Reproductive biology and ecology of Mazatlania fulgurata, a marine gastropod adapted to life on the intertidal of Central American beaches

Danielle R Stevenson, Purdue University

Abstract

Mazatlania fulgurata inhabit the lower intertidal of Playa Avellana, a dissipative beach in Costa Rica. They were first described in 1971 when live specimens were observed with eggs attached to their shell, however their reproductive biology and ecology has never been studied. Northwest Costa Rica is characterized by a rainy and dry season, which correlates with M. fulgurata egg case deposition. Snails are found with egg cases attached to their shells predominantly during the dry season (November to April) and this percentage significantly decreases with the rainy season (May-October). Therefore the breeding season is clearly defined by seasonal fluctuations whereby there is an equal breeding and non-breading period during their reproductive cycle. It is suggested that osmotic pressure plays a role in the egg case production. Futhermore, egg cases were not found on all M. fulgurata specimens so in order to determine if size played any role in egg case deposition snails were collected and measured. A lower percentage of snails carry egg cases and those that do carry egg cases, are on average, larger in size compared to animals without egg cases attached to their shell. This questions whether M. fulgurata are hermaphrodites or if separate sexes exist along with whether egg cases are deposited on one's own shell or on the shells of other individuals within the population. The number of egg cases found on the shell of individuals varied from as few as one to as many as forty-one, however on average the number of eggs/larvae per egg case was twenty-six. Regardless of the number of egg cases that a snail has attached to its shell, the eggs are exclusively found on the apertural side, within grooves and always laid starting at the aperture, extending towards the apex. These observations concerning the pattern of egg case deposition directly questions the statement made by Penchaszadeh et al. (1986) regarding the reproductive strategy of M. aciculata, which states that eggs are laid on the shell of other females in the population of more than 9 mm in length. Mazatlania fulgurata deposit egg cases within grooves, presumably providing mechanical protection of the encapsulated embryos. Alternatively, Vermeij and Hora have proposed hypotheses that suggest depositing egg cases on the shell surface creates `roughness' which may confer an adaptive advantage to the fierce wave action characteristic of the intertidal; specifically by creating turbulence, reducing drag and creating a calm water effect in the animal's vicinity. Experimentation to test these hypotheses revealed that the presence of egg cases has exactly the opposite effect, egg cases increase drag and consequently, snails with attached egg cases fall through the water column at a slower velocity that is significantly different from snails without attached egg case. In addition to the experiment, the Re value was calculated which revealed that shell roughness would have no effect on drag reduction based on the `drag crisis' mechanism. Therefore the presence of egg cases is associated with an increased hydrodynamic resistance and the significant effect that egg cases have on falling velocity remains to be explained. The systematic positioning of egg cases in grooves on the shell surface is presumed to play a role in parental protection. The encapsulated embryos are protected from the stresses associated with the intertidal and upon development within the structures they are subsequently released into the interstitial where they live between sand grains until they metamorphosis into juveniles. Among all beaches, the predator-prey relationship plays a role in the continued existence of a species. Specifically among Playa Avellana Agaronia propatula is a key predator however, in the rocky fringe area where Mazatlania fulgurata aggregate A. propatula are rarely found. To determine if predation pressure could explain this phenomenon, the scent of M. fulgurata and a live specimen were presented to actively hunting Agaronia. Agaronia propatula do in fact accept M. fulgurataas prey and are even able to recognize them through olfactory stimulus; therefore the distribution of the two species on Playa Avellana may be a direct consequence of predation pressure.

Degree

M.S.

Advisors

Peters, Purdue University.

Subject Area

Biology|Ecology

COinS