SINDERMANN: INTERNAL DEFENSES OF CRUSTACEA 



been adequately characterized and the mecha- 

 nisms involved have not been adequately eluci- 

 dated, it may be speculated that in the verte- 

 brates a greater degree of specificity of humoral 

 factors has been added to the nonspecific mech- 

 anisms found in the invertebrates. 



The proper role of specific vertebrate anti- 

 body as a possible augmentation of evolutionarily 

 older nonspecific internal defenses was alluded 

 to by Miles (1962). He stated ". . . it is fairly 

 clear that antibody per se has little effect on 

 the viability or metabolism of microbes with 

 which it combines. It is effective in defense 

 either because it neutralizes toxins, or because 

 it makes the microbe susceptible to non-specific 

 defense factors like complement or the phago- 

 cyte .... We may then properly consider anti- 

 body as accessory to the more fundamental non- 

 specific defense mechanisms . . . ." It should be 

 emphasized, however, that much remains to be 

 learned about the nonspecific humoral factors 

 of vertebrates, as well as invertebrates. 



9. Brown (possibly chitinoid) bodies or cysts 

 in gills characterize later stages of a number of 

 crustacean diseases. The sequence of events, 

 after invasion by microorganisms, may include 

 action of toxic or inhibitory factors in hemo- 

 lymph, accretion of moribund or dead invaders 

 in gill lacunae, phagocytosis of dead organisms, 

 and formation of nodules or cysts containing 

 dead organisms, and gradual phagocytic de- 

 struction of necrotic material. 



10. An important point, as Bang (1967b) 

 mentioned, is that the probability of discovering 

 internal defense mechanisms is greater when di- 

 sease phenomena are studied under natural con- 

 ditions. In experimental work, microorganisms 

 pathogenic to marine invertebrates should con- 

 stitute test organisms of choice; microorganisms 

 found in the environment (and which may be 

 facultatively pathogenic) should be next in order 

 of preference; and microorganisms or proteins 

 which the mai'ine animal is unlikely to encounter 

 seem least instructive. There are valid experi- 

 mental reasons, of course — such as the ease of 

 recognition of bacteriophages — that often lead 

 to selection of test microorganisms other than 

 pathogens or potential pathogens. Whether 



these unusual choices are effective antigens is 

 obviously a most important consideration. 



Another extremely pertinent observation 

 made by Bang (1967b) was: "The limited 

 amount of information [concerning immunolog- 

 ical responses of invertebrates] is, I believe, due 

 mainly to the limited number of studies, and not 

 to any lack of imagination on the part of evo- 

 lutionary forces in developing protective mech- 

 anisms." 



11. One final and very significant thought was 

 proposed by Stauber (1961): "That so few 

 examples of acquired resistance are known 

 among invertebrates may even be quite logical. 

 Because of their relatively short generation 

 times, their usual small size and often enormous 

 reproductive capacities, subsequent epizootics 

 would be much more likely to be circumvented 

 by the appearance of resistant stocks through 

 natural selection .... Even with very high mor- 

 tality rates a residual stock of animals under 

 favorable conditions later might repopulate an 

 area .... If this reasoning is adequate to explain 

 the lack of evidence for the occurrence of ac- 

 quired resistance in most of the invertebrates, 

 perhaps those invertebrates with a long life span, 

 like Liniulus should be investigated more fully, 

 as likely hosts capable of demonstrating ac- 

 quired resistance." 



It is interesting to note that it is precisely 

 those invertebrates with a long life span which 

 have received increased attention during the past 

 several years, and that a few indications of 

 acquired resistance have been reported. 



Information about the internal defenses of 

 crustaceans and other invertebrates may be 

 summarized as follows: 



The weight of evidence indicates a major de- 

 fensive role in invertebrates for phagocytosis, 

 augmented by relatively nonspecific innate or 

 acquired humoral factors. Preformed substances 

 released into the hemolymph from granular 

 hemocytes seem to play a major role in humoral 

 defenses of Crustacea, and probably other in- 

 vertebrates as well. Thus the body fluids of 

 many invertebrates contain natural bacterici- 

 dins, agglutinins, lysins, and occasionally pre- 

 cipitins. Some limited evidence for augmenta- 



479 



