SINDERVIANN: INTERNAL DEFENSES OF CRUSTACEA 



toxic. A final section will attempt a detailed 

 review of the demonstrated systems of internal 

 tlefenses of lobsters and other crustaceans 

 against the microbial pathog-en Gaffkya homaii 

 — which is one of the best examples of a test 

 system for invertebrates for which existing in- 

 formation is adequate. 



CONCEPTS AND TERMINOLOGY 



Before proceeding with an examination of in- 

 ternal defenses of crustaceans, a brief review 

 of some of the terminology may be relevant. 

 "Resistance" and "susceptibility" have often been 

 used interchangeably and reciprocally, but as 

 Schneider (1951) and Stauber (1961) pointed 

 out, insusceptibility and resistance should prolv 

 ably be considered as sejiarate biological phe- 

 nomena. Insusceptibility refers to those exist- 

 ing external or internal features of an animal 

 — morphological and physiological — which deny 

 access to a potential pathogen, or prevent its 

 successful establishment and survival. Resist- 

 ance, on the other hand, has been defined by 

 Read (1958) as "those changes in the physio- 

 logical state of the host which represent a re- 

 sponse to present or jirevious contact with the 

 parasite or with a similar chemical entity." 



Resistance (and its equivalent — "immunity") , 

 defined as host response, can then be considered 

 as "innate" (natural) or "acquired" (induced). 

 Innate resistance includes responses to primary 

 contact with a pathogen, and acquired resistance 

 includes responses that develop after primary 

 contact. The distinction between the two types 

 of response is not, however, always as definitive 

 as might be preferred. Acquired resistance is 

 often characterized by enhanced responsiveness 

 to subsequent contact with a pathogen. 



Resistance, whether innate or acquired, may 

 be cellular or humoral. The cellular defense 

 mechanisms are based largely on activities of 

 leucocytes (hemocytes of invertebrates) and in- 

 clude: phagocytosis — engulfment and often di- 

 gestion of foreign particles, lencocytosis (hemo- 

 cytosis), and leucocytic (hemocytic) infiltiv- 

 tion — the mobilization of hemocytes in the lilood 

 stream and their migi-ation to invaded or injured 

 tissues; coagnkttion — the formation of cellular 



or extracellular clots to close gaps in the circu- 

 latory system and to immobilize microorganisms; 

 and encapsulation — the surrounding of large 

 masses of invading material liy phagocytes and 

 fibrocytes. Humoral defense mechanisms, which 

 probably depend on cellular secretions or cell 

 disrur>tion, and act synergistically with cellular 

 defenses, include agglutinating, lytic, precipi- 

 tating, and bactericidal systems, and other ac- 

 tivities (Figure 1). 



POTENTIAL PATHOGEN IN 

 EXTERNAL ENVIRONMENT 



Figure 1. — Mechanisms of internal defense. 



Antibodies (in the vertebrate sense of specific 

 immunoglobulins) have not laeen demonstrated 

 in invertebrates, although less specific antibody- 

 like activity is common. Since antibodies have 

 not been demonstrated, it is probably technically 

 incorrect to use the term "antigen" with in- 

 vertebrates. The semantics involved will be con- 

 sidered in the discussion section, but for con- 

 venience the term will be used in this paper. 

 Furthermore, it must be made clear that when 

 lysins, precipitins, agglutinins, etc. of inverte- 

 brates are discussed, no af^empt is made to 



457 



