Ambulacral structure in the terrestrial moiety of the intertidal Acari

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AMBULACRAL STRUCTURE IN
THE TERRESTRIAL MOIETY OF THE INTERTIDAL ACARI,
AND ITS RELATIONSHIP WITH THE LIFESTYLE OF THE ACARI
BY
P. J. A. PUGH, P. E. KING and M. R. FORD Y
*
AMBULACRA
AND BEHA VI OUR
AMONG
LITIORAL
ACARI
ABSTRACT : Ambulacral modifications among the terrestrial moiety of the intertidal Acari associated with maintenance of position reflect the lifestyles of these
Acari. The Mesostigmata have strenthened or elongated pulvillar sclerites and
long spatulate setae to aid adhesion. The claws of the genus Ameronothrus
(Cryptostigmata) are proportionately longer in the intertidal compared with the
supralittoral species. The claw mechanism of Hyadesia spp. (Astigmata) is unique ; the mechanism and its mode of action are analysed. Howerver, the Prostigmata show no unusual ambulacral modifications, with ail structures having parallels among non-intertidal species.
AMBULACRES
DES ACARIENS
ET CONDITIONS
DU MILIEU
RÉSUMÉ : Les modifications ambulacraires associées au maintien de la position, au
sein de la partie terrestre des Acariens de la zone des marées, reflètent les modes de
vie d~ ces Acariens. Les Mesostigmata ont renforcé ou allongé les sclérites pulvillaires et leurs longs poils spatulés pour aider l'adhésion. Dans le genre Ameronothrus (Cryptostigmata) les ongles sont proportionnellement plus longs chez les espèces
de la zone des marées comparées à celles de la zone supralittorale. Le mécanisme
de l'ongle des Hyadesia spp. (Astigmata) est unique ; mécanisme et mode d'action
sont analysés. Toutefois, les Prostigmata montrent aucune modification ambulacraire inhabituelle et toutes les structures qu'ils possèdent ont leurs parallèles chez
les espèces qui n'appartiennent pas à la zone des marées.
INTRODUCTION
One of the major factors determining the distribution of the fauna and flora in the intertidal
zone is their ability to withstand mechanical stress
resulting from wave action, and thus to maintain
their position on the shore (BALLANTINE, 1961 ;
NEWEL, 1979). The intertidal Acari may be divi-
ded into two groups ; an aquatic moiety (the
family Halacaridae) which are always submerged
or at least wetted by a water film, and a terrestrial moiety, a mixed assemblage of species belonging to a number of families. These generally
avoid wetting by retreating ahead of the tide, or
withdraw to air pockets trapped in crevices (PUGH
1985 ; PUGH & KING, 1985b).
Many intertidal Acari have ambulacra which
* Marine Biology Research Group, School of Biological Sciences, University College of Swansea, Singleton Park, Swansea, West
Glam. SA2 SPP. Great Britain.
Acarologia, t. XXVIII, fasc. 1, 1987.
4
may help them to maintain their position on the
shore. SCHUSTER (1962, 1965, 1979) described
sorne ambulacral structures present among the
Mesostigmata and Prostigmata. However, the
different lifestyles described among the intertidal
Acari {PUGH, 1985 ; PUGH & KING, 1985b), suggest that these may not be purely adaptations to
submersion, but also to rapid locomotion, particulary in the Prostigmata. The evidence for
these ideas is re-examined.
MA TERIALS AND METHODS
Acari were collected from open rock surfaces
and crevices in the intertidal zone, and from tidal
debris in the supralittoral using an entomologist's
aspirator. Specimens were extracted from intertidal algae, barnacles and lichens using a hypersaline flotation technique (PUGH, 1985 ; PUGH &
KING, 1985a). Non-intertidal species were extracted from soi! and leaf litter by means of Tullgren
funnels {EVANS, et al, 1961), using live cells containing moist tissue paper instead of an alcohol
based fixative.
Specimens for examination under the scanning
electron microscope were fixed in alcoholic
Bouin's fixative (GRIMSTONE & SKAER, 1972),
cleaned in an ultrasonic wash, dehydrated in acetone, coated with gold under vaccuum, viewed
and photographed under a JEOL 35C SEM. Particular emphasis was placed upon claw structure.
In sorne species which were not examined under
the SEM, claw structure was analysed by light
microscopy. The species studied are listed
below:
MESOSTIGMATA
Macrocheles superbus Hull
Thinoseius fucicola Halbert
Onchodellus tesse/atus (Berlese)
Halolaelaps ce/lieus Halbert
H. marinus (Brady)
Saprogamasel/us incisus Hyatt
Arctoseiodes ibericus Willmann
P/atyseius necorniger (Oudemans)
(Macrochelidae)
(Eviphidae)
(Pachylaelapidae)
(Halolaelapidae)
))
))
(S)
(S)
(Ascidae)
))
Note : t : terrestriai non-intertidal/supraiittoral species ;
S : examined in the stereoscan.
Hydrogamasus giardi (Berlese &
Trouessart)
(Cyrtolaelapidae)
H. salinus (Laboulbene)
»
Cyrthydrolae/aps hirtus Berlese
(Veigaiidae)
C. incisus Evans
>>
Parasitus kempersi Oudemans
(Parasitid:;~e)
Vulgarogamasus immanis (Berlese)
>>
V. trouessarti (Berlese)
>>
Pergamasus longicornis (Berlese) (t)
>>
(S)
(S)
(S)
(S)
PROSTIGMATA
Halotydeus hydrodromus Berlese &
Trouessart
Bdella decipiens Thorell
Balaustium murorum (Hermann) (t)
B. rubripes (Trouessart)
Neotrombidium spjJ. (t)
(Penthaleidae)
(Bdellidae)
(Erythraeidae)
))
(Trombidiidae)
(S)
(S)
(S)
(S)
(S)
CRYPTOSTIGMATA
Ameronothrus bilineatus (Michael) (Ameronothridae)
A. lineatus (Thorell)
>>
A. macula tus (Michael)
>>
A. marin us (Banks)
>>
Euzetes seminu/um (0 . F. Müller)
(t)
(Euzetidae)
(S)
(S)
(S)
ASTIGMATA
Hyadesia furcillipes Benard
(Hyadesidae)
(S)
OBSERVATIONS & DISCUSSION
1. MESOSTIGMATA
Ambulacra II to IV of the free-living Mesostigmata consist of a basal pretarsus, paired distal
claws, and a large lobate pulvillus, the lateral
lobes of wich are referred to as the pulvillar
sclerites {Figs. 10, lH & 11) {EVANS & TILL,
1979). Hydrogamasus salinus has modified ambulacra, the pulvillar sclerites are narrow and
acuminate, reinforced by longitudinal ridges, and
are orientated at 90° to the points of the claws
(Figs. lA & lB). It is suggested that, if wetted,
they would cause the clause the claws to be pressed onto the substratum by water surface tension,
and thus would increase the grip, while the renfor-
FIG. 1 : Ambulacral Structure : Mesostigmata.
lA. Hydrogamasus salinus : ambulacrum II, lateral aspect. Note how the claws (c) may act antagonistically against the thumb-pad
(tp) to provide grip. The pulvillar sclerites (ps) and spatulate setae (ss) are flattened to provide adhesion on wet
surfaces. Scale : 10 l'ffi ; lB. H. salinus ambulacrum II, dorsal aspect. Note the large ribbed pulvillar sclerites (ps). Scale :
10 l'ffi ; IC. H. giardi: ambulacrum II, lateroventral aspect. Structure is similar to that of H. salinus (Figs . lA & lB), being
equipped with large claws (c), pulvillar sclerites (ps), spatulate setae (ss), and thumb-pad (tp). Scale : 5 l'ffi ; ID. H. salinus:
spatulate seta shown in Fig. lA. Note the reinforcing marginal ridge (r). Scale 2 l'ffi ; lE. Cyrthydrolaelaps hirtus : ambulacrum II, dorsolateral aspect. The claws (c), pulvillar sclerites (ps) and spatulate setae (ss) are very elongate, considerably more
so than in H. salinus (Figs. lA & lB). Scale : 15 l'rn ; IF. C. hirtus : ambulacrum Il, dorsal aspect. Note the very elongate
claws (c), pulvillar sclerites (ps) and spatulate setae (ss). Scale: 20 l'ffi ; IG, IH & Il Pergamasus longicornis (non-intertidal) :
ambulacrum II in lateral and ventral aspects. Note the short claws (c), broad and leaf-like pulvillar sclerites (ps) ; there are no
prominent flattened or spatulate setae associated with maintenance of position under conditions of mechanical stress, contrasting
with the intertidal species shown above. Ali scales : 10 l'ffi·
6
cing ridges would prevent the sclerite bending.
The ventral surface of tarsi II-IV have a central
pad which may fonction as a fixed " thumb "
against which the two moveable claws may act as
" fingers ", allowing the mite a better grip on
irregular surfaces (Figs. lA & lB). A similar
mechanism is present in Hydrogamasus giardi
(Fig. lC).
ln contrast, the ambulacra of the mesostigmatic
Cyrthydro/ae/aps hirtus Berlese, have flattened,
spatulate pulvillar sclerites (Figs. lE & IF) which
may aid in adhesion to wet surfaces, as suggested
by BERLESE (1905). However, their mode of
action is different from those of Hydrogamasus
salinus because, when wetted, they adhere to the
substratum and increase their grip. Halo/ae/aps
marinus has elaborate ambulacra similar to those
of Hydrogamasus sa/inus (Halbert, 1915) but, in
the related Halo/aelaps celticus Halbert from tidal
debris, they are not so weil adapted. Here the
ambulacra form an intermediate structure between
those of Halolaelaps marinus ( = H. g!abriusculus) and '' the typical gamasid '' of terres trial origin, for example P. coleoptratorum (Figs. lG, lH
& 11) (HYATT, 1956).
Although the ambulacra of sorne intertidal
Mesostigmata show adaptations associated with
grip on wet surfaces, the legs are unmodified
(Figs. 2 & 3). Legs of species occuring in the
lower intertidal zone, namely Ha/o/ae!aps marinus, Hydrogamasus giardi, Hydrogamasus sa/inus
and C. hirtus do not differ in relative lengths
from those of the upper intertidal and supralittoral-zone dwelling species.
2.· PROSTIGMATA
The ambulacra of sorne intertidal Prostigmata
are elaborate. SCHUSTER (1962) suggested that
the modified tarsi of Halotydeus a!bolineatus Halbert, (Penthaleidae) may prevent its dislodgement
by wave action. However, the closely related H.
hydrodromus collected extensively during the present study has similar tarsi (Fig. 4A), but is intolerant of submersion and is immobilised, or even
damaged, by surf (PUGH, 1985 ; PUGH & KING,
1985a). These ambulacra and the modified
empodia of Bdel/a decipiens equipped with spatulate setae (Figs. 4B, 4D & 4F) may aid adhesion
on to wet surfaces, but it is suggested that these
tarsi are adapted for rapid locomotion, and that
the spatulate setae increase adhesion on especially
wet surfaces . However, such ambulacra are not
unique to the intertidal fauna, and occur bath
intertidal and non-intertidal Acari from a number
of different families e.g. the Bdellidae (SCHWEIZER, 1951, WALLACE & MAHON, 1972), and the
Rhagidiidae (ZACHARDA, 1980).
Members of the genus Ba!austium (Erythraeidae) have no empodia but, when running, bath
the claws and the setae on the ventral surfaces of
the tarsi are brought into contact with the substratum to provide traction. These setae possibly
fonction in a similar manner to the spatulate setae
of the empodia of the species mentioned above.
Those of the intertidal species, B. rubripes (Figs.
4C, 4E & 40), are similar to those in the related
non-intertidal B. murorum (Figs. 4H & 41), providing furthur evidence in favour of this being an
adaptation to rapid movement, rather than maintenance of position during submergence.
The majority of the terrestrial moiety of the
intertidal Prostigmata are swift moving predators,
whose elaborate ambulacral and tarsal structures
presumably are adaptations for rapid locomotion,
and not specifically for the maintenance of position when submerged. Field and laboratory data
suggest not only that these mites avoid inondation, but that they are intolerant of it (PUGH,
1985 ; PUGH & KING, 1985b). Thus, adaptations
specifie for the maintenance of position during
submergence would hardly be useful in these
mites.
3. CRYPTOSTIGMATA
SCHUSTER (1965) stated that there are no ambulacral modifications in the intertidal Cryptostigmata. However, SCHUBART (1975) demonstrated
that the genus Ameronothrus, which has bath
intertidal and non-intertidal representatives, exhibits a correlation between the type of substratum
and claw structure. The species occurring on
hard substrata have three claws, and those on soft
a
a
e VI
18oo
Ch*
Ch*
1200
1600
~
1400
E
VIe
•vt
Mse
1000
=800
1200
"'
Cl
....1
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Cl
....1
..
0
0
-"'
1000
Tfe
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.c
r:n
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Hm*
800
Cie
CI
c
Cl
....1
* Hs
c
~
Hs
600
400
Ote
400
1000
800
600
1200
1400
Length of ldlosoma (~ml
•He
SI
4oo,___or~----.-----.-----.------.400
600
1000
800
1200
1400
Length of ldlosoma (~ml
b
1400
1800
b
1600
VI
1200
E
:::....
= 1000
0
1400
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r:n
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....1
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E
:::1....
> 1000
800
=
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Dl
Cl
....1
c
Cl
....1
*
c~
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Pne
0
Hs
.c
Ot
;
Pn•
600
..J
400
Hs
SI
*Hg
Al
SI
200~----,-----,-----,-----,-----,-
400
*
He
Cl
Hg
Tf
•• Cl
• •ot
c
•He *Hm
400
600
800
1000
1200
1400
Length of ldlosoma <rm>
FlOS.
2 & 3.
Graphs showing the relative leg lengths of sorne intertidal
Mesostigmata. Species occurring in the lower littoral zone
are indicated by asterisks (*). The legs of these species do
not differ significantly from the over upper intertidal and
supralittoral species, as shown by the Mann-Whitney test
(Meddis, 1975).
Graphs suggest a high correlation between leg and idiosomal length-regression line 'r' values calculated as follows :
FIG. 2A
3A
Hm*
800
0.968
0.948
FIG. 2B : r = 0.980
3B : r = 0.949
200/+----.-----.-----r----,.----,
1000 1200 1400
800
400
600
Length of ldlosoma lpml
Key: Ai.
Ch.
Ci.
He.
Hg.
Hm.
Hs .
Ms.
Ot.
Pk.
Pn.
Si.
Tf.
Vi.
Vt.
Arctoseiodes ibericus
Cyrthydrolae!aps hirtus
C. incisus
Halolae!aps celticus
Hydrogamasus giardi
Halolae!aps marinus
Hydrogamasus sa!inus
Macrocheles superbus
Onchodellus tesse!atus
Parasitus kempersi
Platyseius necorniger
Saprogamasel/us incisus
Thinoseius fucicola
Vu!garogamasus immanis
V. trouessarti
FIG. 4 : Ambu/acral Structure : Prostigmata.
4A. Halotydeus hydrodromus : ambulacrum II, lateral aspect. Scale : 10 !!ffi ; 4B. Bdella decipiens : ambulacrum II , lateral aspect,
showing the prominent " haired " empodium (emp). Scale : 10 !!ffi ; 4C. Balaustium rubripes : ambulacrum II, lateral
aspect. Note the dense mat of empodial processes on the ventral surface which acts as a " foostsole " to increase
traction. Scale : 5 !!ffi ; 40. B. decipiens : ambulacrum II, lateral aspect of empodial processes shown in 3.2 Note that the tips
of the processes are flattened to increase substrate adhesion. Scale : 2 !!ffi ; 4E . B. rubripes : tarsus Ill, detail of setae on the
ventral surface, which give traction (compare with Fig. 4J). Scale : 5 !!ffi ; 4F. B. decipiens : ambulacrum II, dorsal
aspect. Note the empodium (emp), and the setate claws (c). Scale : 10 !!ffi; 4G . B. rubripes: tarsus IV, lateral aspect. Note
the setae on the ventral surface used to provide traction (compare with Fig. 4E) . Scale : 20 !!ffi ; 4H . Ba/austium murorum
(non-intertidal) : tarsus IV, lateral aspect. Note the similarity with B. rubripes (compare with Figs. 4C & 4G). Scale 20 !!ID ;
41. Neotrombidium sp. (non-intertidal) : tarsus IV, lateral aspect. Note the similarity with the tarsi of Ba/austium spp. (compare
with Figs. 4C, 4G & 4H). Scale: 20 !!ffi ; 4J. B. murorum (non-intertidal) : tarsus II, detail of setae on the ventral surface of
the tarsus, (compare with Fig. 4E). Scale : 5 !!ffi .
9
substrata have only one. The claws of A. marinus are structurally unmodified, compared with
those in related non-intertidal species (SCHUBART,
1975) ; in both groups the lateral claws are thin
and serrated, while the medians are massive and
simple (Figs. 5A & 5B), (SCHUBART, 1975).
Although the claws of A . marinus are structurally
similar to those of sorne of the non-intertidal
Ameronothridae, they are larger both in absolute
length, and in length relative to that of the whole
animal, than are those of non-intertidal species.
A second intertidal species, A. lineatus, also has
relatively large claws (Fig. 6).
4. ASTIGMATA
Hyadesia is the only genus of the Astigmata
normally encountered in the intertidal zone. The
claws of this genus are of unusual structure, as
shown by MICHAEL (1901) and BENARD (1961)
(Figs. 7E & 7F). BENARD gave a brief description of locomotion in Hyadesia . During the present study, additional observations were carried
out on H. furcillipes to ellucidate stepping
sequence and claw function during locomotion.
The legs of H. furcillipes are relatively thick
when compared to the empodial stalks (especially
empodia 1 and II (Figs. 7D & 7E)), and probably
function as compression members. The structure
and position of the single heavy spine on tarsi 1
and Il , and the smaller trio of spines on tarsi III
and IV, suggest th at they are points of contact
with the substratum and that the animal " walks
on tip toe '', and th at the mite '' stands '' on the
tarsal claws of legs 1 and II and the spines of
tarsi III and IV. The pulvillar stalks are thinner
and equipped with axially deflected hook-like
claws, features suggesting that the stalks are tension members.
In the Halacaridae, the body is wetted and
usually partially supported by water. The legs
are used to pull the animal along, so that the
claws function simply as hooks or grapnels .
Thus legs 1 and II are thicker than III and IV
(Fig . 7B) because they provide the majority of
thrust (PUGH, 1985). In the terrestrial moiety,
(excluding Hyadesia), because they move in air,
legs III and IV are thicker and provide thrust.
In sorne species of Mesostigmata, leg 1 is sens ory,
and may Jose its ambulacrum and locomotor
function to become antenniform, (most Macrochelidae for example). ln these cases legs Il are heavier and may compensate for legs 1 in supporting
the weight of the body. ln many male Mesostigmata, legs II are thick and armed with heavy
spurs, though this is a secondary sexual characteristic.
In Hyadesia, legs 1 and II are heavier than III
and IV (Fig. 7C), suggesting that locomotion may
involve pulling the body along in a manner similar to that described for the Halacaridae. This
idea was supported by crushing individual tarsi
with a pair of fine forceps ; specimens with damaged tarsi 1 or Il, showed greater difficulty in
moving than those with damaged tarsi III or IV.
Locomotion in H. furcillipes was observed, and
the sequence of leg movement was as follows :
1) Leg 1 on one side is extended and the empodial claw changes orientation from 90° to the axis
of the leg, to it lying along the axis of the leg,
which is then lowered until the tip of the tarsal
claw is brought into contact with the substratum.
2) Leg II on the opposite side repeats this process
al most simultaneously. 3) Legs III and IV also
move in an alternate pattern, with the body supported on the terminal tarsal spines . Thus, the
overall pattern of leg movement is : Left 1 and
right II rn ove together, followed by a short time
!ag, followed by left III and right IV together,
time !ag, right 1 and II together, time !ag, right
III and left IV together. The process then
repeats itself (Fig. 7F) . A similar pattern was
noted in Tyrophagus putrescentiae (Schrank)
(Acaridae) by BAKER & SMITH (1968) .
If H. furcillipes is mechanically stimulated by
water movement, the body is pulled forward on
legs 1 and Il, with the weight supported by the
tips of the tarsal claws. Leaving the claws on
tarsi III and IV hooked onto substratum irregulanties, " Jeans forward ", applying downward
pressure on tarsi 1 and Il. This locks ali 8 claws
onto the substratum. The Jock may be released
by relaxing legs 1 and II.
The evidence presented here reinforces the
FIG.
5 : Ambulacral Structure : Cryptostigmata and Astigmata.
5A. Ameronothrus marinus: ambulacrum IV, lateroventral aspect. Note the serrated lateral ctaws (le), and large smooth median
ctaw (mc). Scale : 10 l'rn ; 5B. A . marinus : ambulacrum II, lateroventral aspect. Scale : 10 l'ffi ; 5C. Euzetes seminulum (nonintertidal) : ambulacrum III, lateral aspect. Scale : 10 l'ffi ; 5D. E. seminulum (non-intertidal) : ambulacrum III, lateral
aspect. Scale : 10 l'ffi ; 5E . Hyadesia furci/lipes : tarsus II, lateral aspect. Note the empodial ctaw (ec), empodium (emp), tarsus (tar) and tarsal ctaw (tc) . Scale : 10 l'rn ; 5F. H. furcillipes : ctaw IV, lateral aspect. Scale : 5 l'ffi·
-11-
50
•
-
E
:a..
A. llneatus
• A. marinus
~
-"
( .)
c
"
'til
Cl
-
40
•regression une·
::E
0
• A. schneider!
• A. blllneatus
• A. maculatus
.c
0)
c
• A. schusterl
_,Cl
304-----------~------------~----------~~----------~-650.
800
700
600
750
Length of ldlosoma <rm>
FIG. 6.
Graph to show the relative median claw length among members of the genus Ameronothrus (Ameronothridae :
Cryptostigmata) •. The " regression line " divides the species into two groups, and suggest that the intertidal species A. /ineatus
and A. marinus have proportionately larger claws. This is further supported by the Mann-Withney test (Meddis, 1975), as shawn
below:
Species
A.
A.
A.
A.
A.
A.
Length
of
ldiosoma
Claw
Length
(pm)
(pm)
769
784
633
682
700
784
45
34
36
34
36.5
lineatus
marinus
macula/us
bilineatus
schusteri
schneideri
50
Ranked
Claw
Index
(ld/Claw)
15.38
17.42
18.62
18.94
20.59
21.60
Habitat
lntertidal
Supralittoral
and
Terrestrial
The claw index ratio values of the intertidal species are outside the range of those of the supralittoral and non-intertidal species,
therefore there is a highly significant difference between the two groups.
• : additional data modified from
SCHUBART
(1975).
Ill
f
t
FIG. 7.
Locomotion in Hyadesia furcillipes.
Acari of the Terrestrial Moiety usually have legs III and IV thicker than I and Il, eg .- Bdella decipiens (7A) ; compare with the
aquatic moiety where legs I and II are the thickest, eg. Ha/acarellus basteri (7B) ; Hyadesia spp. most resemble the aquatic moiety in
this respect (7C) ; 70. The grip mechanism of H. furcillipes. The tip of the tarsus is in contact with the substratum, with the
pùlvillar claw (pul) hooked over an irregularity in the substratum . A downward pressure on the leg (1), causes the point of the tarsus (2) to act antagonistically with the hook of the claw (3) ; 7E. Leg III of H. furcillipes. The body of the mite may be supported on the distal tarsal spines (!) ; the claw is used to grip omo irregularities (2) ; 7F Stepping sequence in H. furcillipes . Dots
represent tarsi in contact with the substratum ; direction of movement of legs not in contact with the substratum shown by the
arrows.
(7C is modified after BENARD, I961).
13
information regarding lifestyles observed among
the Terrestrial Moiety of the intertidal Acari
(PUGH, 1985 ; PUGH & KING, 1985b). The
Mesostigmata, and the genera Ameronothrus
(Cryptostigmata) and Hyadesia (Astigmata), are
tolerant of inundation, and have ambulacra adapted to increase adhesion on wet surfaces. However, the Prostigmata, which are intolerant of wetting, show no such adaptations and rely upon
speed to forage ahead of the tide. Their ambulacra are adapted to give maximum traction at
speed, contrasting with the aquatic moiety (family
Halacaridae), which show a range of adaptations
to cope with the mechanical stresses resulting
from wave action (PUGH, 1985 ; PUGH, et al. in
press).
ACKNOWLEDGEMENTS
We would Iike to thanck Professors E. W. KNIGHTJONES and J. S. RYLAND for providing research facilities, and one of us (P .J .A.P .) is grateful to the Natural
Environment Research Council and the Gulf Oil Refining Company of Milford Haven for their financial
support.
BIBLIOGRAPHY
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