Mite life cycle
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Abstract
The life cycle of Gammaridacarus brevisternalis is documented for
first time. The life cycle appears to follow that of a typical Gamasida
mite with two exceptions: it skips the prelarva stage and appears to
undergo an adult molt. The life cycle is divided between beach wrack
and beach amphipods of the genus Orchestoidea. Only the deutonymphs
and adults are found on Orchestoidea. Gravid femalis are found almost
exclusively on beach wrack. It appears that the wrack somehow induces
fertility in the mites.
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Mite life cycle
Introduction
The higher intertidal zone of the beaches along the east coast of
the Northern Pacific is often inhabited by sandy beach amphipods of the
genus Orchestoidea, commonly known as beach hoppers. They burrow into
the sand during the day and at night forage along the beach for food.
Although omnivores, Orchestoidea prefer to graze on beach wrack composed
fo the giant kelp Macrocystis pyrifera.
The fact that ectoparasitic mites infest these amphipods was first
documented by Hall (1912), who described a mite which appears to fit the
description of Gammaridacarus brevisternalis Canaris, 1962, as infesting
Orchestoidea californiana Brandt, 1851. Canaris (1962) described adult
males and females and deutonymphs of G. brevisternalis found on 0.
californiana. McClurkin (1953) and Bowers (1964) mention the presence of
mites on the ventral wurface of 0. californiana and 0. corniculata Stout,
1913, during their studies of the amphipods, but conclude the mites had
little effect on their host. Scurlock (1975) reported that the mites are
found attached exclusively to the ventral body surface and the gills of
the Orchestoidea, and that the mites also could be found on decaying beach
wrack.
The purpose of this investigation was to examine the relationship of
G. brevisternalis to both the Orchestoidea and the beach wrack, and to
document the life cycle of the mite. Until this study, only two of the
six life stages of a typical Gamasida mite (Krantz 1978), the deutonymph
and adult, had been documented for this species. All lab work was done at
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Hopkins Marine Station of Stanford University, Pacific Grove, California.
Collections were made at the Monterey Boat Works and West Beaches of Hopkins
Marine Station and Asilomar Beach, Pacific Grove. Amphipods were
identified using the field key of Bowers (1963).
Location of mite habitat
Three methods were used to determine the presence of mites in various
substrates. The first was simply a visual inspection of the substrates.
The second technique was to release mite-free Orchestoidea in the substnate.
G. brevisternalis is known to actively seek a host Orchestoidea (Scurlock
1975). Thus the Orchestoidea should become infested if mites are present.
The final method of detection involved placing the substrate in a Berlese
funnel and driving the mites down into a collection jar containing either
moist filter paper, for live collection, or 75% ethanol.
Neither of the first two methods yielded mites with a wide variety
of sand as the substrate. Sand was not placed in the Berlese funnel as
it fell into the collection jar. All three methods produced mites when
beach wrack was used as the substrate. Use of the Berlese apparatus
with ethanol in the collection chamber was the best method of locating
mite concentrations. Mites were most commonly found on partially buried
wrack above the mean high tide line consisted; primarily of seagrass and
kelp of the genera Phyllospadix and Macrocystis, respectively. They
preferred moist decaying wrack. Wrack with a high concentration of mites
might contain up to 100 mites per liter. Wrack which was older and very
decayed contained few, if any, mites.
Pill bugs and isopods were also examined for mites. Examination of
10 Amardilidium vulgare Latreille, 1804, collected near a beach where
the infestation rate among 0. corniculata was 85% showed no mites. Also
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15 Alloniscus perconvexus Dana, 1856, were collected from a wrack sample
showing 902 infestation of the 0. corniculata in the wrack. These
Isopods also carried no mites.
Location of eggs
Sixty Orchestoidea, consisting of infested and uninfested amphipods
of both species, were examined visually and found to contain no mite
eggs. Examination of approximately 20 500 ml samples of wrack also showed
no eggs.
In the lab, approximately 80 mites were removed from the Orchestoidea
and divided between two vials containing only a small piece of filter paper
moistened with seawater. It was hoped that the mites would lay eggs on
the filter paper as it was the only substrate provided. No eggs were
observed in a three week period.
The experiment was repeated, but the mites were not removed from
their host in order that they could continue to recieve food. Five 0.
corniculata carrying at least 10 mites each were placed in individual
vials. These vials contained a small piece of glass fiber filter moistened
with seawater which the amphipods did not eat. During the experiment
the amphipods were fed only small pieces of wrack which they immediately
ate. In a two week period no eggs were observed.
In another experiment, eight infested 0. corniculata were placed in
similar vials. Cross sections of Macrocystis stipe were rubbed against
the fiber filter in four of the vials. The amphipods were not fed.
Within two and a half weeks all of the amphipods were dead, apparently
due to starvation. No eggs were laid by the mites and dissection of
24 females showed no sign of egg production.
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Fifty four of 57 G. brevisternalis females from wrack samples had
swollen abdomens. These 54 contained a single, pearl white egg measuring
between 160 by 140 and 220 by 180 microns, nearly a third of the mite's
total body length. Only one egg was found in dissection of 54 females
which were removed from Orchestoidea.
Five hundred ml of wrack with many mites was placed in a jar and
the jar filled to approximately three-fourths capacity with 75% ethanol.
The jar was shaken vigorously about one-fourth of the time for half an
hour. The wrack was removed and the remaining material was swirled and
immediately decanted through a two mm mesh screen into a finger bowl,
leaving most of the sand behind. After the filtrate settled, the upper
liquid layer was siphoned off and the sediment observed under a dissecting
microscope. Approximately 50 eggs were visible. The ratio of eggs tom
females was approximately 2:1, a finding which was approximated in
10 other wrack samples.
Visual examination of another portion of this same wrack sample
revealed only I egg which was found attached to Smithora, an epiphyte
of Phyllospadix. Subsequent visual examination of other wrack samples
shown by the extraction technique to contain mites revealed only two
more eggs, each on Macrocystis.
Rearing eggs
Thirty eggs were extracted from the wrack by the washing method
above
describedéwith seawater substituted for ethanol. The eggs were placed
on damp filter paper lining the bottom of a petri dish. A small
(1.5 x 1.5 cm) piece of Macrocystis blade was added to provide food for
any hatching mites. In order to maintain high humidity, the petri dish
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was floated on a bowl of water and the bowl covered with parafilm.
Nine of the eggs eventually hatched into hexapod larva. No prelarva,
which are found in many species of mites (Krantz 1978), were found. Of
the eggs that did hatch, five did so within threerdays. The larva moved
to the Macrocystis and three days later molted into eight-legged
protonymphs. Although small, the protonymph resembles the adult stage
more closely than the deutonymph. The duration of this stage is not
known, but it lasts at least five days.
Sixteen gravid females collected off wrack were placed in individua
vials containing moist filter paper and a small piece of Macrocystis
blade. Four laid eggs within five days. Dissection of two females
three days after laying showed no sign that a new egg was forming in the
mite.
Distribution of adults and deutonymphs
Initial studies of adult sex ratios gave the percentage of males
as 728 on Orchestoidea and 65% on the wrack. The largest size class of
adult mites had relatively equal numbers of males and females. The
smaller mites, which were much more difficult to sex, tended to possess
exclusively male characteristics. While adult mites of most species
usually don't molt (Krantz 1978), it was observed that old molts from
smaller adult mites were found in vials containing only adult mites.
addition, larger adult mites were observed which had fresh, uncolored
exoskeletons. In subsequent sex ratio studies, only the larger adults
with easily distinguishable sexual characteristics were sexed. While
female counts are relatively easy, it is often difficult to determine
if male mites have undergone their final molt. Females compose 418 of
this population on Orchestoidea and 61% on the wrack.
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Studies of the distribution of young undifferentiated adults
show that they make up nearly equal percentages of the total mite
populations on the wrack (318) and on the Orchestoidea (322). Deutonymphs,
on the other hand, make up 272 of the total mite population on the wrack,
but only six percent on the Orchestoidea.
Discussion
The life cycle of a typical Gamasida mite, the suborder to which
G. brevisternalis belongs, is shown in figure A. Figure B shows the
suggested life cycle of G. brevisternalis. It appears that the female
mite lays a single, large, pearl white egg on the wrack. The requirements
for the wrack are that it be moist and decaying. Being partially buried
will also protect the wrack from drying out. Wrack consisting of a mix
of Macrocystis and Phyllospadix seems to be the wrack with the highest
concentration of mites. This seems logical as Macrocystis is the primary
food of Orchestoidea. Thus the young have easy access to their host
amphipod.
The fact that few eggs were spotted when visually examining wrack
known to contain many eggs tends to suggest that the eggs are tucked into
pockets and folds in the wrack. While protection from predation may be
one benefit of such strategy, the primary benefit is likely that it prevents
dessication of the eggs.
In a random sample of eggs, one would expect that after half the
incubation period has passed, half of the eggs would be hatched. Approximately
one-half of the eggs which hatched did so two to three days after collection
indicating an incubation period of five to six days. However, given that
only about one third of the eggs collected actually hatched, the incubation
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period could be longer: eggs which were less mature at the time of collection
could be a high proportion of those which did not hatch.
All mites of the suborder Gamasida go directly from a protonymph
to a deutonymph (Krantz 1978). Since both proto- and deutonymphs were
observed, but no unaccounted stage which may come between them, it can
reasonably be assumed that this is the course of the life cycle.
Most Gamasida also go directly form a deutonymph todan adult. As
these are the only two life stages previously described in G. brevisternalis,
and no others have been observed in this study, this appears to be the
course of development.
The deutonymph is the earliest stage which is found on the
Orchestoidea, although it predominately inhabits the wrack. Young adults
are distributed relatively equally between the Orchestoidea and the wrack.
This data indicates that these are the life stages where the transition from
wrack to Orchestoidea occurs.
The wrack seems to induce egg formation in the mites. Only one
female mite on Orchestoidea was ever found to contain an egg. Even mites
observed for two weeks on Orchestoidea produced no eggs. On the other hand,
958 of the females extracted from the wrack were found to contain eggs, a
surprisingly high percentage since it was shown that females which have
just laid an egg do not contain another egg. Possibly females crawl
back to an Orchestoidea immediately after laying.
Fertilization most likely occurs on the wrack. If fertilization
occurred on the Orchestoidea, there would appear to be no reason for the
adult male to leave the amphipod, which is their source of nourishment.
Generally, small parasites experience a low rate of survival to
adulthood which they counter by the production of many eggs. G.
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brevisternalis have been observed to drown within half an hour when
submerged, and die of dessication within a few hours when placed on
dry filter paper. Wrack containing all stages of the mite life cycle
can easily be totally dried out or completely submerged by the tides.
Logically, each female would produce many eggs to combat these harsh
conditions and insure that at least two would survive to adulthood.
However, the fact that each female lays one egg at a time and does not
produce another for several days means that these mites actually have
very low fecundity. Apparently they are adapted to their environment
very well.
Acknowledgements
I'd like to thank Gary Wagenbach of Carlton College for sharing
his knowledge of parasitology. My advisor, Robin Burnett, also has left
me forever indebted by showing the patience to allow me to discover
science for myself. It was an experience l'll never forget.
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Literature cited
Bowers, D.E. 1963. Field identification of five species of amphipods.
Pacific Science, 17:315-320.
Bowers, D.E. 1964. Natural history of two beach hoppers of the genus
Orchestoidea (Crusteacea:Amphipoda) with reference to their complementa!
distribution. Ecology, 45:677-696.
Canaris, A.G. 1962. A new genus and species of mite (Laelaptidae)
from Orchestoidea californiana (Gammaridea). J. Parasit., 48:467-169.
Hall, H.V. 1912. Some marine and terrestrial acarina of Laguna Beach.
First annual report of the Laguna Marine Laboratory. Pamona
College Press, 218 pp.
Krantz, G.W. 1978. A manual of acarology. Oregon State Univ. Bookstores,
Inc., 509pp.
McClurkin, J.1., Jr. 1953. Studies on the genus Orchestoidea (Crusteacea:
Amphipoda) in California. Unpublished Ph.D thesis, Stanford Univ.
Pub1., 5803 Univ. Microfilms, Ann Arbor, Michigan.
Scurlock, D. 1975. Infestation of the sandy beach amphipod Orchestoidea
corniculata by Gammaridacarus brevisternalis (Acari:Laelaptidae)
Bull. So. Calif. Acad. Aci., 74:5-9.
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Figure captions
The life cycle of a typical Gamasida mite. The prelarva stage
A.
is often bypassed. Occasionally mites give birth of live young.
The life cycle of Gammaridacarus brevisternalis. The life stages
in the shaded portion occur on the wrack. The life stages in
the dotted portion are transition stages found on both Orchestoidea
and the wrack. These appear to be the stages where the mites move
from the wrack to the Orchestoidea.
A
LIFE CYCLE OF A
TYPICAL GAMASIDA
adult-
cutchon
prelarva
protonymph
larva—
LIFE CYCLE OF
G. BREVISTERNALIS
gravid female
-
mature
adult
9
.
.
young
larva
adult . ..


protonym
deutonymph