ANIMAL AGGREGATIONS 



409 



each variety of P. aurelia, for example. 

 Individuals of a given mating type would 

 not conjugate with members of their own 

 type or with members of a different sex 

 variety. Given proper conditions, conjuga- 

 tion took place readily between representa- 

 tives of different mating types of the same 

 variety. Knowledge in this field is grow- 

 ing rapidly, and its complexity is becom- 

 ing more apparent. Intervarietal conjuga- 

 tions are now known in P. aurelia and P. 

 bursaria, but these do not take place so 

 readily as do those between different sex 

 types within the same variety. Even under 

 these conditions, potent blocks to the free 

 exchange of genes include "hybrid" non- 

 viability and low viabiUty (Sonneborn and 

 Dippell, 1946). Evidence is growing that 

 some varieties can so condition the medium 

 in which they Hve that it will induce 

 clumping and conjugation both between 

 animals of different varieties and even be- 

 tween members of the same clone (Chen, 

 1945). This is a phase of ecology in which 

 continuing rapid progress is to be ex- 

 pected; the present account is already par- 

 tially out of date. 



A variety of animals show consecutive 

 sexuaUty, one and the same individual be- 

 ing first a male and then a female. With 

 some, maleness is the more juvenile, and 

 femaleness the more adult, condition. 

 There may be the increased complexity 

 that the duration of the male phase de- 

 pends on whether the animal has female 

 associates. This situation is developed al- 

 most diagrammatically with certain marine 

 snails of the genus Crepidula (Coe, 1936), 

 except that in all species of Crepidula so 

 far studied critically, a few males do not 

 change into females, in contrast with the 

 hermaphroditic males constituting the bulk 

 of the male population. 



Bonellia, an aberrant, Mediterranean an- 

 nelid worm, illustrates another variation of 

 the same general principle. The large fe- 

 male harbors small parasitic males within 

 her uterus. The fertilized eggs are shed 

 into the surrounding sea water and develop 

 into free-swimming larvae possessing both 

 male and female potentiaUties. If these 

 free-swimming young settle on the probos- 

 cis of a female Bonellia, they develop into 

 the minute, parasitic males living in poly- 

 andric relation with this female host. Those 

 that do not settle on a female, normally 



become functional females themselves, able 

 to produce maleness in any Bonellia larvae 

 that come their way. The sexually indiffer- 

 ent larvae receive some substance from the 

 female proboscis that retards their growth 

 and induces maleness. The external, social 

 relations— or lack of them— determine 

 which of the alternative paths of sexual 

 development will be followed (Baltzer, 

 1925). 



In other well-studied cases, population 

 density determines the sex ratio of in- 

 dividuals with labile sexual potentialities, 

 MonstrilUd copepods (Malaquin, 1901) 

 and certain nematode parasites of grass- 

 hoppers (Christie, 1929) and of chirono- 

 mid larvae (CauUery and Comas, 1928), 

 among others, illustrate this effect. In this 

 variation of sex determination by popula- 

 tion pressure, isolated parasites or those in 

 small populations per host tend toward 

 equality in sex ratios or have a preponder- 

 ance of females. In denser populations the 

 sex ratio is unbalanced toward maleness, 

 and in some instances (CauUery and 

 Comas, 1928) intermediate densities yield 

 intersexes. 



We can give quantitative data. Christie 

 (1929) fed known numbers of eggs of a 

 nematode Mermis to common New Eng- 

 land grasshoppers. Four grasshoppers fed 

 twenty to thirty eggs each— 100 in all- 

 produced eighty-six mermid parasites, all 

 males. In contrast, twenty-one grasshoppers 

 —all but three of the same species that had 

 been tested with heavy infestations— fed 

 four or five eggs each, 102 in all, yielded 

 seventy-three parasites, of which 92 per 

 cent were females. We are deaUng here 

 with labile sexual potentiahty rather than 

 with differential mortality. 



Another variant of the modification of 

 sex by crowding— the last we shall mention 

 —comes from the close study of sexuaUty 

 in cladoceran crustaceans. Moina, Uke 

 many other Cladocera, lives in ponds and 

 pools of fresh water. The populations 

 usually consist of parthenogenetic females. 

 Outbreaks of bisexuality occur from time 

 to time, often at the onset of a drought or 

 in autumn. The resulting fertiUzed eggs 

 are more resistant than are the partheno- 

 genetic ones and carry the stock over 

 periods of environmental adversity. With 

 the coming of better conditions, these so- 

 called winter eggs hatch into partheno- 



