1988) which reports that particles are passed along the gill axis. Once the particles have reached the margin of the foot, they are passed posteriorly along the margin until they reach the anal opening of the girdle. Particles are then expelled through this opening by water currents, which are created by cilia and by coordinated, flushing movements of the entire gill “curtain.”
Examination of the location of gill cilia by scanning electron microscopy supported my assertion that particles were passed along the inter-ctenidial septum, and not along the axis, as Russell-Hunter reported. Cilia were found to be sparsely distributed along the axis and more densely packed along the lamellar ridges (ridges of adjacent gills make up one septum), indicating that this is where particles are more likely to be passed by ciliary action. Further inspection revealed that the gill epithelium in chitons is composed of simple columnar epithelium. These epithelial cells have numerous microvilli at their apices, and some cells have clumps of cilia sprouting from their apices. Although amounts of cilia may vary between the gills of different species, the relative abundance of the cilia on any individual gill occurs in the same proportions. Thus, regardless of the species being considered, chiton gills have dense bands of cilia across the middle of the lamella and along the lamellar ridges.
Though Fischer (1990) performed a SEM study of one species of chiton’s gills and reported no specialized cilia for particle transport, my observations of live chitons extruding particles from their gills by ciliary action in repeatably the same fashion suggests otherwise. Closer inspection of those cilia by SEM revealed that their location and density corresponded with my observations of particle transport, suggesting that the gills of chitons are specialized in their ability to efficiently expel particles in concentrations far beyond those found in most natural environments. Further study is currently being performed, using transmission electron microscopy, to document the direction of beat of each type of cilium.
References
- Fischer, F.P., M. Alger, D. Cieslar, H. Krafczyk 1990. The Chiton Gill: Ultrastructure in Chiton olivaceus (Mollusca, Polyplacophora). Journal of Morphology 204: 75-87.
- Russell-Hunter, W.D. 1988. The gills of chitons (Polyplacophora) and their significance in molluscan phylogeny. American Malacological Bulletin 6(1): 69-78.