Art. XIX. — Notes on the Occurrence and Habits of the Fresh-water Crustacean Lepidurus viridis Baird.
[Read before the Philosophical Institute of Canterbury, 6th December, 1916; received by Editors, 30th December, 1916, issued separately, 16th August, 1917.]
In some of our rain-water pools formed in gravel or shingle pits during the spring rains the interesting crustacean Lepidurus viridis occasionally makes a sudden appearance. For a few weeks it flourishes and may be collected in large numbers, then as the hotter months of summer come the pool dries up and the animal disappears, to reappear perhaps in a similar way at the same time of the following year, or, it may be, to be seen no more in that locality for several years. In this paper are recorded observations made recently on collections of living specimens kept for two or three weeks in the Biological Laboratory at Canterbury College, and the information available from previously published records is collected. These records are apparently very scanty.
Lepidurus viridis belongs to a group of the crustaceans commonly known as the Phyllopoda, and is interesting not only on account of its size and structure, but also because Bernard (1892) attempts to make of the group a connecting-link between the Annelida and the higher types of Crustacea. The Phyllopoda (so called because the numerous appendages are membranous and foliaceous, thus serving the double purpose of limbs and respiratory organs) is further subdivided into families, to the lowest of which—namely, to the Apodidae—this genus belongs. This is a small family consisting of only two genera, Apus and Lepidurus. These agree in having a long segmented body, from ¾ in. to 2½ in. or 3 in. long, partly covered with an oval shield-like dorsal carapace, and bearing at the posterior end of the body two jointed appendages, or “cercopoda,” which vary in length. The chief difference between the two genera is the presence in Lepidurus of a long produced telson and a larger carapace (Packard, 1882, p. 316). This large plate or caudal lamella of the telson is shaped “like a beaver's tail, and must give it an advantage over Apus in extricating itself from muddy places” (Packard, 1882, p. 380).
The species Lepidurus viridis was described and figured by W. Baird (1850, p. 254). His description was based on the examination of one animal from Tasmania, 2 in. long and 1 in. broad, “of a fine green colour.”
In 1866 two more specimens were sent to Baird from “rain-pools on the Gawler Plains, north of Adelaide, South Australia” (1866, p. 122). This animal was just 1 in. in length; the caudal setae were rather more than half the length of the body, and in spirit the carapace was a pale horny colour. Baird therefore made of it a new species, Lepidurus angasii
G. M. Thomson (1879, p. 260) described two new species based on specimens obtained from three different localities in New Zealand. The first, L. kirkii, from Wellington, was pale olive-green in colour, with carapace broadly oval, very membranous, caudal setae more than half the length of the body, total length 1.25 in. The second, L. compressus, from Waikouaiti and Queenstown, Otago, was dark olive-green, with carapace oval and more or less arched, caudal setae not half the length of body, and a total length of only 0.8 in. Thomson expressed some hesitation in forming these new species, and considered it likely that they, with L. angasii Baird, might well be only varieties of a very widespread species, Lepidurus viridis.
Tate described (Proc. Roy. Soc. S. Aust., 1878–79, p. 136) a species which he called Lepidurus viridulus. I have not been able to consult this paper, but probably the species is only a variety of L. viridis, being given as a synonym by Sayce (1903, p. 242).
Spencer and Hall, in their Report of the Horn Expedition to Central Australia (1896, pt. ii, “Zoology,” p. 234), mention Lepidurus viridis as being found near the coast and not inland.
In 1903 Sayce mentions the species in his list of Phllyopoda of Australia, and gives synonyms (1903, p. 242); and it is again cited in Hutton's Index Faunae Novae Zealandiae (1904, p. 267).
Sayce (1903, p. 242) has been followed in classing L. angasii, L. kirkii, L. compressus, and L. viridulus all as synonyms of L. viridis Baird. The species, therefore, appears to be widely distributed in Australia and New Zealand.
The earliest recorded specimen, as already mentioned, was from Tasmania (Baird, 1850). Sayce (1903, p. 242) gives its distribution thus: “Inland and coastal areas of N.S.W.; northern and southern areas of Victoria; southern area of South Australia; Tasmania; and New Zealand.”
The only published record of the species given in New Zealand is that given by G. M. Thomson (1879, p. 260), and he gives the habitat as Wellington, Waikouaiti, and Queenstown.
In the collections of the Biological Laboratory of Canterbury College are specimens received from Ashburton (October, 1902), Cashmere (December, 1904), Linwood (September, 1916), Springston (October, 1916). Dr. F. W. Hilgendorf has found them at Lincoln, and Mr. G. E. Archey in pools near New Brighton.
Occurrence and Habits.
Lepidurus viridis, in common with most of the Phyllopoda, is found in stagnant shallow water in pools sometimes “only a few square yards in area” (Calman, 1909, p. 161). These pools are formed by the spring rains in gravel-pits or clay mudholes. Dr. Hilgendorf has found specimens of Lepidurus at Lincoln in very small holes formed by the hoof of a horse or cow. The sudden appearance of these animals is due to the rapid development of the “winter” or “resting” eggs. Their rate of growth must be very great, for Spencer and Hall (1896, p. 228) speak of collecting full-grown specimens of the allied genus Apus, 2 in. or 3 in. in length, “certainly not more than two weeks after a fall of rain”; and Packard (1882, p. 328) speaks of Apus himalayanus, a species found in North India, as being collected from a stagnant pool in a jungle, four days after a shower
of rain had fallen, following a drought of five months. The animals make the most of their brief existence by producing numbers of parthenogenetic eggs, so that the pools swarm with animal life for a short time, and then, to preserve the species, the “winter” or “resting” eggs are produced, which can resist desiccation for many months.
As the pools dry up in the hotter weather the density of the water increases considerably, but these animals seem adapted to withstand such increase. This was noticed of the living specimens kept in the laboratory. The water was not renewed at all, and there was a decided scum formed on the surface of the water, possibly of calcium carbonate from the water. The animals did not seem to suffer in any way from this.
Spencer and Hall (1896, p. 229) have recorded of Apus a tendency on the part of the animals, as the water receded in the pool, to swim out towards the shallower edges and bury themselves in the mud, instead of keeping in the deeper parts of the pool. Probably, as is suggested, the eggs are protected in this way, although the animals themselves die.
When once the pool is quite dry the mud may remain caked and hard, and the eggs therefore stay embedded in it till next rainy season and then develop. Sometimes it may happen that the mud is broken up into dust through various agencies, and the dust so formed scattered by the wind. Then the eggs are carried from this spot to one far distant, and next year there may be no sign of the animal in this particular pool, though in another locality where it was formerly unknown it may be found in abundance. This accounts for its intermittent appearance. Wind is not the only means of dispersal of the eggs. Wading-birds and water-birds may remove them in the mud which cakes on their legs, and so carry them to another of their haunts.
It seems necessary, however, that the eggs should be subjected to longer or shorter periods of desiccation for their proper development. “The eggs should be dried first and afterwards placed in water. Many eggs float when placed in water after desiccation, the development taking place at the surface of the water” (Weldon, 1909, p. 32).
All the attempts to hatch out the eggs in October, 1916, in the laboratory have been unsuccessful.
In New Zealand Lepidurus viridis usually makes its appearance about September or October, though those collected at Cashmere bear the date December, 1904. There is, however, no further record of them. Mr. G. E. Archey tells me that in 1912 he saw in shallow pools near New Brighton many specimens of Lepidurus swimming about under a thin coating of ice, but he has kept no record of the month.
The first collection brought alive into the laboratory consisted of about eighteen animals from Linwood on the 23rd September, 1916. Six of these were placed in a large aquarium-jar containing water about 3 in. deep, together with a small quantity of river-weed, Elodea canadensis. The jar was left in a window facing south. Most of the specimens were females bearing a large number of deep-orange-coloured eggs in the ovisacs. Some of these eggs were discharged into the water, and it was noticed that the animals that discharged them died very shortly afterwards. These conditions were evidently not suitable, for all the animals were dead within five days.
The greater number of this first collection was left in a large enamel bowl on a bench in the centre of the large laboratory away from direct
window-light. A small quantity of mud from an aquarium-jar was also added, as no direct information concerning the proper food could be found. On the mornings of the 25th and 26th four cast exoskeletons were found in the water, only one of them, however, being perfect. All these animals lived for over a week, then began to die off, and were removed as soon as they were noticed to be dead. The last one died on the 13th October, having lived in the laboratory just three weeks.
On the 14th October another larger supply of Lepidurus viridis was brought in from Springston, and all were put together into a large enamel bowl with plenty of mud. These specimens were much smaller than those of the last collection, and varied considerably in size and colour. The number was roughly estimated to be about thirty-five. On the 18th nine specimens were put in another smaller bowl in clear water. On the following day one animal (A) was noticed to be dead, and on closer observation one side of its carapace was seen to be very irregular in outline. Another animal (B) was apparently dying, lying on its back and moving its legs very feebly. Two hours later another animal was noticed in close proximity to A, and when it moved away more than an hour later the other side of the carapace of A was quite broken and irregular. Before long A was visited by two others, and by that time more than half of it had disappeared. The suspicion aroused by the great decrease in numbers since the collection had been brought in was now confirmed. There could be no doubt that the animals, deprived of their natural food, were eating the bodies of their fellows. As this promised to be interesting, four others were taken from the large bowl, in which the water was too muddy for observation, and placed in a third bowl of clear water.
The numbers were carefully counted next day, with the following results: The original bowl (I) now contained seven animals; in the second bowl (II) only seven out of the nine put in two days before were alive, nothing was left of A, and only part of the head of B; in bowl III there were still four, but one of them was lying on its back, moving its legs very feebly, most active whenever any other animal came near it, as though conscious of its danger. This made a total of eighteen left out of the original thirty-five after six days. There could be no doubt about Lepidurus being carnivorous. On each of the following days the numbers were counted and were found to be less and less, till on the 27th there were three left, one in each bowl. Of the four originally put into bowl III the smallest was left, a dark-green animal. In the other two bowls the largest were left, and both were mottled green. On the 28th the one in bowl III died; on the 30th that in bowl II was found to be dead; the last one, in the original bowl (I), died on the 3rd November, having lived in the laboratory three weeks.
These animals had, of course, been living under unnatural conditions, and this may have induced this method of preying on one another. References to their natural food are very scanty, there being, in fact, no references to Lepidurus itself, but in describing Apus lucasanus, a species of the allied genus, Packard says (1882, p. 324), “The food of this species appears to be Crustacea, as in dissecting the mouth-parts of one of this species the legs of an Asellus-like crustacean were found partly swallowed. Hence they are quite predaceous in their habits.” Weldon (1909, p. 19) describes the food of the Branchiopoda as consisting of “suspended organic mud, together with Diatoms and other Algae and Infusoria; the larger kinds, however, are capable of gnawing objects of considerable size, Apus being said to nibble
the softer insect larvae and even tadpoles. Many Cladocera may be seen to sink to the bottom of the aquarium with the ventral surface downwards and to collect mud, or even to devour the dead bodies of their fellows.” Bernard, in his work on the Apodidae (1892), endeavours to trace the origin of the Crustacea through the Apodidae from a carnivorous annelid; consequently it is evidently taken as a fact that the Apodidae are carnivorous, but so far no record has been given of their food other than in general terms as given in the quotation above.
The animal when feeding usually lies on its back, with its food above it. All the limbs are kept moving. Whether the posterior ones just serve to keep the animal in its inverted position by their paddling movements, and only those near the mouth are used for preparing the food, cannot quite be ascertained, but this is probably the case. In seeking for smaller pieces of food the animal burrows into the mud with its shovel-shaped carapace, and the mud is passed along the central groove underneath the body. This groove is formed between the closely ranked legs of either side, and closed above by the segmented body. When the animal is on its back the movement of the legs is seen to begin from the posterior limbs and pass on in regular waves to the anterior. Any particles of food would be passed on in this way to the mouth.
The water in the bowls in which the animals were kept was about 1 in. deep. They seemed to prefer to move about ventral surface down, though occasionally they would swim about on their back or on the side. When placed in a bowl of fresh water they would often hang to the surface almost suspended on one side, part of the carapace being out of the water. At other times they moved along at the bottom of the bowl, head down and body inclined at an angle as though standing on the front flattened portion of the head. When they were very active they were most interesting to watch: their movements were very quick; they darted about guiding themselves by the caudal segments, which twisted and turned in every direction as the animal pleased. On one occasion one was seen to turn a complete somersault several times. They did not appear to interfere with each other, except that one would sometimes seem to be nibbling at the caudal setae of another and would be quickly shaken off, the victim swimming away. Still, the gradual and unequal shortening of these tail setae, which was noticed even among the first specimens, before it was discovered that the animals did actually eat the dead bodies of their fellows, must have been due to their attempts to appease their hunger. The length of the caudal setae, therefore, can hardly be considered as a good distinguishing specific character, since accidental shortening may easily take place.
The colours varied considerably amongst individuals brought from the same pool. Those brought from Linwood were perhaps the most uniform in their green colour. Among them, however, were three or four which showed some variation in the way of mottled markings of the carapace, and these proved to be the longest lived. Still more variety was shown amongst the individuals from Springston. These were dark green, lighter green, mottled green, and several small ones were of a uniform pale colour, perhaps the horny colour of L. angasii (Baird, 1866, p. 122). As in the previous case, the mottled individuals were the last to die. Specimens from Melbourne in spirit in the Biological Laboratory show a uniform pale colour. The New Zealand specimens remain green.
The sizes varied considerably also. In the following measurements the total length has been taken from the front ridge of the head to the posterior end of the caudal lamella, the caudal setae, as mentioned before, being frequently broken or eaten off by other animals; where these have apparently not been injured in any way-their measurements have been given also.
[The section below cannot be correctly rendered as it contains complex formatting. See the image of the page for a more accurate rendering.]
|Length, Total.||Length, Carapace.||Length, Caudal Setae.||Breadth.|
Another interesting performance of which some notes were taken was the process of ecdysis, or moulting. As mentioned above, several cast shells were found on certain mornings, the process of casting them having taken place, as is usual in such cases, during the night. One morning, however, a small pale-coloured animal was discovered to be just in the act of casting its exoskeleton, and a close watch was kept. When first noticed it had freed the posterior portion of the body from the old shell, including ten body-segments, the caudal lamella, and the caudal setae, and was then busily engaged in the task of freeing its sixty-odd pairs of limbs. The caudal setae were bent, and appeared to be attached to the lamella. The segments were pale grey in colour, with lighter markings at the division of the segments. The exoskeleton had apparently been torn just about the level of the seventh segment to allow these parts to be drawn out.
At first the animal was very active. Lying on its back, it moved the appendages one after another, beginning with those at the posterior end. After a minute or two of these movements it would jerk the body violently, at the same time moving the tail portion very rapidly round and round. Then the first position was resumed, and again the jerking and twisting followed. The animal seemed to be endeavouring to free its body from the exoskeleton by its series of movements, and apparently tested the results at intervals in its violent struggles to draw some of its limbs out of their covering. While it was watched it made some progress. As far as could be seen with the aid of a magnifying-glass, all the limbs were
finally freed from the exoskeleton, though only about half of them were drawn out. At the end of four hours the movements were growing very feeble, and at the end of the fifth hour, as a fairly long struggle produced little result, the animal evidently was too exhausted to continue. There was just a feeble movement kept up for another hour, and then the struggle was over and the animal dead. With its partially cast shell it has been preserved carefully for future reference.
The following morning, in another bowl, another animal was seen to be dragging round with it an exoskeleton attached to one corner of its carapace. The exoskeleton was rather imperfect, as though in its endeavours to free itself the animal had become entangled in it and broken it. An attempt to assist the animal to get rid of its burden resulted in its death; probably the soft tissues of the carapace were injured. In this case the animal remained on its back, feebly kicking, for more than a day, and then died
In one or two instances the cast shells removed from the water were complete; in other cases the carapace had broken away from the body-cuticle. In one such case, on removing the carapace-cuticle from the water, it was seen to be in two layers, which were joined at the posterior end but completely separated right round the edge of the curve from one posterior angle to the other. In the lower layer was a tear in the tissue to allow of the body being drawn through.
I have not been able to find any record of the method of moulting for Lepidurus or Apus, but Packard (1882, p. 412), quoting from a letter from Dr. Gissler, “who has raised the young Apus from the egg,” says, “I am certain that the larvae of Apus (from skins examined) split across or just in front of the eyes, and with two or three jerks the animal rids itself of the underlying skin.” According to Packard, then, this corresponds to the method of moulting in Limulus.
Most of these lower forms of Crustacea usually swim on their back, a method adopted probably for the protection of the soft tissues of their limbs from the attack of their fellows or other inhabitants of the water Spencer and Hall (1896, p. 228) report an instance of this from Australia: “While watching Apus swimming about, one was seen to come suddenly to the surface struggling violently, and on being caught was found to have three water-beetles tearing its soft appendages These beetles are always darting up and down in search of food, and if the Apus swam with its ventral surface downwards it would probably more often fall a prey to such voracious enemies.”
In the case of the living animals of the genus Lepidurus kept in the laboratory, they were more often to be seen ventral surface downwards, swimming or paddling along in the mud at the bottom of the bowl At first some of them were very active, and their movements were interesting to watch as they darted here and there in the water, steering themselves by the posterior portion of the body, and avoiding as far as possible their fellows. Occasionally they would attempt to nibble at the cercopoda or caudal setae of another, and would then be shaken off
In conclusion, it might be well again to draw attention to the fact that several zoologists have expressed the opinion that the number of the species of Lepidurus would bear reduction. Some of the specific characters used vary considerably in individuals taken from the same locality and even
from the same pool, and are probably only different stages in the development, due to the age of the individual. This is borne out in the study on which this paper is based.
Lubbock (1863, p. 206) notes in the case of a character Baird used—namely, the extent to which the body of the animal is covered by the carapace — that his specimens taken from gravel-pools in the north of France showed a great amount of variation in this respect. In some instances the carapace left seven posterior segments uncovered, in others it covered part of the caudal lamella. He notes that the shape even of the caudal lamella varied.
G M. Thomson (1879, p. 261), at the very time of describing his two new species, expresses as his opinion that these may be only varieties of a widespread species, and would “be inclined to include under one species L. productus Bosc., from Europe, L. viridis Baird, from Tasmania, L. angasii Baird, from South Australia, and perhaps even L. glacialis Kroyer, from North America.”
Spencer and Hall (1896, p. 233), after examining a large number of specimens of Lepidurus, come to the same conclusion, that a comparative study of all the species known from different parts of the world “would result in materially reducing the number of species.”
The specific characters used by different writers appear to be—(1) Size of animal; (2) extent to which carapace covers body; (3) shape and thickness of carapace; (4) colour; (5) shape of caudal lamella; (6) length of caudal setae or cercopoda. Most of these have been mentioned, and have been shown not to be constant characters, but in this paper I make no attempt to discuss the specific distinctions of the specimens examined.
Baird, W., 1850. Description of Several New Species of Entomostraca, Proc. Zool. Soc., 1850, p. 254.
— 1866. Description of Two New Species of Phyllopodous Crustaceans, Proc. Zool. Soc., 1866, p. 122.
Bernard, H. M., 1892. The Apodidae.
Calman, W. T., 1909. Ray Lankester's Treatise on Zoology, pt. vii, Appendiculata; Third Fascicle—Crustacea.
Hutton, F. W., 1904. Index Faunae Novae Zealandiae.
Lubbock, Sir John, 1863. Some Fresh-water Entomostraca, Trans. Linn. Soc, vol. 24, pp. 205–9.
Packard, A. S., 1882. Monograph of the Phyllopod Crustacea of North America.
Sayce, O. A., 1903. The Phyllopoda of Australia, Proc. Roy. Soc. Vict., p. 242.
Spencer, B., and Hall, T. S., 1896. Report of the Horn Expedition to Central Australia, pt. ii, Zoology.
Thomson, G. M., 1879. New Zealand Crustacea, with Descriptions of New Species, Trans. N.Z. Inst., vol. 11, pp. 260–1.
Weldon, W. F. R., 1909. Cambridge Natural History, Crustacea, chap. ii.