Egg Shells Prevent Microbial Fouling

green and brown sea shell next to green algae

Biological Strategy

White rock shell

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Living systems must share resources with other living systems. Often, this can be done in cooperative ways but at other times, there is competition for resources such as water, nutrients, and sunshine. By consuming resources, plants can threaten other living systems. In response, threatened living systems must discourage this competition in ways that don’t harm the threatened system. For example, trees in tropical rainforests have strategies to prevent climbing vines from attaching to their trunks, eliminating competition from the vines for sunlight above and nutrients at the roots.

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In living systems, microbes play important roles, such as breaking down organic matter and maintaining personal and system health. But they also pose threats. Bacteria can be pathogens that cause diseases. Some bacteria create colonies called biofilms that can coat surfaces, reducing their effectiveness–for example, inhibiting a leaf’s ability to photosynthesize. Living systems must have strategies for protecting from microbes that cause disease or become so numerous that they create an imbalance in the system. At the same time, living systems must continue living in harmony with other microbes. Some living systems kill microbes. Others repel without killing to reduce the chances that microbes will adapt to the lethal strategy and become resistant to it. For example, some pea seedlings exude a chemical that inhibits biofilm buildup.

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Gastropods

Class Gastropoda (“stomach-foot”): Snails and slugs

There are at least 40,000 known species of gastropod, with many more yet to be discovered, making it the largest class of mollusks. These animals exhibit torsion, meaning their internal organs are twisted 180° during development. The majority of gastropods have shells, and most open up on the right side. The largest gastropods are the size of a small child (3 feet, or 1 meter) while the smallest are barely visible to the naked eye.

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The eggs of the white rock shell snail ward off microbial attack with a series of physical, mechanical, and potentially chemical defenses

Multicellular marine organisms face a constant onslaught of microbes and other small organisms seeking structures upon which to adhere. Whether they are fungi, algae, biofilm-forming pathogens, or other lifeforms, they lead to biofouling on the surface of the larger organism that can cause serious complications. The white rock shell (Dicathais orbita), a type of sea snail, produces eggs with remarkable anti-fouling adaptations. In early stages of development, the exterior of the egg capsule is covered in uniform ridges separated by 1 – 5 microns. Unlike irregular nano-textures observed on the surfaces of eggs from other marine organisms, ridges that are regularly-spaced sufficiently close together are believed to minimize potential contact points for fouling organisms, making it harder for them to attach and settle. Over time, however, bacteria will attach and take root on the surface of the egg capsules. To combat this inevitable biofouling, later stage eggs shed their exterior crust completely to reveal a fresh layer underneath. After this shedding of the outer layer, lipophilic (lipid-loving) droplets are extruded from pores on the egg surface and seem to exert some kind of antiseptic effect. This series of anti-fouling steps keeps biofilms and parasitic microbes from harming the egg until it is developed enough to hatch.

Last Updated September 25, 2017

References

“[E]arly stage egg capsules of Dicathais orbita (Neogastropoda) are relatively free of surface microorganisms. Egg capsules during the trocophore stage had a regularly ridged microtexture, but as capsules matured, shedding of the outer wall was observed, followed by the extrusion of unidentified droplets, which then accumulated on the capsule surface in association with bacteria…colonization was significantly less on D. orbita egg capsules…D. orbita appears to use a combination of physical, mechanical and possibly chemical defense mechanisms to reduce fouling on their egg capsules.” (Lim et al. 2007:275)

“[S]essile invertebrates and algae are exposed to a constant onslaught of potentially detrimental microbes. These include biofilm-forming bacteria along with single-cell diatoms that rapidly settle, attach and form colonies on any surface placed in the marine environment. The formation of a microbial biofilm promotes the attachment of algal spores, protozoa, barnacle cyprids and marine fungi, followed by the settlement of other marine invertebrate larvae and macro- algae…Heavy surface fouling could lead to the accumulation of toxic wastes, a reduction in oxygen and nutrient availability and increased drag, which can cause sessile organisms to become dislodged from benthic substrata in strong currents…egg capsules appear to be highly resilient multilaminate biomaterials…egg capsules can remain in the marine environment for several months, and thus would also be vulnerable to surface fouling. Nevertheless, previous studies indicate that these egg capsules remain axenic and remarkably free of surface macrofouling…surface composition and microtexture can influence the rate of biofouling…homogeneous surfaces are capable of deterring attachment by limiting space available for fouling organisms…egg capsules of neogastropods, including D. orbita, were significantly less fouled than a range of gelatinous egg masses.” (Lim et al. 2007:276)

“Dicathais orbita possessed a thin layer of crust over a microtopographical structure on the outer-most surface. The surface was almost free from any bacteria and algae. As the egg capsules matured (1 to 3 wk of age), the crust began to breakdown, exposing the microtopographical features. This layer has regular homogeneous ridges separated by 1 to 5 μm…during the third week, densities of these bacteria increased and other fouling organisms such as filamentous algae were observed, forming a mixed biofilm community. In the later stages of development (>4 wk old veligers), the fouled outer wall structure began to dissociate and shed from the capsule, leaving behind a naked capsule without any texture…Unidentified droplets ranging from 8 to 20 μm appeared on the surface of mature egg capsules after the majority of the outer wall dissociated. These droplets were either solitary or clumped in association with attached bacteria.” (Lim et al. 2007:279)

“[D]roplets appeared to be secreted through pores in the wall as the outer capsule degrades. These droplets were clearly not cellular…They also did not appear to be membrane-bound…This suggests that they were hydrophobic…they may contain lipophilic compounds such as the indole dimer tyriverdin, which is a potent bacteriostatic agent previously reported from extracts of D. orbita egg capsules. The droplets were observed to aggregate with bacteria on the capsule surface and…were frequently associated with dead bacteria, suggesting that they may have antimicrobial properties…a combination of all these defense mechanisms to defend its egg capsules, including a surface texture not suitable for bacterial attachment, followed by shedding of the outer layer to remove existing microbial colonization and then exudation of unidentified chemical droplets that aggregate and possibly interfere with bacterial growth on the capsules’ surface.” (Lim et al. 2007:285)

Journal article

Comparison of surface microfouling and bacterial attachment on the egg capsules of two molluscan species representing Cephalopoda and Neogastropoda

Aquat. Microb. Ecol. | 09/08/2007 | NSH Lim, KJ Everuss, AE Goodman, K Benkendorff

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