THE ECOLOGY OF PATELLA LINNAEUS FROM THE CAPE PENINSULA, SOUTH AFRICA, 3. GROWTH-RATES

Summary

An analysis was undertaken of the growth-rates of Patella cochlear, P. granatina, P. granularis, P. longicosta and P. oculus. Shell growth is rapid in the first year and then decreases. A negative linear relationship exists between initial length and increment per year. Increases of shell length eventually cease and this may be important in determining longevity. Shell thickness increases throughout life. Body growth is sigmoidal, maximum absolute growth occurring after about a quarter of the life-span is complete.

P. oculus and P. granatina grow very rapidly, P. granularis at an intermediate rate, and P. cochlear and P. longicosta very slowly. These relative rates are probably related to the nutritive value of the different food sources. P. granularis grows faster on the cold west coast than in warmer waters. This may be due to the relative availability and nutrient value of organic depositions. In P. cochlear high densities probably override the effects of other environmental factors, but temperature and wave action were positively correlated with growth.

Only P. longicosta shows significant seasonal patterns in growth. Smaller animals grow slowest in March to November, coincident with gonad maturation and low temperatures. Larger animals decrease growth from November to March, but there is no logical explanation for this. In other species gonad maturation and low temperatures alternate (Kalk Bay), or sea temperatures are relatively constant (Kommetjie), and this may explain the lack of seasonal patterns in growth. Longevity is inversely related to growth-rate on both an interspecific and an intraspecific basis.

Life tables and survivorship curves were constructed for P. cochlear, P. granatina, P. granularis and longicosta. All have a basically ‘diagonal’ survivorship, although in P. cochlear and P. longicosta this is modified by massive mortality at a specific point in the life cycle. This is related to a transition between habitats. The population structure of P. oculus does not lend itself to the construction of life tables, and is strongly suggestive of a biannual settling.