CHAPTER V 



LIFE HISTORY (ZOOPARASITICA) 



Parasites of Invertebrates 



J. R. CHRISTIE, U. S. Horticultural Station, Beltsville, Md. 



Introduction 



There are many different types of association between nema- 

 todes and other invertebrates and it is difficult to draw a line 

 between what should and what should not be regarded as para- 

 sitism. Most of the nematodes that live within the bodies of 

 invertebrates are customarily referred to as parasites though 

 there is little evidence that some of them interfere materially 

 vpith the well-being of their "hosts." We know very little, 

 however, about the effects of these nematodes on the animals 

 that harbor them unless the manifestations are pronounced and 

 obvious. The only feasible procedure is to regard as eligible for 

 inclusion in this chapter all nematodes that regularly spend 

 part of the life cycle within the bodies of invertebrates regard- 

 less of the precise character of the association. Species for 

 which vertebrates serve as definitive hosts and invertebrates 

 only as intermediate hosts are dealt with in the following 

 chapter. 



In general the parasites of invertebrates and those of verte- 

 brates are not found in the same phylogenetie groups and in 

 those cases where both belong to the same group the vertebrates 

 involved are almost always amphibians and reptiles. However, 

 the Thelastomatidae and the Oxyuridae have very close affini- 

 ties. 



Arthropods, annelids and mollusks are the invertebrates most 

 commonly parasitized by nematodes though scattered cases have 

 been reported where other invertebrates, even nematodes them- 

 selves, serve as hosts. There are surprisingly few records of 

 marine invertebrates harboring nematodes and most of these 

 apparently deal with cases where the association is erratic or 

 accidental or where some vertebrate serves as definitive host. 



Included among the nematodes harbored by invertebrates are 

 species where a parasitic mode of life is only now being ac- 

 quired and others where it is of great antiquity. There is great 

 diversity in the types of life cycles and to simplify discussion 

 and facilitate comparison the nematodes are divided into three 

 groups. 



The first of these groups is made up of nematodes that are 

 more or less closely related to free-living species and in the life 

 cycles we often find a combination of saprophagous and "para- 

 sitic" habits. In one line of evolutionary development the 

 nematodes live and reproduce in the carcass of the "host," to 

 the death of which they may or may not have contributed. Life 

 cycles are simple, perhaps the most outstanding feature being 

 the frequent occurrence of dauer larvae,* a characteristic that 

 has been carried over from a free-living to a parasitic mode of 

 life. Another line of evolutionary development seems to have 

 culminated in a life cycle where the nematode may pass through 

 one or more free-living generations, then gain entrance to the 

 host and pass through one or more parasitic generations. 



The second group comprises tliose nematodes, not included in 

 the first group, that inhabit the alimentary tract. Life cycles, 

 so far as known, are simple. With perhaps an occasional ex- 

 ception (i. g., Ccphalobium microbivoriim) , only the egg stage 

 occurs outside the host, a characteristic shared by verj' few 

 species in the otlier two groups. 



The third group includes the body-cavity and tissue para- 

 sites. In contrast to the fir.st group, these nematodes are highly 

 specialized, obligate parasites and, in contrast to the second 

 group, they pass, at the most, only a transitory period in the 

 alimentary tract of the host. Five families are included in 

 this group. The Drilonematidae and Mycnchidae have received 

 little attention and our knowledge regarding life cycles is very 

 meager. The Tetradoneraatidac, Mermithidae and Allantone- 

 matidae have been somewhat more adequately studied. The 

 nematodes belonging to these three families have been parasites 

 for a very long time and many of them have complicated life 

 cycles that are highly adapted to individual requirements. Of 



*'nie term dauer larva is used in tliis te-\t to designate a larva, in a 

 particular stage of development, that is especially adapted to withstand 

 adverse conditions and, when a dauer stage is not obligatory, that differs 

 from a larva of the same stage that develops when conditions are favor- 

 able and food is abundant. The term is not new, having been used by 

 Fuchs and others with approximately this same meaning and, while not 

 of classic origin, it is short, expressive, appropriate and useful. Dauer 

 larvae are of common occurrence in the Rhabditidae and Diplogasteridae 

 and are more characteristic of free-living than of parasitic species, hence 

 the term is not synonymous with "infective larva." 



the various factors that have influenced these life cycles, two 

 stand out as being of great importance. 



One of these factors is the necessity for the infective stage 

 to reach and gain entrance to the host. This, of course, is a 

 requisite in the life cycle of every parasite but for the allan- 

 tonematids and merraitliids there are certain restricting condi- 

 tions with which many of the others do not have to contend, at 

 least not to an equal extent. Some of the hosts are insects that 

 develop in seasonal cycles and where the total life span of the 

 individual may be only a few months. It is frequently neces- 

 sary that the parasite enter when the host is in a particular 

 stage and this stage may be available only at restricted times 

 of the year. As a result the life cycles of many of these para- 

 sites have become closely correlated with the life cycles of their 

 respective hosts. 



The other factor is the ability of the nematode to take food 

 only during restricted periods. The fact that for many of these 

 parasites the free-living stage may be of considerable duration 

 and that during this period the nematodes take no food, but, 

 nevertheless, pass through important phases of the life cycle, 

 has had a profound effect on development. In many cases the 

 larval mermithid, during a comparatively short period of para- 

 sitic life, must make a phenomenal growth and store sufficient 

 nutrient materials to carry the adult through its relatively 

 long, free-living period of sexual activity and reproduction. The 

 larval allantonematid that develops to maturity outside the host 

 after only a very brief period of parasitic life, must exercise 

 the strictest economy in the utilization of its limited supply of 

 stored nutrients. Since, as a rule, only the female again be- 

 comes parasitic, the male must produce and mature its sperma- 

 tozoa though the production and maturation of the eggs by the 

 female is postponed. There can be little or no increase in 

 l)ody size during this free-living period, hence the adult, im- 

 pregnated female, after entering a new host, undergoes a pe- 

 riod of rapid growth. In the Sphaerulariinae a prolap.sus of 

 the uterus has resulted through the inability of the small, un- 

 derdeveloped bodj' of the young female to keep pace with the 

 rapidly growing reproductive organs. 



Novitious Parasites and Semiparasites 



Among these nematodes two lines of evolutionary develop- 

 ment seem to stand out more or less distinctly though it is ob- 

 viously improbable tliat they account for the origin of all the 

 different types of parasitism or semiparasitism encountered in 

 tills heterogeneous group. 



One line of evolutionary development appears to have been 

 initiated when certain saprophagous nematodes utilized other 

 invertebrates, frequently saprophagous insects, as vehicles for 

 transportation. These "hitchhikers," first seeking protection 

 from desiccation in crevices on the external surface, eventually 

 entered the bodies of their "hosts." In the life histories of 

 species representing an intermediate step in this line of develop- 

 ment, larval nematodes, after gaining entrance to the body of 

 the "host" and becoming established therein, do not at once 

 grow to maturit3' and reproduce but remain in a more or less 

 quiescent condition. These larvae do not appear to interfere 

 materially with the life processes of the animal that harbors 

 them but when the animal dies from otlier causes the nematodes 

 immediately resume development and reproduce in the carcass. 



In some cases, however, this type of relationship has evolved 

 to a point where it is no longer passive but where the nema- 

 todes are an important factor in bringing about the death of 

 the animal whose body they enter. Even though present in 

 small numbers, some species of NeoapJectana are said to kill 

 their insect hosts in a very short time. 



The parasitic or semiparasitic relationship between these 

 nematodes and tlicir respective "hosts" is not always obliga- 

 tory. Johnson (101,3) concluded that entrance into the body 

 of an earthworm is not necessary in the life cycle of Rhahditis 

 maupasi but if larvae, during their sojourn in the soil, find suit- 

 able decaying organic matter they will develop and reproduce 

 therein. Neither is Pristionchus acrivora dependent on en- 

 trance into a termite or some other insect to complete its de- 

 velopment as it has been found reproducing in a number of dif- 

 ferent habitats including decaying plant tissues. Neoaplectana 

 fliaseri, on the other hand, appears to be an obligate parasite 

 that, in nature, develops only after entering the living body of 

 its insect host. 



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