Triops longicaudatus

Abstract: In the discussion on arthropod phylogeny, the structural evolution of compound eyes and optic ganglia in Crustacea and Insecta is an important topic. On the one hand, many morphological features as well as developmental aspects of the visual system in Insecta and Crustacea correspond in so much detail that eye design in these two groups is likely to have a common euarthropodan ancestor. On the other hand, however, some authors advocate a convergent evolution of the crustacean and insect visual system founding their arguments on differences in the arrangement of the visual neuropils and the fibre connections between Malacostraca and Entomostraca (the "entomostracan enigma"). Therefore, information about cellular aspects of visual system formation in entomostracan Crustacea is likely to enliven this debate, but is not yet available. To fill this gap, we examined the proliferation of neuronal stem cells in the developing visual system of the tadpole shrimp Triops longicaudatus (LeConte, 1846) (Entomostraca, Branchiopoda, Phyllopoda, Calmanostraca, Notostraca) by in vivo incorporation of the proliferation marker bromodeoxyuridine and subsequent immunohistochemical detection. Our results indicate that in the developing visual system of T. longicaudatus, three band-shaped zones containing neuronal stem cells are present corresponding to the proliferation zones found in Malacostraca. We therefore conclude that the ontogenetic mechanisms of visual-system formation are evolutionarily conserved (homologous) in Branchiopoda, Malacostraca, and Insecta.

Abstract: The side effects of candidate mosquito control agents against nontarget organisms were investigated in the laboratory and outdoors in artificial containers, intermittent ponds, and irrigated pastures. The compounds used were TH 6040 [1-(4-chlorophenyl)- 3-(2, 6-difluorobenzoyl)-urea], H 24108 [3-butyn-2 yl N - ( p -chlorophenyl) carbamate], and Altosid® [isopropyl 11-methoxy-3, 7, 11-trimethyl-2, 4 dodecadienoate]. In the laboratory, crustaceans, especially the tadpole shrimp, Triops longicaudatus LeConte, clam shrimp, Eulimnadia spp., and water fleas, Daphnia and Moina spp., were susceptible to TH 6040 at levels below 0.01 ppm. Copepods, Cyclops and Diaptomus spp., showed some tolerance at 0.01 ppm, whereas seed shrimp, Cypricerus and Cypridopsis spp., were tolerant to as much as 0.5 ppm. Among aquatic insects tested, mayfly nymphs, Callibaetis spp., and chironomid midge larvae, Goeldichironomus holoprasinus (Goeldi), were sensitive. However, beetles, Thermonectus basillaris (Harris), Laccophilus spp., Hydrophilus triangularis Say, and Tropisternus lateralis (F.), demonstrated a strong tolerance. Mosquitofish, Gambusia affinis (Baird and Girard), also showed no effect at high dose levels. In artificial containers, TH 6040 suppressed the cladocera and copepod populations, but they recovered again. There was no effect on seed shrimp and the sideswimmer, Hyalella azteca (Saussure). TH 6040 also suppressed reproduction of the backswimmer, Notonecta unifasciata Guerin. There was no adverse effect on the backswimmer and mosquitofish colonies when the SR-1O formulation of Altosid was applied. In field tests, TH 6040 suppressed populations of tadpole shrimp, clam shrimp, water fleas, and copepods; but these recovered soon. Adult emergence of mayflies and aquatic midges was affected by TH 6040 treatments. Some affected midge pupae were noticed when Altosid was applied.

Abstract: The Notostraca are phyllopod Crustacea in which the carapace is shield-shaped and covers the anterior part of the body; together with other phyllopods, they are considered to be relatively unevolved Crustacea-there is a high, but irregular, number of appendages (forty to sixty pairs are usual) and the segmentation of the body is anameristic. It is characteristic of phyllopods that they occur in temporary rain pools, generally in arid or steppe country where such pools are common, and are unable to maintain themselves in the face of competition from higher Crustacea or Insecta (Ard6, 1947). The survival of such primitive and vulnerable animals poses an interesting problem-the key to which may lie in their main adaptation to a temporary habitat, the drought-resistant egg. The possession of such an egg, from which hatching normally occurs only after desiccation, is an obvious prerequisite for the exploitation of a temporary freshwater habitat by a group of animals which do not have the capacities of aerial migration which the Insects possess. These drought-resistant eggs of phyllopods are the only possible means of dispersal for the group, for the pools in which the animals occur are isolated from one another, and are only rarely part of a stream system along which dispersal of adults would be possible. The eggs are small, light in weight because of their dry alveolar shells, and are sticky when laid in at least one genus (Triops, Notostraca); further, the eggs of some Anostraca are known to retain viability after passing through the guts of Amphibia (Mathias, 1937). These facts suggest several possible means of dispersal-in or upon larger animals, attached to dry pieces of wind-blown vegetation, or airborne singly in wind currents (a desert dust-devil will whirl particles much denser than the eggs of phyllopods high into the air). That 'these passive means of dispersal are effective is quite clearly shown by the ubiquitous occurrence of phyllopods in suitable pools wherever these occur. Notostraca occur on such isolated oceanic islands as the Hawaiian group, and in the few suitable pools that the rainy English climate permits. The phyllopods show another characteristic of passively distributed invertebrates-that of very widely spread, sometimes almost cosmopolitan species; in a recent study of the Notostraca for a systematic revision I found that Triops longicaudatus, which Linder (1952) showed was the only species of the genus in the Americas, extends from the West Indies through Central America and right across the Pacific to Japan by way of the Galapagoes, Oahu and New Caledonia. In all this area there is virtually no morphological change (Longhurst, 1955). These facts of dispersal and geographical distribution seem to me to explain the survival of the phyllopods. The initial arthropod invasion of temporary pools may well have been performed by the primitive fresh-water Crustacean fauna and has radiated into the forms of phyllopods we see today-the carnivorous Notostraca, the "pelagic" Anostraca, and the burrowing, detritus-feeding Conchostraca. This early fauna would have spread and occupied at least a high proportion of the suitable pools, and such a cosmopolitan occupation would have effectively prevented any geographically iso-

Abstract: Field and laboratory evidence demonstrates that differences in tolerance to salinity and temperature of phyllopods notably affect their habitat types and seasonal occurrence. Oxygen concentration and pH are of lesser ecological consequence. The 11 phyllopods studied are categorized according to their seasonal occurrence and habitat salinities. For example, based on the tolerance of both eggs and adults, Branchinecta lindahli is eurythermal and euryhaline, whereas Triops longicaudatus is stenothermal and stenohaline. Though some adult phyllopods endure substantial ranges in salinity and temperature, their eggs are not necessarily so adapted. In some instances egg hatchability is impeded by environments not hostile to adult shrimp. Phyllopod eggs are, therefore, instrumental in defining when and where a particular species occurs. Ephemeral ponds with moderately high magnesium concentrations may be hostile to some phyllopods. See full-text article at JSTOR