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The heterocystous cyanobacterium Richelia intracellularis Schmidt 1901 is well known for its capability to fix atmospheric nitrogen (N2) in oligotrophic waters. Symbiotic associations of Richelia intracellularis with the diatoms Rhizosolenia hebetata, Rhizosolenia clevei, Rhizosolenia cylindrus and Hemiaulus membranaceus is reported for the first time from the coastal waters of South Andaman, India. In these the symbiotic associations, variations were observed in the number of vegetative cells, trichomes, and the shape of the heterocysts. The highest number of trichomes was observed in Rhizosolenia clevei (14) per host. The trichomes consisted of 9–10 vegetative cells and a spherical heterocyst that was orientiated toward one end of the host diatom cell. In Rhizosolenia hebetata, three trichomes of Richelia intracellularis were present, one trichome at one end with 7–8 vegetative cells and two trichomes at the other end. In Rhizosolenia cylindrus, two trichomes of Richelia were observed, with a spherical heterocyst and 5–6 vegetative cells. An association between Hemiaulus membranaceus and Richelia intracellularis was also observed but with less frequency. Trichomes were observed in the centre of the diatom. By using the cell-specific rates, the amount of new N provided by the Diatom-Diazotrophic Associations was estimated. This is the first record of a symbiotic association of Richelia in diatoms from Andaman coastal waters.

The influence of salinity on growth, survival and byssal fibre attachment of the blacklip pearl oyster, Pinctada margaritifera spat was assessed. P. margaritifera enjoys a coastal distribution in the intertidal reef flats of Andaman and Nicobar Islands, with maximum density at 5 -10 m depth, where drastic changes in environmental parameters such as salinity, temperature and turbidity occur. Spat showed comparatively good growth in terms of instantaneous growth rates (IGRs) of dorso-ventral measurement (DVM), antero-posterior measurement (APM) and total weight (TWT) in a wide range of test salinities from 19 to 37 psu; IGRs were drastically decreased in spat reared at lower and higher salinities outside of this range. The highlight of the present study was better growth performance and 100% survival of P. margaritifera spat at lower salinity levels (22-28 psu) than in the ambient salinity (31 psu). The study also showed that sudden exposure to a higher salinity (above 40 psu) will be helpful for detachment of P. margaritifera spat from settlement tanks in the hatchery to facilitate stress-free spat transfer to sea farming systems. This will be useful in adopting the Japanese technique on post-operative culture for producing high quality pearls from P. margaritifera.

The settlement of hatchery produced spat of the blacklip pearl oyster Pinctada margaritifera was investigated with respect to time of introduction of settlers, type of cultch material and the larval (pediveliger) densities. The study clearly indicated that the ideal time for deployment of spat collectors in the hatchery is when the pediveligers are beginning to appear in the larval rearing tank. In similar larval stocking densities, the settlement was more on the spat collectors which were deployed when the larvae had not metamorphosed to spat. Six different types of cultch materials were tested for their efficiency in three larval densities. The study showed that better spat collection of P. margaritifera can be achieved in hatcheries by providing darkness, and employing dark coloured, rough-surfaced, corrugated and conditioned spat collectors such as the garden shade spiral at higher larval densities of 1.0 nos/ml.

Biodiversity is the fundamental subject of ecology, and its knowledge becomes an integral part for the assessment and conservation of any type of biota, maintains a healthy biosphere and provides direct and indirect value to human beings. Marine environment basically consists of many important biodiversity-rich ecological units. Human beings are highly depending on flora and fauna of the marine ecosystem for the various needs in their life. However, human activities have destroyed the biodiversity in these ecosystems and are mostly responsible for the degradation of all resources directly or indirectly in the earth. Reduction of a single species can affect the energy flow in the various trophic levels and resulted in reduction of biodiversity which might have some serious and long-term effects on the biosphere. To cater the increasing demand for animal protein in the world, catch of fish species are inevitable and leads to overexploitation. Sustainable use of all these resources along with effective execution of strategies and resources management techniques is essential for the conservation of biodiversity marine realm.