Embryos often encounter pathogenic microorganisms before they have developed a functioning immune system. One way for embryos to gain immediate protection from these pathogens is by forming commensal relationships with other organisms. The embryos of the shrimp Palaeon macrodactylus are extremely resistant to infection by the marine fungus Lagnidium callinectes, a known pathogen of many crustaceans. The surface of these shrimp embryos are consistently infected with the bacterium Alteromonas, which produces the antifungal compound 2,3-indolinediol.
In laboratory studies (Gil-Turnes et al. 1989), Palaeon embryos can be bred with or without the bacterial symbiont and exposed to the fungal pathogen. When exposed to the fungus, the bacteria-free embryos die rapidly, whereas the embryos reinoculated with Alteromonas (or treated with 2,3-indoleinedione) survive well. Therefore, it appears that the commensal bacteria protect the shrimp embryos from fungal infection.
Several other systems in which bacteria have been found associated with embryos are being studied (see, for instance, Biggs and Epel , on bacteria that were found in the egg sheath of the squid Loligo opalescens). One interesting speculation (Shostak 1993) is that the cnidocysts that protect cnidarians from predation originally came from protistan symbionts of cnidarians and were later incorporated into the cnidarian itself. In this scheme, the interstitial cells (I-cells) were originally an independent organism.
Biggs, J. and Epel, D. 1991. Egg capsule of Loligo opalsescens. Structure and association with bacteria. J. Exper. Zool. 259: 263-267.
Gil-Turnes, M. S., Hay, M. E., and Fenical, W. 1989. Symbiotic marine bacteria chemically defend crustacean embryos. Science. 246: 116-118.
Shostak, S. 1993. A symbiogenetic theory for the origins of cnidocysts in Cnidaria. BioSystems 29: 49-58.