Report on a Centenary Research Grant for 2003

Fouling and Gastropod Egg Masses

Rachel Przeslawski, Biological Sciences, University of Wollongong, NSW 2522, Australia.

rachelp@uow.edu.au

The intertidal is a very harsh environment, and the organisms that live there must adapt daily to abruptly changing and often extreme conditions. Nevertheless, many gastropods enclose their offspring within benthic egg masses deposited in the intertidal where they are exposed to desiccation, high temperatures, changing salinity, and sunlight. Embryos within these egg masses may also be vulnerable to surface fouling (Biermann et al. 1992), particularly when combined with other environmental stresses (Przeslawski 2004).

Every surface in the ocean is potentially a substratum for colonisation by the settlement of water borne adults, spores or larvae (Davis et al. 1989). Molluscan egg masses are fouled by a range of surface fouling organisms, including macroscopic filamentous and encrusting algae, some marine invertebrates, diatoms and protists. Potential relationships between surface fouling and species, in conjunction with their associated egg mass structure or spawning habitat, have yet to be explored.

In this study, egg masses from 18 gastropod species  collected from southeastern Australia were held in outdoor recirculating seawater aquaria. Both gelatinous and capsular egg masses were collected, and egg masses came from a variety of habitats: rock platforms in the full sun, the undersides of boulders in full shade, and partially shaded habitats (fig. 1). Egg masses were cultured under three spectral filters with various transmission properties: (1) full spectrum, (2) UV-blocking, and (3) dark. When egg masses showed signs of hatching, they were removed for examination. Embryonic mortality was recorded for all egg masses, and encapsulation period for gelatinous egg masses. The presence of protists was noted, and the proportion of egg mass surface colonised by algae was quantified.

Unsurprisingly, algal fouling on egg masses was higher in the light treatments than in the dark. However, algal growth was inhibited by UV, with greater algal fouling on egg masses under UV-blocked treatments than under full spectrum treatments. This suggests that egg masses spawned in habitats partially shielded from sunlight may be more susceptible to algal fouling than those spawned in full sun with associated higher UV intensity.

In order to eliminate UV as a confounding factor, the remainder of the analyses focused only on egg masses cultured under UV-blocked treatments. The rate of algal fouling varied between species: gelatinous egg masses had significantly higher rates of algal fouling as well as a higher proportion of egg masses colonised by protists. Thus, gelatinous masses appear to provide a more favourable substrate for biofilm formation, which promotes early algal settlement and subsequent protist colonisation. Leathery egg capsules, on the other hand, are proteinaceous with little or no glycosylation; this may explain why they are a less suitable substrate for the settlement of fouling organisms. In contrast, fouling was not affected by spawning habitat.

The effects of fouling on embryonic mortality are species specific: algal fouling significantly increased embryonic mortality in the naticid Conuber (Polinices) sp, the nudibranchs Austraeolis ornata and Hoplodoris nodulosa, and the littorinimorph Bembicium nanum. Algal fouling did not significantly affect any of the other 14 species examined. Furthermore, protists were associated with higher embryonic mortalities and higher fouling levels. This suggests that protists only settle on the surface of egg masses after a reasonable amount of algal colonisation has occurred. Alternatively, the protists may be attracted to dead or dying embryos already affected by high algal fouling. The effect of fouling on encapsulation period is also species specific, with only two species, the aplysiad Aplysia sydneyensis and the nudibranch Dendrodoris fumata, showing a significantly negative correlation between algal fouling and encapsulation period. Protists had no significant effect on encapsulation period for any species.

Overall, this study has shown that surface microfouling can play a significant role in the development of encapsulated molluscan embryos. However, fouling levels and effects on encapsulated molluscan embryos vary greatly between species and egg mass structures. Because of this high variability, generalisations about fouling effects based on one species should be avoided. Instead, we recommend incorporating multiple species or narrowing the hypothesis to a particular species. Furthermore, future studies examining the embryonic effects of environmental stresses should consider any possible interactions with the effects of surface fouling, particularly in relation to the species and egg mass type being examined.

Most of this research will be published as an article in a forthcoming issue of Journal of Molluscan Studies

References

Biermann, C.; Schinner, G. & Strathmann, R. 1992. Influence of solar radiation, microalgal fouling, and current on deposition site and survival of embryos of a dorid nudibranch gastropod. Marine Ecology Progress Series, 86: 205-215.

Davis, A.R., Targett, N.M. McConnell, O.J. and Young, C.M. 1989. Epibiosis of marine algae and benthic invertebrates: Natural products chemistry and other mechanisms inhibiting settlement and overgrowth. In: Bioorganic Marine Chemistry, Vol. 3.( P.J. Scheuer, ed.), 85-114. Springer-Verlag, Heidelberg.

Przeslawski, R. 2004. Effects of environmental stress on embryonic development within intertidal molluscan egg masses. Molluscan Research, 24: 43-63..