32
R. H. ‘MTT.T.bR
FIQ. 16. HypGtozoa jaameriana. Annual cycle of sexual and asexual reproduction
(redrawn from Brewin, 1966af.
laboratory studies raise doubts about how far the observed events
correspond to those taking place in nature. A few observations indicate
the longevity of certain species (Table 11).
The causes of death in compound forms, other than by the activity
of predators (see p. 48) are little known, but presumably are similar to
those applying to simple ascidians. Intraspecific competition may sometimes eliminate the less vigorous specimens, according to Bancroft
(1903), who also noted that colonies of Botryllus lost vigour and shrank
before dying, apparently from old age.
A further cause of death, which affects compound and solitary forms
dike, is detachment from the substratum, when the animals may drift
away to unsuitable conditions or be cast ashore. Schwartz et al. (1960)
studied Aplidium cowtellatum (Verrill) in Sinepuxent and Chincoteague
Bays, which are shallow landlocked areas on the east coast of the U.S.A.
Specimens were common but unevenly distributed, and in the more