Does Cichlid Predation Affect Gastropod Reproductive Strategies? Feeding Choice of Neolamprologus Tretocephalus on Two Lavigeria Species
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Does cichlid predation affect gastropod reproductive strategies? Feeding choice of Neolamprologus tretocephalus on two Lavigeria species Students: Stephanie Miller & Mbalassa Mulongaibalu Mentor: Ellinor Michel Introduction The reproductive strategies of Lake Tanganyika gastropods vary widely. Among closely related endemic Lavigeria species, average brood size varies from 25 individuals to 150 individuals between species (Kingma & Michel, 2000). Offspring size also varies between an average of 2.5 mm to 0.75 mm at hatching between species. Gastropod reproductive strategies may be affected by environmental factors including the variability of the environment, primary production, and physiochemical factors (Thorp 1991). In Lake Tanganyika, Lavigeria grandis and Lavigeria coronata are largely sympatric, yet vary markedly in their reproductive strategies. Mean brood size of L. grandis is approximately 50 individuals while mean L. coronata brood size is approximately 110 individuals (Kingma & Michel 2000). Mean size at hatching is 2.5 mm for L. grandis while 1.5 mm from L. coronata. Because these species are subject to similar environmental conditions, reasons for this variation are unclear. Whereas crabs are the dominant predator of adult Lavigeria, they are less likely to consume immature snails (Michel, personal communication). Several Tanganyikan fish species may crush and consume small snails; two cichlid species are known to consume these snails regularly. Gut contents analysis of Neolamprologus tretocephalus reveal that small gastropods are a dominant prey item and that Lavigeria species are the dominant genera consumed (Nduwarugira 1999, Michel et al 1999). The ability and preferences of these fish may affect reproductive decision-making in prey snails. To test whether fish vary in their ability and preference for snail size, we performed 10 feeding trials using individual Neolamprologus tretocephalus. Previous feeding trials (n=7) reveal that feeding cichlids prefer smaller snail prey (Zorich pers. comm., Michel et al. 1999). Methods Ten individual feeding trials were preformed using Neolamprologus tretocephalus. Fish were in collected in good condition (by professional aquarium collectors, the Kazumbes) and starved for 24 hours before trials began. For each trial, one fish was placed in a 10-gallon aquarium and allowed to acclimate for four hours. Twenty late-stage babies (stage 5, Kingma & Michel 2000) each of L. grandis and L. coronata were placed in the aquarium in a 40 mm diameter petri dish. Fish were allowed to feed for 18 hours, with a 13- hour dark period mimicking a dry season photoperiod. After 18 hours, fish were removed and remaining snails were counted. Snails not in the aquarium were assumed to have been consumed. Fis h were measured and classified as either small (less than 10 cm including tail fin) or large (greater than 10 cm). Results The number of snails remaining after each feeding trail varied greatly. However, no significant difference in number of snails eaten (t-test, p=0.37) was found between snail species (Figure 1). Discussion In the previous study smaller snails were the preferred prey (Zorich, pers. comm.; Michel et al., 1999). However, our results did not support the previous trend. We suggest that differences in fish size may have been responsible for this difference, as we may have used larger fish than in the 1999 study. Additional trials using smaller fish may yield a better understanding of the effect of cichlids on snail reproductive strategies. Continued analysis of fish gut contents will indicate whether juvenile gastropods are significantly affected by cichlid predation. Fig. 1: Survivorship of Lavigeria hatchlings under N. tretocephalus predation 25 20 15 10 5 0 grandis coronata -5 hatchlings surviving hatchlings -10 -15 -20 12345678910 Predation trial (hungry on the left, stuffed on the right) References Kingma, I. and E. Michel. 2000. Life history variation in the gastropod genus Lavigeria (fam. Thiaridae) of Lake Tanganyika_. Corresp.-blad Ned. Malac. Ver. 312:15-24. Nduwarugira, F. 1999 “Contribution a l’etude du regime alimentaire de Lobochilotes labiatus et Neolamprologus tretocephalus.” Nyanza Project 1999 Annual Report. Thorp, J.H., and A.P. Covich. 1999. Ecology and Classification of North American Freshwater Invertebrates. Academic Press, Inc. Michel, E., J.A. Todd, I. Kingma, F. Nduwarugira, and N. Zorich. 1999. Is life history variation in a gastropod species flock driven by cichlid predation? Brood and embryo size differences in Lavigeria from Lake Tanganyika. Euro. Soc. Evol. Biol Ann. Mtg., Barcelona 1999. .