1.0 Introduction
Snail is the common name that is applied most often to land snails, terrestrial pulmonate gastropod mollusks. However the common name snail is also applied to most of the members of the molluscan class gastropoda that have coiled shell that is large enough for the animal to retract completely into (Wikipedia.com. 2012). Snails are invertebrates which mean that they lack a backbone. They belong to a large and highly diverse group of invertebrates known as the Phylum mollusca (also known more commonly as ‘mollusks'). The Phylum mollusca include slugs, clams, oysters, mussels, squids, octopuses and nautiluses, in addition to snails (About.com, 2012).
They belong to the class Gastropoda. Gastropoda are a major part of the phylum mollusca and are the most highly diversified class in the phylum. Snails belong to the order stylommatophora (Fryda et al, 2005). Snails are terrestrial and marine shell-bearing animals of approximately 80,000-100,000 species of the phylum Mollusca (Cooper and Knowler, 1991). They are the second largest phylum in the animal kingdom. Europe and Africa are two important regions notable for snail production and consumption involving mainly the snail families Helicidae and Achatinidae, respectively. However, the giant African land snails Achatina achatina and Archachatina marginata are the most popular edible snails in the West African high forest zones (Ogunsami et al., 2003, Cobinah et. al, 2003).
During dry spells free-living snails withdraw into their shells and conserve water by sealing the shell opening with an epiphragm and aestivate by reducing mobility reproductive behaviour and growth. Water loss is further retarded by the use of discontinuous breathing pattern; the pneumostome opens intermittently to allow a rapid exchange of CO and O (Hermes-lima et. al, 1998). When humidity falls below 75% (as witnessed during the dry season: October to Mid-March in West Africa), A. achatina becomes inactive and seals itself into its shell with a white calcareous layer and aestivates in order to prevent loss of water from the body (Omoyakhi, 2007). A. marginata were observed to form epiphragm more readily and replace them more frequently than A. achatina (Omoyakhi, 2007). Snails are said to survive many months without food and water under aestivation (Akinnusi, 1998). The aestivated snails draw on their reserve of fat and glycogen at much reduced rate, which implies an imminent reduction in weight and loss of valuable growing time as growth during aestivation is said to reduce.
The success of gastropod molluscs in terrestrial habitats has been due to various structural, physiological and behavioural specializations (Riddle, 1983). One specialization that is well developed among the pulmonate land snails is the capacity to enter the dormant state of aestivation during periods of hot and dry environmental conditions (Rees and Hand, 1993). Snails can be found in a very wide range of environments, including ditches, deserts, and the abyssal depths of the sea. Although many people are familiar with terrestrial snails, they are in the minority. Several species of the genus Achatina and related genera are known as giant African land snails (GALS); some grow to 15 in (38 cm) from snout to tail, and weigh 1 kg (Frederick, 2010).
1.1 Justification of Study
Haemolymph is the blood analogue found in all arthropods and most molluscs which have an open circulatory system (Ebroadcast.com 2012). It is composed of water, inorganic salts (mostly Na, Cl, K, Mg and Ca) and organic compounds (mostly carbohydrates, proteins and lipids). Muscular movements by the animal during locomotion can facilitate haemolymph movement, but diverting flow from one area to another is limited (Harris, 2000). Responses to high or low temperatures in the snail induce aestivation and hibernation.
The haemolymph is an important medium for the transport of nutrients to and wastes from various organs of the snail. Aestivation, being a structural, physiological and behavioural response to desiccation, probably plays a role in the dynamics of haemolymph changes in land snails as haemolymph ionic concentration is said to show seasonal fluctuations, strongly influenced by hydration, feeding and acid/base balance (Langley, 2004). There are limits to the duration of aestivation that can be tolerated by land snails and mortality eventually increases as aestivation is prolonged (Rees and Hand, 1993).
Comprehensive values of haemolymph ionic parameters for the giant African land snails are now popularly reported in literature but none for fasting snails. Reference range values for haemolymph ionic parameters from non-aestivating, aestivating and fasting Archachatina marginata could therefore be useful for assessment of physiologic and pathologic alterations in wild as well as captive snails and establish their possible application in the evaluation of health and disease status. The information may also be useful in domestication, management and bio-conservation initiatives involving this species.
