Most of us accidentally cut our feet when we walk barefoot on rocky beaches. This is normally first introduction to the barnacles to the visitors of beaches!  The sharp edged shells of barnacles bring tears to our eyes. But if you observe these carefully without cursing them, they are quite fascinating beautiful creatures for a close photograph. The animals live within the shell. You would also notice them on the man-made structures in saline waters like bridges, hulls of ships, etc. Because of this characteristic they are also called ‘fouling' organisms and therefore are problem species. Most of the balanomorph barnacles are hermaphrodites, usually capable of producing eggs and sperms. Exchange of sperms between adjacent individuals, leading to cross fertilization, appears to be the rule among acorn barnacles. These were initially considered as the mollusks because of the presence of shell around the animal but during early nineteenth century, when the larval stages were discovered first, they were then rightly classed under crustaceans. The larval stages include naupliar stages followed by a pre-settling cypris larva.
 Since these animals are found almost world over, they have been of interest to researchers and studied at large for their settlement behaviour, recruitment, biology, ecology, etc. The researchers studying barnacles from the temperate waters indicate that phytoplankton blooms, especially spring blooms, influence larval release. The phytoplankton (especially, Skeletonema costatum and Chaetoceros calcitrans ) is the most favourable food for naupliar larvae. The understanding is that adult barnacles release naupliar larvae only when there is enough food in the environment. The blooms trigger adult individuals to release larvae undergo development to cypris larval stage, and settle down on a substratum to complete their life cycle. In temperate regions, with clear seasonal changes, such a signal can work in favour of release of larvae in food rich environment and successful recruitment at a later stage. All this seems to be highly synchronized. Interestingly, this kind of synchronization is also noticed in other animals like sea urchins and mussels.
In order to find out whether such a synchronization exists in tropical waters too, recruitment process was studied by the scientists from NIO in Indian waters. A commonly found barnacle species, Balanus amphitrite in the typical tropical waters at Dona Paula Bay, Goa, India was studied. Dona Paula Bay is situated at the mouth of the Zuari estuary along the central west coast of India. Rhythmic exchange of marine and estuarine plankton populations takes place in this bay due to perennial connections of this estuary with the Arabian Sea. South-west monsoon influence this area significantly and therefore, a year can broadly be classified into pre-monsoon (February to May), monsoon (June to September) and post-monsoon (October to January) seasons.
Regular observations, data collection and analysis from marked quadrats in this area revealed that the recruitment process is largely non-seasonal. This is so because food (other than phytoplankton blooms) is available throughout the year. In fact, during monsoon, the recruitment was low compared to the non-monsoon months though phytoplankton blooms are reported during the monsoon. Of course, mature barnacles did respond to favourable conditions of food (in the form of blooms) in the environment during monsoon, but the recurrence of unfavourable conditions did not allow all the settled juveniles to continue their life. The scientists also attributed this failure to other physical factors such as currents, turbidity and low salinity during the monsoon.
The food for adult organisms is also an important factor in survival of the larvae released during different seasons. Considerable variation was observed in the larvae collected at two peaks – September and November. The larvae released in November were comparatively larger than those collected in September i.e. immediately after monsoon. These differences also indicate that adult conditioning and inter-brood variation are important factors in the larval ecology of this organism. A principle that healthy parent gives birth to a healthy progeny.
The growth rate of barnacles during monsoon was also noticed to be lower when compared with other seasons. This was attributed to the turbid conditions and low food availability during monsoon. The greater land runoff during monsoon, in the bay, increases suspended load, which interferes with the feeding mechanism and respiratory organs. Higher growth rates after monsoons are attributed to stable marine conditions.
All these observations indicate that the synchronized release of larvae in food rich environments, their settlement and growth may not hold true in all environments. |
