A. Ercolini

Zonal recovery mechanisms in talitrid crustaceans

Abstract Zonal orientation in the seashore ecotones particularly that of littoral talitrids depends on several mechanisms, but primarily on the sun compass which compensates for the suns apparent motion (chronometric solar photomenotaxis). The details of this mechanism have been studied in Mediterranean and North European talitrids, but above all in populations where the astronomic conditions (different apparent motion of the sun in different periods of the year) complicate this dependance on the sun compass. Another aspect of the mechanism is its innate component: laboratory‐born young of both Mediterranean and intertropical populations, when exposed to the sun for the first time, show a statistically significant directional tendency which is ecologically correct and in compliance with the original population. Crosses indicate the presence of a genetic control of behaviour with a continuous circular variability, and of a genetic difference in the escape direction of animal populations capable of chronom...

Environmental factors influencing the zonation and activity patterns of a population of Periophthalmus sobrinus Eggert in a Kenyan mangrove

The activity rhythms and zonation of a population of Periophthalmus sobrinus were studied along the muddy mangrove banks of the Tana river delta (Kenya). A transect, perpendicular to the water channel, was subdivided into sections 1 m in width parallel to the river bank. The number of surface-active mudskippers and their zonation was recorded over several tidal cycles. Fishes found resting on their nests were also recorded. The observations were carried out over two periods of 48 consecutive tidal hours corresponding to a spring and a neap tide. Environmental variables such as tidal level, air temperature and relative humidity were also recorded. Periophthalmus sobrinus was active both diurnally and nocturnally and during spring tide all the observed mudskippers were active on the mud surface. At neap tide more than half of the total number of animals sighted were found resting on their nests. During this synodic phase, daytime sightings decreased drastically because the mudskippers took refuge inside the nests. Since P. sobrinus normally tends to avoid water, its mean hourly zonation at spring tide was more variable than at neap tide. Differences in mean hourly zonation were also found between night and day of the spring tide. Activity patterns and zonation of the mudskippers were directly influenced by the synodic and tidal cycles and depended more on environmental factors such as air temperature and relative humidity than on the diel light cycle.

AN OPTICAL DIRECTIONAL FACTOR IN THE SKY MIGHT IMPROVE THE DIRECTION FINDING OF SANDHOPPERS ON THE SEASHORE

SUMMARY Adult specimens of the sandhopper Talitrus saltator Montagu (Crustacea Amphipoda) from populations differing in theoretical escape direction were tested for sun orientation. Tests were performed on the home beach, on beaches with opposite or different orientations, or inland far from the sea. The results show a better orientation performance for animals tested on their home beach than in any other experimental condition. These differences might be due to the influence of an auxiliary optical factor on the sea, which animals can perceive, and which interferes with the accuracy of direction finding.

RESEARCHES ON THE PHREATOBIC FISHES OF SOMALIA: ACHIEVEMENTS AND PROSPECTS

SUMMARY An up to date outline of the morphology, ecology and ethology of Somalia phreatobic fishes, with special regard to the recent contributions, is presented. Guidelines are given for future work with the aim of providing better knowledge of the evolutionary changes related to the adaptation of fauna to a phreatobic environment.

Zonal Recovering in Equatorial Sandhoppers: Interaction Between Magnetic and Solar Orientation

An orienting mechanism based exclusively on a sun compass runs into considerable difficulties in the Tropics (see Braemer, 1960; Lindauer 1960; Ercolini, 1964) on account of the annual change in the sun’s declination.

Direction-finding ability in a mudskipper from the delta of the Tana river (Kenya)

In a mangrove near the delta of the Tana river (Kenya) the capacity of orientation was tested in a mudskipper belonging to the genus periophtbalmus Bloch & Schneider 1801. Release experiments showed that the fish were capable of a zonal recovery. This phenomenon can be interpreted as the initial phase of orientation towards a preferred site. The possible mechanisms that underlie the phenomenon are discussed.

MORPHOLOGY AND RESPONSE TO LIGHT OF BARBOPSIS DEVECCHII DI CAPORIACCO (CYPRINIDAE), MICROPHTHALMIC PHREATIC FISH FROM SOMALIA

SUMMARY Barbopsis devecchii Di Caporiacco, microphthalmia phreatic fish from Somalia, has markedly reduced optic lobes in comparison to a systematically close epigean species with normally developed eyes (Barbus macrolepidotus Cuvier & Valenciennes). Chiasma, optic nerves and retina are always found. Slightly microphthalmic fish show the typical retinal layers and have both cones and rods. In extremely microphthalmic fish, the typical retinal layers are not always recognizable. In response to achromatic light, Barbopsis accustomed to complete darkness proved to be totally photophobic while those accustomed to a periodic cycle proved to be substantially photoindifferent. In response to four wavelengths of monochromatic light, Barbopsis accustomed to periodic cycle with shadow zones in the aquarium proved to be completely photophobic.

OXYGEN CONSUMPTION IN HYPOGEAN AND EPIGEAN CYPRINIDS (PISCES): PUBBLICAZIONI DEL CENTRO DI STUDIO PER LA FAUNISTICA ED ECOLOGIA TROPICALI DEL C.N.R.: CCXCVI

SUMMARY The oxygen consumption in the hypogean cyprinids from Somalia Barbopsis devecchii Di Caporiacco, 1926 and Phreatichthys andruzzii Vinciguerra, 1924 has been examined and compared to the related epigean species Puntius lateristriga (Cuvier & Valenciennes, 1842). Both in the test with a respirometer and under asphyxia, a lower oxygen consumption was found in the hypogean forms, the lowest consumption being shown by the species more adapted to the hypogean environment.

SENSITIVITY AND RESPONSE TO LIGHT IN THE LABORATORY OF THE LITTORAL AMPHIPOD TALITRUS SALTATOR MONTAGU

SUMMARY In the laboratory, the phototactic response of Talitrus saltator Montagu (Crustacea Amphipoda) to monochromatic and polychromatic light is predominantly positive with a clear preference for the shorter wavelengths. In the presence of two monochromatic lights of equal wavelength and differing intensity, this Amphipod prefers the light of greater intensity. It reacts indifferently to both polychromatic and monochromatic polarized and diffused light. During the course of the day, a variation occurs in the positive and negative phototactic response to the green and red bands of the population studied (TFD towards the West).

RESEARCH ON THE NON-VISUAL ORIENTATION OF LITTORAL AMPHIPODS: EXPERIMENTS WITH YOUNG BORN IN CAPTIVITY AND ADULTS FROM A SOMALIAN POPULATION OF TALORCHESTIA MARTENSII WEBER (CRUSTACEA AMPHIPODA): PUBBLICAZIONI DEL CENTRO DI STUDIO PER LA FAUNISTICA ED ECOLOGIA TROPICALI DEL C.N.R.: LXX

SUMMARY Experiments were made on a population of Talorchestia martensii Weber from the sandy beach of Gesira, located 10 km S of Mogadiscio (STFD = 132°; LTFD = 312°) with the aim of controlling whether these Amphipods are capable of a directional orientation in experimental conditions of complete darkness and aridity. These experiments were made with adults collected a few days or immediately before each test as well as with young born and raised in the laboratory. The tests were made in the spring of 1969 in the phase of positive solar declination, both 60 km inland and on the home beach, at various hours during the day. Although of the test conditions were definitely motivating for the animals, no evidence was obtained to support the view that the population studied is capable of correct orientation towards sea or land when visual cues are excluded.