The behaviour of the Pacific oyster Crassostrea gigas in polluted and unpolluted environments

Abstract

Several aspects of the behaviour of the Pacific oyster Crassostrea gigas inhabiting areas of different environmental quality were studied. The study of the reproductive behaviour gathered information related to the cyclic fashion in which the oyster accumulates reproductive material throughout the duration of the study. Histological examination of reproductive tissue in C. gigas indicates that oysters in three sites in the Mahurangi estuary tend to concentrate their eggs around the three main sections covering the gut area (hind, mid and fore gut). During winter, a withdrawal of eggs toward sections located within the mid-gut area occurred. Oysters collected in the Half Moon Bay Marina sites show eggs distributed evenly throughout the mantle; this distribution shows virtually no change between seasons. Quantitative analysis of egg numbers shows a strong seasonal tendency in egg production and egg size density of C. gigas in the five study sites. In these sites there is an accumulation of eggs throughout the year. Large concentrations of eggs appeared in summer and conversely lower concentrations during winter. From all the studied oyster populations, only one population showed periods of reproductive resting in which immaturity was recorded during the winter. Overall, the production of eggs in the five study sites was cyclic however, the colonies in the Mahurangi estuary showed larger numbers of eggs compared to those oyster populations inhabiting areas in which pollution was suspected. Egg size density was also noted to differ between the two main study sites with oysters in the Mahurangi estuary showing much larger eggs than those in the Half Moon Bay Marina study sites. In the Mahurangi estuary, oysters exhibited constant changes in egg size density with small eggs showing high densities throughout the year. Large eggs were always less numerous and their absence in certain months suggests the likelihood of spawning in late summer and early autumn. Oysters in Half Moon Bay Marina show evidence of levels of tributyltin (TBT) by the presence of deformed chambers in their shells. These oysters were characterized by smaller eggs and the tendency to retain specific egg size without noticeable change in size for several months. This condition suggests uncompleted oogenesis or immaturity and is probably associated with the poor environment of those sites. C. gigas also showed differences in sex ratio between populations, females being predominant in large colonies and males dominating small ones. Such differences appear to be related to oyster density. Several aspects of the settlement and recruitment of C. gigas were studied. Using wooden traps, larval spatfall was recorded in an oyster farm in the Mahurangi estuary. Recruitment was recorded at three depth levels for one year and at one depth level a year later. A seasonal pattern in recruitment was observed at all levels with a slight preponderance for oyster larvae to recruit at the middle level (2 m at high tide). In addition, the recruitment of the barnacle Elminus modestus was also recorded. The barnacle also shows a seasonal pattern in recruitment with some depth preference, however this factor was not consistent through time. When examining data on recruitment for both oyster and barnacle larvae, a significant positive recruitment correlation was found in both plankters. Oyster larval recruitment was also observed on a daily basis for long time periods in the Mahurangi estuary oyster farm. The results indicated a distinctive semilunar rhythm in larval recruitment consistent for two consecutive years surveyed. The greater number of larvae settled on spring tides. Experiments were then conducted to observe larval settlement on a hourly basis. This settlement tended to be concentrated at night and at high tide, even when spat collectors were submerged at low tide. Manipulation of light regimes in the field was achieived by covering some of the spat collectors with black polythene. Under such circumstances oyster recruitment showed to be photonegative and skototactic. A model for the settlement of oyster larvae is proposed. The first component in the model assumes that the larvae use tidal currents to maintain themselves close to the shore, utilizing the large volume of water associated with spring tides to disperse. The second component of the model invokes a switch from a semilunar mechanism to a circatidal one by which the larvae attain settlement. An ethological approach was used to study different aspects of the oysters' rhythmic behaviour. Field observations were conducted on the duration and frequency of gaping behaviour in the Mahurangi estuary and in Half Moon Bay Marina. Animals were observed for extended periods of time during day and night. Observations revealed that gaping takes place during day and night and individual oysters show variations in gape width. A quantification of the duration of gape state for three consecutive years show oysters in the Mahurangi estuary opening their valves 170 minutes during the four hours of immersion; this frequency shows no apparent differences between years, seasons or day and night. The oysters observed in Half Moon Bay Marina exhibited a reduction in the duration of gape. These oysters opened their valves 47 minutes during the four hours of immersion and showed a decrease in gaping activity during winter. The effect of tidal cycles on gaping activity was also investigated. It was noted that in the Mahurangi estuary a large percentage of oysters gape during the first three hours of immersion, with most animals closing their valves one hour before the tide recedes. No differences were noted in this behaviour between day and night or periods of spring or neap tide. Oysters in Half Moon Bay Marina gape at different hours during immersion with no polarization in gaping in relation to the beginning or the end of the tidal period. The frequency in faecal production was investigated in an oyster farm in the Mahurangi estuary for three consecutive years. A random population of 30 animals was randomly monitored for faecal production. Observations were made at day and night time, at periods of spring and neap tide, and during summer and winter. Results show animals producing more faecal material at night and during summer. No differences were found in faecal production between periods of neap and spring tide. An examination of rhythmic aspects in the field revealed an ultradian pattern in oyster gaping behaviour. Individual animals were monitored during periods of immersion at day and night time. Results show that intertidal oysters exhibited an ultradian rhythm in valve closure of mostly 7.5 minutes of frequency. Subtidal oysters show an ultradian rhythm of 12.0 minutes of frequency. Translocation of 12 subtidal animals to the intertidal resulted in a reduction in the frequency of the period, with translocated animals displaying a frequency typical of intertidal oysters (7.5) after being subjected to two periods of exposure. While testing ultradian rhythms in the laboratory, it was found that only animals subjected to artificial tide cycles were able to sustain ultradian rhythmicity. Animals kept constantly submerged became totally arrhythmic. Endogenous rhythms of activity were tested in the laboratory. Animals were tested for gaping activity in an activity recorder for periods of up to 100 days. Animals showed free running rhythms for long periods of time. These rhythms were characterized by longevity, spontaneous changes in the FRP (resulting in a zig-zag appearance), rhythmic clarity and independence of food availability. The effects of tributyltin (TBT) on translocated healthy C. gigas was investigated in two Auckland marinas. Healthy oysters were placed in cages located at 1 station in the marina and two stations at 1 km and 3 km away from the marinas respectively. Oysters were inspected on a monthly basis for TBT induced chambers and mortality. TBT chambers were recorded in all the stations two months after translocations were found. The oysters located closer to the marinas developed more TBT induced chambers and had higher mortality rates than stations located at some distance from the moorings. An examination of gaping frequency in TBT affected oysters showed that while immersed oysters with more than five TBT induced chambers exhibited frequencies of about 20 minutes in gaping behaviour this frequency contrasted with oysters having less than five TBT induced chambers which exhibited frequencies of about 80 minutes. Oysters showing no evidence of TBT induced chambers exhibited frequencies in gaping of about 160 minutes.