Some 80 adult and over 250 juvenile A. tenebrosa, the total natural population on 2.5 m² of a stable rock substratum, were tracked for up to 738 days. In this period 24% of adults and 10% of juveniles moved > 250 mm, Some adults and many juveniles showed no discernible locomotion at all, although most juveniles survived less than 20 days. Large, directed movements of up to 1.6 m were sometimes associated with such external factors as impact injuries from moving rocks or logs, physical shocks, repeated desiccation, interference from molluscs, or wounding from intraspecific aggression. Acrorhagi are the nematocyst-bearing structures, present in most A. tenebrosa larger than about 10 mm column diameter, which are used in intraspecific conflicts. Some 44 % of adults were involved in fights with other adults over the 2 years of observations. None of those fights was fatal. Contact of two adult A. tenebrosa did not always result in an aggressive conflict, but after intraspecific aggression, in both field and laboratory situations, the wounded anemone moved directly away from the site of wounding.
Synonymy and geographical distribution of A. tenebrosa are discussed. Reproductive cycle and fecundity were examined between 1972 and 1975 inclusive at Kaikoura, New Zealand. At any one time, up to 77% of the adult population developed gonads and 20-94% of adults were brooding young. The main periods of gonad development, November-April inclusive, coincided with the warmest annual sea temperatures. Over 99.4% of brooded embryos dissected from 1851 adults were tentaculate young; the rest were planulae. It is suggested that the normal reproductive mode of the observed population is cross-fertilizing labile gonochorism, in which adults change from one sex to the other within each breeding season. Brooding adults would therefore be the maternal parents of their brooded embryos, even though subsequently the brooders would appear to be asexual or could become functional males.
The upper limit of the zone occupied by Actinia tenebrosa is determined
primarily by the effects of desiccation on the juvenile stages after settling. Juveniles
are especially prone to death from these effects at relative humidities lower than
about 70%. This susceptibility is mainly a consequence of high surface area to
volume ratios in juveniles, and is further enhanced by a higher rate of evaporation
from the column walls of juveniles as compared with adults. A rapid rate of evaporation
will cause the anemones to show decreased responses, but within the usual
range of environmental conditions death probably results from a net loss of water
from the ectoderm and mesogloea. Normally the effects of temperature are unimportant,
although adult anemones may move away from areas of unfavourably high
temperatures. During emergence evaporation from the ectoderm may decrease the
surface temperature of adult anemones by over 5°C from that of the surface of the
adjacent substratum. Dark-adapted A. tenebrosa show responses to light stimuli of
high intensity after 20-40 sec, but these are immediate responses and there is no
evidence either of phototaxis or that light affects the intertidal zonation of the
anemones.
In all, 82 adults and about 600 juvenile A tenebrosa were tracked for up to 3 years These intertidal sea anemones were free-living on the rocky coast at Kaikoura, New Zealand Regular measurements of column diameters were taken for 2 years Annual growth increments were small for all sizes of A. tenebrosa observed most increments were in the range -2 to 4 mm of column diameter per year, with the smallest anemones havmg the fastest relative mean growth rates From the growth data, A tenebrosa reaches a column diameter of 40 mm in 8-66 years after settlement From mortality data, the observed adults have a predicted mean longevity of at least 50 years and a predicted maximum longevity of at least 210 years.
An isolated subpopulation of A. tenebrosa was studied, for three years, on the rocky intertidal coast at Kaikoura, New Zealand. The number of adults remained very stable: 94% of adults first seen in November 1973 survived until the observations were concluded. Adults died mainly from impact injuries caused by moving rocks and logs during gales. The number of juveniles in the colony fluctuated markedly from season to season, between 81 and 225 % of the number first seen. Most settlement was in the summer months, but some occurred throughout the year. The main causes of juvenile mortality were, in order of decreasing importance, failure to successfully attach leading to probable predation by benthic invertebrates, desiccation at low tides, dislodgement or crushing by grazing molluscs, impact injuries during gales, and exposure to combinations of excessive desiccation and high temperatures at low tides. Adult mortalities appeared to be independent of size. The rate of mortality of juveniles in their first 20 days after release from brooding adults is largely independent of juvenile size. After that, the mortality rate is negatively correlated with size until juveniles exceed about 15 mm column diameter: the smaller juveniles are more susceptible to both desiccation at low tide and to fatal interference from grazing molluscs.
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