Studies of the biology and ecology of the high shore South African limpet, Helcion pectunculus (Mollusca : patellogastropoda)

Gray, David Richard (1997) Studies of the biology and ecology of the high shore South African limpet, Helcion pectunculus (Mollusca : patellogastropoda). PhD thesis, Rhodes University.

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Investigations were carried out into aspects of the bio!ogy and ecology of Helcion pectunculus along the coait of South Africa. These included studies of the distribution, density and biomass of the limpet at six sites along the east coast plus one site on the west coast; the growth of H pectunculus on both west and east coasts; a comparison of the reproductive biology of the east and west coast populations; the foraging activity and feeding behaviour of H pectunculus and the driving forces behind the rhythmic behaviour of this limpet; the importance of the crevice environment in the biology and ecology of H pectunculus. Helcion pectunculus has a restricted zonation, with the majority of animals residing in crevices in the upper Balanoid zone during diurnallowtides, although individuals were occasionally found in the lower Balanoid zone on shores with a gently sloping aspect. This limpet occurs in higher densities (50-lO0 individuals/m2) on shores which have large numbers of crevices and boulders i.e. quartzitic sandstone shores. On most shores, the ratio of males to females differed significantly from a 1: 1 ratio with the highest ratio being obtained on the west .. coast (3 males: 1 female). At all sites, the populations of H pectunculus exhibited strong sexual dimorphism. Males and females were always found to differ in size, with individuals of < 20 mm shell length generally being male whilst limpets with a shell length of> 22 mm were generally female. Helcion pectunculus grows allometrically, increasing in height faster than length, which is expected of a high shore gastropod mollusc attempting to reduce evaporative water loss. Growth rates were similar on both east and west coasts regardless of the differing oceanographic conditions. The theoretical values of Lmax were also similar being 30.86 mm and 30.71 mm respectively. Micro-growth bands are laid down within its' shell which have the same periodicity as the tidal cycle and these enabled age estimates to be made. Younger individuals were male whilst older animals were female, suggesting that H pectunculus is a protandrous hermaphrodite. Histological examination proved, unequivocally, that this limpet undergoes a protandric sex change, changing from male to female when they are about 2 years old. Both east and west coast popUlations had a marked reproductive cycle, exhibiting two spawning periods a year, one in April and another in November. The possibility that the reproductive pattern exhibited is now phylogeneticallyconstrained is discussed. It is suggested that H pectunculus has evolved a reproductive cycle which will allow its planktonic larvae to utilise the valuable phytoplankton bloom food source whilst using onshore winds to ensure that larvae are not transported out to sea and lost. The number of foraging excursions carried out by individuals of H pectunculus was found to have a significant effect on Gonad Index and hence potential reproductive output. The activity pattern of H pectunculus varied depending upon micro-habitat; animals inhabiting both east and west facing rock surfaces are active during nocturnal low tides whilst animals on west facing rock surfaces are also active during daytime low tides whilst in the shade. Limpets travel further during foraging excursions in winter (X = 85.53 cm) than in either spring (x = 55.7 cm) or summer (X = 48.8 cm) and also during spring low tides (x = 89.8 cm) compared with neaps (x = 40.9 cm). This limpet exhibits rigid homing to a fixed scar within a-crevice and feeding excursions were found to of three distinct phases, a rapid outward phase, a slower foraging phase and a rapid homeward phase. Foraging was always highly directional, with a mean vector which took limpets onto an area of the rockface with the highest microalgal biomass and also the smoothest rock surface. Helcion pectunculus exhibits a free-running endogenous rhythm of locomotor activity with both circadian and circatidal components and it is suggested that this rhythm plays a role in allowing the limpet to avoid unfavourable environmental conditions. The exogenous entrainment factor of this endogenous rhythm was the time of exposure to air in the field. There was found to be an organized distribution of limpets within crevices with smaller, younger limpets being towards the back of the crevice and larger, older limpets towards the crevice mouth. It is hypothesized that juvenile limpets of this species actively select and settle at the backs of crevices responding to chemical cues of adult conspecifics. The crevice refuge supplies the limpets with a stable and buffered environment with higher relative humidities (X = 72.3%) and lower rock surface temperatures (X = 19.7°C) than adjacent exposed rock surfaces (X = 64.5%; x = 22.9°C). Limpet body temperatures were significantly lower in crevic~refuges compared to limpets on exposed rock surfaces. Body temperatures never exceeded the rock surface temperatures. It is suggested that this is the result of morphological adaptations such as shell ornamentation and allometric growth. Light levels above 1000 J.1E.m-2.s-1 inhibited foraging activity in H. pectunculuswhilst limpets subjected to 30-50% shade foraged even during daytime lowtides. This limpet is one of the least tenacious (2.75 ± 0.13 ) of all South African limpets and the possibility that wave activity governs both the activity patterns and homing behaviour of this limpet is discussed. Limpets deprived of a crevice refuge experienced extremely high mortalities, with 45% of the limpets being lost during the first high tide period. A hypothetical model of the hierarchy of exogenous factors controlling limpet foraging activity is introduced and discussed in relation to the results of this study. Finally, it is suggested that in addition to the "migratory" and "non-migratory" groups of limpets present on southern African shores a third group of limpets seem to be present which may be classed as "specialized non-migratory" species. These are species that do not migrate, garden or aggressively fight off like conspecifics. They have overcome the competition for space and food on intertidal rocky shores by adapting to a particular habitat which is exclusive to them alone. From the combined results of these studies, it can be stated that H. pectunculus has adapted physiologically, morphologically and behaviourally to successfully survive the extreme conditions in the upper Balanoid zone.

Item Type:Thesis (PhD)
Uncontrolled Keywords:Helcion pectunculus, Patellogastropod, Limpets, Molluscs, South Africa
Subjects:Q Science > QL Zoology > Invertebrates
Divisions:Faculty > Faculty of Science > Zoology & Entomology
Supervisors:Hodgson, A. N.
ID Code:3284
Deposited By: Mrs Carol Perold
Deposited On:04 Sep 2012 14:19
Last Modified:04 Sep 2012 14:19
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