120
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
1938, 1941a,b) and Rothschild and Rothschild (1939). Rothschild
(1936) has reported that Hydrobia (= Peringia) ulvae parasitized by
Cercaria oocysta and C. ubiquita are larger. She has expressed the
opinion that the abnormal increase in size is not due to the ingestion of
greater quantities of food, as suggested by Wesenberg-Lund, but " it
seems more likely that this is brought about by the destruction of the
gonads and other glands " thus suggesting that gigantism is a manifestation of castration. In 1938 Rothschild extended her observations
on parasitized Hydrobia ulvae and was convinced that gigantism does
occur. Moreover, she has concluded after studying the correlation
between host size and number of cercariae produced, that :
The gigantism of the host, which involves an increase in the soft parts o.
the body as well as the shell, is thus of great advantage to the parasite
The faculty of producing this increase in size is a character which is presumably most susceptible to selection and its widespread occurrence among
the Trematoda is therefore not surprising.
Rothschild and Rothschild (1939) reported that the sizes of H . ulvae
raised in the laboratory varies with the habitat, either in glass bowls
or glass tubes, with those maintained in bowls growing considerably
faster. They have also pointed out that uninfected females grow faster
than uninfected males. Relative to the effects of parasitism, they found
that parasitized snails grow faster than uninfected ones.
M. Rothschild (1941a) has further confirmed her earlier finding that
parasitized H . ulvae demonstrate gigantism. These grow to lengths of
9-10 mm while uninfected snails of comparable age measure 6.75 mm
long. M. Rothschild (1941b) has considered three additional factors
which might have accounted for the correlation between the higher
incidence of parasitism in larger Littorina neritoides in addition to
gigantism due to parasitism. She postulated that (1) young snails are
" unattractive " to miracidia, (2) infections may be lethal to young
snails and hence these are killed off, and (3) the growth rate of snails
may be so greatly slowed down after attaining a certain size that the
time factor alone could have accounted for the greatly increased
percentage of infection in the larger size groups. She considered all
three of these to be feasible auxiliary factors but, nevertheless, she
considered parasite-induced gigantism to be real. By plotting the
infection rate in L. neritoides which serve as the first intermediate host
(cercarial production involved) and in specimens which serve as the
second intermediate host (metacercarial encystment), Rothschild has
demonstrated two different curves. The first reveals a low rate of
infection in the small size groups, but with a steep upward slope rising
to 91 % in the large size groups. The second curve reveals an increasing
MARINE MOLLUSCS AS HOSTS FOR SYMBIOSES
1938, 1941a,b) and Rothschild and Rothschild (1939). Rothschild
(1936) has reported that Hydrobia (= Peringia) ulvae parasitized by
Cercaria oocysta and C. ubiquita are larger. She has expressed the
opinion that the abnormal increase in size is not due to the ingestion of
greater quantities of food, as suggested by Wesenberg-Lund, but " it
seems more likely that this is brought about by the destruction of the
gonads and other glands " thus suggesting that gigantism is a manifestation of castration. In 1938 Rothschild extended her observations
on parasitized Hydrobia ulvae and was convinced that gigantism does
occur. Moreover, she has concluded after studying the correlation
between host size and number of cercariae produced, that :
The gigantism of the host, which involves an increase in the soft parts o.
the body as well as the shell, is thus of great advantage to the parasite
The faculty of producing this increase in size is a character which is presumably most susceptible to selection and its widespread occurrence among
the Trematoda is therefore not surprising.
Rothschild and Rothschild (1939) reported that the sizes of H . ulvae
raised in the laboratory varies with the habitat, either in glass bowls
or glass tubes, with those maintained in bowls growing considerably
faster. They have also pointed out that uninfected females grow faster
than uninfected males. Relative to the effects of parasitism, they found
that parasitized snails grow faster than uninfected ones.
M. Rothschild (1941a) has further confirmed her earlier finding that
parasitized H . ulvae demonstrate gigantism. These grow to lengths of
9-10 mm while uninfected snails of comparable age measure 6.75 mm
long. M. Rothschild (1941b) has considered three additional factors
which might have accounted for the correlation between the higher
incidence of parasitism in larger Littorina neritoides in addition to
gigantism due to parasitism. She postulated that (1) young snails are
" unattractive " to miracidia, (2) infections may be lethal to young
snails and hence these are killed off, and (3) the growth rate of snails
may be so greatly slowed down after attaining a certain size that the
time factor alone could have accounted for the greatly increased
percentage of infection in the larger size groups. She considered all
three of these to be feasible auxiliary factors but, nevertheless, she
considered parasite-induced gigantism to be real. By plotting the
infection rate in L. neritoides which serve as the first intermediate host
(cercarial production involved) and in specimens which serve as the
second intermediate host (metacercarial encystment), Rothschild has
demonstrated two different curves. The first reveals a low rate of
infection in the small size groups, but with a steep upward slope rising
to 91 % in the large size groups. The second curve reveals an increasing