The family of primitive prawns, Atyidae, are freshwater animals with a circumtropical distribution, but additionally some have penetrated into temperate regions. An intriguing aspect of their distribution is that, although they are, and have long been, freshwater crustaceans, they have succeeded in colonizing many oceanic islands. The West Indies is an area of interest, as representatives of several genera sometimes occur on one island. For its size, Dominica is particularly rich in this respect. The fauna of the island includes the most primitive living atyid (a West Indian endemic, Xiphocaris elongata) and 2 representatives of the most advanced genus, Atya. Each of the other 3 spp belongs to a separate genus. The feeding behaviour of the Dominican atyids is discussed in this article

Fryer, G.

English

Aquatic Commons

4 8 FORTY-FIFTH ANNUAL REPORT 
THE ATYID PRAWNS OF DOMINICA 
G. FRYER 
That the fauna of Europe in general, and of Britain in particular, gives 
a very imperfect idea of the importance of several groups of freshwater 
animals is perhaps insufficiently appreciated by European workers. An 
excellent example of such faunal poverty is provided by the decapod 
crustaceans - the group that includes crabs, crayfishes and prawns. Of this 
group there is only one truly native British species, the common crayfish 
(Austropotamobius pallipes). In warmer regions, and especially in the 
tropics, the situation is often different. Here prawns and crabs are some-
times important constituents of the freshwater fauna. In few places can 
this be more clearly seen than in the West Indian island of Dominica. 
Here, where a copious rainfall is reflected by much rich forest and many 
perennial streams, the number of species of prawns greatly outnumbers 
that of fishes, which they replace as the dominant large animals of flowing 
water. The streams also harbour aquatic and semi-aquatic crabs thus 
emphasizing crustacean dominance. Of the prawns six species belong to 
the family Atyidae (Fig. 1) and five to the Palaemonidae. It is the former 
family that concerns us here. 
Atyids, which students of the Crustacea regard as primitive prawns, 
have a circumtropical distribution but additionally some have penetrated 
into temperate regions, sometimes in surface waters, sometimes as 
cavernicolous forms. An intriguing aspect of their distribution is that, 
although they are, and have long been, freshwater animals - fossils are 
known from freshwater deposits of Cretaceous age, and they have 
produced endemic genera in Lake Tanganyika — they have succeeded in 
colonizing many oceanic islands. The West Indies is an area of particular 
interest to the student of the family as representatives of several genera 
sometimes occur on one island. For its size Dominica is particularly rich in 
this respect. Especially interesting is the fact that its fauna includes not 
only the most primitive living atyid (a West Indian endemic, Xiphocaris 
elongata) but also two representatives of what is almost certainly the most 
advanced genus, Atya. Each of the other three species belongs to a 
separate genus. 
All Dominican atyids are detritus feeders. Their food comes from the 
forest, leaves from which fall continuously into the streams and there 
break down into particles sufficiently small to be collected by the 
specialized food-gathering devices which they employ. If the forest is 
destroyed the atyids will disappear, either as a result of the lack of food or 
by scouring and drying up of the streams. 
The primitive Xiphocaris, which reaches a length of 7 cm if one includes 
the long rostrum, is a lightly-built, transparent prawn with a well-defined 
ecological niche. It frequents quiet pools in streams over whose bottoms it 
tiptoes delicately, picking up individual small particles of detritus with its 
two pairs of chelipeds ('nippers') that are specialized for this purpose. It is 
PLATE 3 Atya innocous advancing over a stone and collecting food by sweeping as it does 
so. The white particles are a proprietary breakfast food laid as bait. (See Plate 4.) 
PLATE 4 (above) As Plate 3. The animal has advanced further and its chelipeds are shown 
at different phases of the cycle of movement. (below) Atya innocous facing a current with 
all its cheliped bristles spread to form four contiguous filtering baskets that passively collect 
suspended food particles. 
DIRECTOR'S REPORT 49 
also an efficient swimmer, and employs a very spectacular escape 
mechanism. When approached it leaps backwards by flexing its abdomen 
with such force as to propel it out of the water and through the air for con-
siderable distances. On alighting it may make one or more additional leaps, 
presumably by further flexions of the abdomen, before disappearing 
beneath the surface. 
In its method of food collection it differs from all other atyids in that its 
chelipeds are used directly for seizing food particles, as in the familiar 
crabs and crayfishes, whereas its relatives employ rows of highly 
specialized cheliped bristles. Although primitive, Xiphocaris employs 
complex machinery for dealing with the collected food in its passage from 
the chelipeds, via the mouthparts, to the mouth. A second complicated 
mass of apparatus located in the fore-gut is then used for sorting and 
processing the collected material. 
All other Dominican atyids are crawlers rather than swimmers, and all 
have an elaborate method of food collection, again employing the 
chelipeds, but in a different way from Xiphocaris. In these species each 
'finger' of the pincer of the cheliped, itself highly modified, is fringed by 
FIG. 1. The atyid prawns of Dominica. 
50 FORTY-FIFTH ANNUAL REPORT 
rows of elaborate bristles whose form, number and arrangement differ 
from species to species. In all cases these bristles are expanded into an 
opposing pair of fans as the cheliped is extended, and can then be placed 
against a stone or other object and brought together. In so doing each fan 
sweeps up small particles and when the two come together, closing as they 
do so, they enclose an accumulation of such material. This can then be 
passed to the mouthparts. 
Such is the basic outline of the process. Many refinements are possible 
and have been exploited. The way in which the fans are expanded and 
closed also demands attention. Clearly, simple bristles will be less effective 
at sweeping than those which are modified by the development of ridges or 
side bristles so it is not surprising to find many such modifications nor to 
find that different species have specialized in different ways to enable them 
to exploit different habitats and niches. For example, the small Jonga 
serrei (generally less than 2.5 cm in length) habitually lives in slow-flowing 
water, usually near stream mouths, not because it needs brackish water 
but because in the mountainous Dominica it is only here that such slow 
flow is to be found. Here it scrapes both hard objects on the bottom and 
marginal vegetation and its bristles have become highly specialized as 
scrapers and brushes. Furthermore they are arranged in arcs so that, as 
the pincers close and the two fans sweep towards each other, the inner arcs 
of beautifully formed scrapers pass over the substratum first. These are 
followed by larger brushes that sweep up any material that is dislodged but 
not passed on directly by the scrapers. Thus are surfaces efficiently swept. 
Specialized in a different way but employing similar principles are the 
two species of Atya- A. innocous (to 12 cm+) which is very common, and 
A. scabra (to at least 10 cm) which is much rarer. Both have extremely 
long cheliped bristles, so much so that the tip of a closed cheliped 
resembles the business end of an artist's paint brush. In order to permit 
this bunching together of the bristles there have had to be fundamental 
changes in the organization of the distal segments of the cheliped such as 
are not to be found in other decapods but which need not concern us here. 
When these bristles are expanded they make very large fans which can be 
used to collect material from stones in a similar manner to the shorter 
scrapers and brushes of Jonga (Plates 3, 4 above). They can, however, be 
used in another and even more remarkable way. The fans can be expanded 
and kept in this position as long as required. If expanded fans are turned to 
face the oncoming current they form a passive filter (Plate 4 below) whose 
'pores' are reduced because the spaces between adjoining bristles are 
spanned by fine setules. Thus a fan can act as a filtering basket and catch 
drifting particles. The efficiency of this device is further enhanced by 
placing the four pairs of fans together, thus making up a large, continuous 
filtering surface. In the Malacostraca such a device is unique to Atya and 
its relatives. It bears remarkable convergent similarity to that employed by 
larvae of simuliid flies but the origin and anatomy of the various 
components is, of course, very different. 
DIRECTOR'S REPORT 51 
Both the Dominican species of Atya frequently filter in this way, often 
for many hours at a time, but aquarium observations suggest that A. 
scabra does so more persistently than A. innocous and that, while the 
latter regularly sweeps up particles, the former seldom does so. This 
difference is reflected in slight differences in the nature of the bristles. A 
few of those of A. innocous have fine teeth, detectable only under the 
microscope, which will help it to sweep up particles, but such are lacking 
in A. scabra. This difference helps us to understand the ecological 
preferences of the two species and may go some way to explaining why A. 
innocous is much more common than A. scabra. Its greater efficiency as a 
scraper may give it an advantage. 
Another Dominican atyid is Micratya poeyi which, with a maximum 
length of only about 2.3 cm, is in many respects a miniature version of 
Atya. This is an extremely abundant species and, like Atya, can both 
scrape and filter passively. The least known species is Potimirim glabra 
whose cheliped bristles are specialized for sweeping and show few signs of 
being used for passive filtration. 
The food collecting mechanism thus briefly described presents many 
problems to the zoologist. Particularly intriguing is the way in which the 
fans of bristles are expanded. Examination of the way in which the bristles 
are inserted on the cheliped segments reveals that no muscles whatsoever 
are attached to any of them - yet they are clearly under the precise control 
of the animal. The secret is revealed by the way in which they are attached 
to the hollow cheliped segments. Each bristle is tubular and sits over a hole 
in the cheliped cuticle (Fig. 2). Thus its cavity is in direct continuity with 
the fluid-filled cavity of the cheliped, itself continuous with the haemocoele 
of the trunk. Further, each bristle is beautifully pivoted so that it can 
swing freely, but only in one direction. Apart from the pivot the gap 
between the bristle and the cheliped cuticle is spanned by cuticle, thin in 
part but, opposite the pivot, thrown into a fold. Now, if haemocoelic 
pressure is locally increased, pressure in each bristle increases and, because 
it is pivoted, it swings in a predetermined direction until it can go no 
further. Thus is the fan expanded. At this point the convoluted cuticle is 
straightened out and is under tension. When the pressure is reduced this 
bit of cuticle acts as a spring, returning to its previously convoluted form 
and, in so doing, pulls the bristle back to its 'closed' position. Extension of 
the bristles is thus effected by a hydraulic mechanism, closure by the 
utilization of energy stored in a spring. It is easy to see that the fan will 
remain expanded so long as pressure is maintained. It is also easy to see 
that, by employing such a mechanism, all the bristles of a cheliped extend 
simultaneously and at the same rate. Each cheliped can, however, operate 
independently of its fellows. 
Notwithstanding the fact that these prawns take great care of their long 
and slender bristles there is always a risk that one may be broken. Should 
this be so leakage of body fluids would be a potentially serious hazard were 
it not for the fact that a simple but effective safety device prevents such a 
DIRECTOR'S REPORT 53 
Food collected by Atya and its relatives, whether by sweeping or by 
passive filtration, is passed by the chelipeds to the mouthparts which are 
exceedingly complex. Even minute particles can be swiftly and efficiently 
removed from the chelipeds. These are then manipulated by various 
processes, including one akin to teaselling, and transferred to mandibles 
that display a curious blend of primitive and specialized attributes in both 
anatomy and musculature. Of them it is possible to say that they retain 
features to be seen in some of the most primitive crustaceans on which 
have been superimposed specializations as remarkable as any to be seen in 
the Malacostraca. 
Many large decapods, such as crabs and crayfishes, seize large masses of 
food which are sheared into manageable lumps by the mandibles - used in 
atyids for sweeping fine particles into the oesophagus and scarcely at all 
for grinding - and store them in the fore-gut. Here they can be masticated 
at leisure by stout 'teeth' (ossicles) developed in its walls which are pulled 
together by powerful muscles. Being feeders on fine particles A tya and its 
relatives have no need of such a device but face different problems, namely 
those that involve the manipulation, sorting and posterior transfer of these 
particles. Here therefore the ossicles are elaborated as complex combs and 
sweeping devices. Further, like other decapods, atyids have, as an integral 
part of the fore-gut, a complicated structure called a gland filter which 
allows the entry of only exceedingly fine particles and material in solution 
which passes ultimately to the hepatopancreas - here an organ of both 
secretion and absorption. 
Study of these animals in both field and laboratory has not only thrown 
light on the way in which they are constructed and how they utilize the 
complex structures that they have evolved, but has given a glimmer of 
understanding of the way in which they manage to live together. All eat 
basically the same kind of food - detritus derived from leaf litter - but the 
various species collect it in different ways and from different habitats 
within the streams. They also manipulate it in different ways though this 
may be irrelevant from the ecological standpoint. 
Life histories are inadequately known. In A. innocous at least, mating is 
successfully accomplished in aquaria and fertile eggs are then carried by 
the females. These produce zooea larvae that have never been reared 
beyond the age of 10 days, but given spacious conditions this should be 
possible. Their fate in nature is not known but one doubts whether a 
period of marine life is necessary, as evidence of successful reproduction in 
an ornamental pond was obtained in Dominica. 
The two species of Atya are eaten in large numbers by the people of 
Dominica, as are the local palaemonids, and are an excellent source of very 
palatable protein. Basketfuls of such prawns are sometimes collected from 
suitable reaches, or pools on streams that can be dammed and drained to 
facilitate this, and casual collection is practised elsewhere. A possible role 
for these prawns elsewhere is the utilization of detritus in heated power-
station waters. In themselves they are innocuous, pacific animals and the 
52 FORTY-FIFTH ANNUAL REPORT 
FIG . 2. (a) Longitudinal section through the base of a cheliped bristle of Atya scabra. (b) Semi-diagrammatic transverse section of such a bristle at the level x—x of figure 2(a). 
tragedy. Not far from its base the lumen of the bristle is occluded by a 
chitinous plug (Fig. 2) so that the long distal portion, while hollow, is 
completely cut off from a shorter basal region. Thus, if broken anywhere 
in the most vulnerable region beyond the plug, no loss of fluid ensues. The 
plug is sufficiently far from the base not to interfere with the mechanism 
for extending the bristle. 
54 FORTY-FIFTH ANNUAL REPORT 
only safeguard necessary before introducing them to such waters would be 
to ensure that they did not serve as hosts to pathogenic micro-organisms. 
They are incapable of surviving an English winter in unheated waters. 
A more detailed account of these prawns, with 121 illustrations, is given 
in a paper published in Phil. Trans. R. Soc. (B), 277, 57-129 (1977). 
Thanks are given to The Royal Society for permission to use, sometimes in 
a modified form, illustrations from this paper. 


The atyid prawns of Dominica

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The atyid prawns of Dominica

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