The reproductive cycle of the sea urchin Arbacia lixula in northwest mediterranean: potential influence of temperature and photoperiod

We studied the reproductive cycle of the sea urchin Arbacia lixula in a subtidal population from northeast Spain over four years using a gonadosomatic index (GSI) and gonad histology. Our results show that the GSI of A. lixula follows a seasonal cycle which peaks in May-July and attains its lowest values in October-November every year. The time course of the GSI matched closely the photoperiod cycle. We also found a remarkable inter-annual variability in the maximum value of GSI, which correlated with mean water temperature during the gonad growth period (winter and spring). Gonad histology was also in agreement with a single gametogenic cycle per year in this species. We explored the application of circular statistics to present and analyse gonadal development data, which allowed us to adequately handle the high intra-individual variability detected, with several developmental stages commonly found within the same gonad. The picture that emerged is one of a gametogenic timing driven by photoperiod, while the amount of reproductive output is determined by temperature. This is coherent with the tropical origin of the species and lends support to recent warnings about an increase in the abundance of this species in the Mediterranean as a result of global warming, with associated increased impact potential in sublittoral communities

One hundred years of inorganic crystal chemistry – a personal view

It was suspected by Leucippus (fifth century BCE) that atoms had to be ordered, but no relationship between atomic order and crystals has been handed down to us from antiquity. In modern times, beginning with Kepler, 1611, crystals were thought to be composed of ordered small particles, but not necessarily atoms. This changed with Dalton, ca. 1800. Barlow made in 1883 a successful prediction of alkali halide structures, while Alfred Werner understood in 1893 already coordination compounds and coordination numbers. By the time of the experiments of the Braggs, 1913, atomism was mostly accepted. The first important crystal chemistry paper was published in 1920 by W. Lawrence Bragg, the son: he found that interatomic distances in crystals obeyed an additivity rule. In 1926, Goldschmidt had amassed a large amount of crystal structure data and deduced from that what he called the laws of crystal chemistry, thus he became the founder of crystal chemistry. On the basis of W. L. Bragg's crystal structure determinations and of Goldschmidt's laws Machatschki unravelled in 1927, the principles of the constitution of the silicates, a problem that had vexed mineralogists and inorganic chemists during the nineteenth century. In 1928/1929, Pauling published his famous rules concerning the principles determining the structures of complex ionic crystals. His second rule, the electrostatic valence principle, which essentially says that the charges of the ions in a crystal structure are balanced locally, was particularly useful. The first book on crystal chemistry was published by Hassel in 1934. Wells showed beginning in 1954 in a series of papers and books that one could systematize crystal structures on the basis of three-dimensional periodic nets of bonds. After the 1950s, the increasing accuracy of crystal structure determinations made it possible to look for interpretations of bond length variations and distortions of coordination polyhedra. Barnighausen constructed family trees of the group-subgroup relationships of topologically related crystal structures. A general method for predicting crystal structures of inorganic compounds from a knowledge of the chemical composition alone is not yet available. Empirical crystal chemistry remains a valuable tool for searching for methods of representing and classifying structures