Growth and biomass production of an amphibious snail Pomacea urceus (Muller), from the Venezuelan savannah
ALBERT J. BURKY
Instituto de Zoologia. Tropical, Facultad de Ciencias, Universidad Central de Venezuela
The life history of Pomacea urceus (Muller) is considered in relation
to the annual rainy (generally May to November) and dry (generally December
to April) seasons of the Venezuelan savannah. Oviposition is at the beginning
of the dry season, mainly in December-January, after females have burrowed into
the surface mud. Clutches of 50 to 200 eggs are normal with eggs of 6-7 to 15-5
mm in diameter. The spat (mean shell length of about 10mm) hatch in January-February
and aestivate beneath the female until the rains start about 4 months later.
Growth of the new generation (starting in June) is rapid and adult size is achieved
by November - December, 6-7 months later. Adult size range (shell length, 85-135
mm) is essentially the same throughout the year and it is indicated that 85
mm is the minimum size for aestivation. Circumstantial evidence indicates a
life span of 2-5 to 3-5 years for adults of two and three breeding seasons respectively.
Biomass and egg production are assessed as total carbon (equivalent to calorific measures). Adults build up carbon (growth) during the rainy season. At the beginning of the dry season there is a marked loss of carbon due to breeding and oviposition; there is continued loss of carbon during aestivation. Spat show negative growth (carbon loss) during aestivation; this is followed by positive growth (carbon accumulation) after the start of the rains. Energy balance for the adult segment of the population is evaluated as growth, egg production, and respiration in relation to life history and seasons. The energetics and adaptive significance of the production of large eggs are discussed.
Proc. malac. Soc. Lond. (1974) 41, 127.