Four slide specimens Buccinium spp (Whelk), Conus spp, Calliostoma spp(Subtidal snail), Patella spp (Limpet) and Littorina spp (Periwinkle), to determine under the light microscope at times ten magnification the differences in each radula.
A Mytilus edulis (Mussel) and Cerastoderma edula (Cockle) were both used as specimens for observation and recording of their various body parts. Using a sharp scalpel the posterior adductor muscle was cut. Pulling apart of the mantle lobe from the right valve, eventually the anterior adductor muscle could be cut exposing the soft body inside. The retractor muscles were also cut and the soft body was removed from the right shell half. In doing so the soft body was completely on the left side of the shell. This allowed observation and identification of the various body parts and a look at the gills between the two specimens, these were both drawn. A further three slides were presented Filibranch Mytilus gill, Folded Filibranch ctenidium Pinna and Katelysia Eulamellii branch gill. Protobranch was already seen with the Cerastoderma edula (Cockle) specimen.
Results.
Fig.1 and Fig.2 show the dorsal observation and the longitude section of the Patella specimen and a clear view of its various body parts. Fig. 3-7 shows the difference in radula structure in relation to their gastropod feeding strategies and habitat type. Thus comparison could be made with the dissection and observation of the morphology of the mytilus spp Fig.8 and Cerastoderma spp Fig.9 of bivalves Table. A
Table A
Much difference was also seen in the frozen slides of three different gill types Filibranch Mytilus gill Fig. 10, Folded Filibranch ctenidium Pinna Fig. 11 and Katelysia Eulamelli branch gill Fig. 12. These differences may have been a result again of their feeding strategy and habitat type.
Discussion.
Comparisons could be made between the radula of the gastropods and the bivalve feeding systems. In the specimen gastropods, Buccinium the radula specimen slide showed very sharp points indicating that yes this species is a carnivore. Previous evidence has shown that they are indeed carnivores and they tend to feed on crabs and such like. Via mechano and chemo receptors they are able to detect food which they move towards using their muscular foot. The radula is extended from the buccal cavity and if he prey has a shell, the teeth of the radula drill a hole through the shell, almost like a zipper action to gain entry to the food source underneath. Again with the Callistoma spp radula which is also a carnivore it had sharp points on their radula. With regards to Patella spp and Littorina these species have been seen to be herbivores which can be detected by the formation of their radula. These species graze by rasping or scraping algae from the surrounding rocks. As grazers, they are continually scraping their radula and as a consequence they have found a means to lower the risk of wear and tear by the mineralization of the tips of their radula with the addition of iron oxide and silicone which hardens them. Food is ingested by repeated movement of the radula controlled by the buccal muscles, these muscles cause the radula to protract out of the mouth to the food and pull the food back into the buccal cavity, or rasp with retraction movement (Elliott and Susswein 2002). They also rotate their radula in order to reduce the wear and tear imposed upon them during feeding. The Conus spp is very different from the rest of the specimens in that under the light microscope it could be seen that they have a single tooth that is barbed and hollow, this species fill the hollow of the radula before they harpoon the tooth onto their prey, thus injecting the prey with the venom. There are many different types of gastropods that all react in different ways to gain access to food. Some carnivores have a prey capture phase, such as pleurobranchaea that have a proboscis that they use which can drill, wedge (Chipping), and asphyxiate their prey.
The gut structure is not dissimilar in the gastropods and the bivalves. In gastropods they have an oesophagus that is dilated to form a crop to store their food. Both have a pear shaped stomach with diverticular branching into the digestive gland. Their stomachs both contain a style sac containing mucus and enzymes. However feeding in bivalves is very different to that of gastropods. Bivalves are filter feeders, they contain ctenidium (Gills) as do gastropods except terrestrial livers but bivalves use their gill for not only gas exchange but also for food supply. The Protobranch seen in the cockle is stacked primitive unmodified ctenidium were by they use proboscis and large palps to aid in food supply. With regards to the mussel, they have large sheet like ctenidium with inhalant and exhalent siphons. The lateral cilia create water current which moves they water, and then the laterofrontal cilia capture food particles from the filtered water which has passed over the mucus covered gills. The frontal cilia transfer this captured food to the food groove which in turn transfers it to the mouth. It is the palps that are the final sorters of the food. They determine what should be ingested and what should be rejected, which is spat from the body in the form of psuedofaeces. Observations of the Pinna filibrancium gill which is folded showed that the ctenidia are highly modified. This may be to increase the surface area of the gill known as Plication. Other benefits that these species use is to expand or extract their gills in response to sediment and quality of their food (Roberts 2006). Katelysis Eulamelli branch gill showed comprised lamellabranch sheets of tissue again with laterofrontal, lateral and frontal cilia to aid the movement of the food particles.
Conclusion.
It can be seen from these observations that the feeding systems of gastropods and bivalves are very different even though they still retain their three main body parts. Evidence suggests they will be selected for their best survival (Feeding) and reproductive strategies and will continue to evolve along their separate branches.
References.
Campbell and Reece
Biology
Sixth Edition
Pearson Education 2002
ISBN 0-201-75054-6
Elliott C. J. H and Susswein A. J.
Comparative neuroethology of feeding control in molluscs.
Journal of experimental biology (2002) 205,877-896
Roberts. D (Lectures on Gastropodes and Bivalves) 1st and 2nd February 06