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

Abstract

Many organisms naturally defend themselves against microbial attachment and biofouling in the marine environment. In this study, we investigated microbial fouling on 2 molluscan egg capsules using scanning electron microscopy (SEM), two-photon laser scanning microscopy (TPLSM) with bacterial viability staining and bacterial attachment experiments with the biofilm-forming Pseudoalteromonas sp. S91 in flow chambers. Results indicated that early 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. By comparison, the egg capsules of Sepioteuthis australis (Cephalopoda) were found to have an irregular surface with many hills and valleys that accommodate colonization by a variety of microorganisms. At the later stages of development these squid egg capsules become heavily colonized by algal spores. Cross sections of egg capsules revealed that S. australis capsule walls were about 12 times thicker than D. orbita egg capsules. Staining the egg capsules with BacLight™ also revealed a significantly thicker biofilm, with more live and dead bacteria on S. australis capsules than on those of D. orbita (p < 0.05). Flow chamber experiments indicated that the surface of S. australis capsules provided a suitable substrate for colonization by Pseudoalteromonas sp. S91, whereas colonization was significantly less on D. orbita egg capsules after 24 and 72 h (p < 0.01). These experiments indicated that D. orbita egg capsules are better defended against fouling microbes than are the eggs of S. australis. D. orbita appears to use a combination of physical, mechanical and possibly chemical defense mechanisms to reduce fouling on their egg capsules.