(^) (After Nagasawa 1984.)
(^) Nagasawa was too polite to contradict an old and widely accepted idea put forward
by Kuhl in 1938, that chaetognaths only produce one “brood”, the lifetime egg output
being the oocyte counts in larger ovaries at collection. Rather she redefined Kuhl’s
term “brood” as lifetime egg output, so that spawning only occurs once in that sense.
Clearly, spawning recurs over many cycles of oocyte production, maturation, and
release. The Sagittidae are all free spawning, but the deep-living Eukrohnia
bathypelagica and Eukrohnia bathyantarctica carry their eggs (up to >60 and 6 per
sac, respectively) and young in “marsupial” sacs very similar to those of copepods or
euphausiids. Eggs are extruded into a film-like substance secreted near the gonopores,
forming the sacs. Embryos develop and hatch there and are retained until yolk
absorption is complete (e.g. Kruse 2009). Evolution in mesoplankton groups has
repeatedly favored both free-spawning and egg protection strategies.
Appendicularians are Semelparous
(^) Because they can be maintained in culture, it is possible to measure appendicularian
egg production. A common species in temperate waters is Oikopleuroa dioica that,
unlike most members of the group, has male and female function in separate
individuals. After a growth period of about 10 days at 14°C, ova zip through
vitellogenesis in under one day. Then the eggs burst through the anterior end of the
trunk, killing the mother. Thus, at least one group of zooplankton is semelparous (one
clutch of eggs, then death). Fertilization is external. Fenaux (1976) found a correlation
between egg number and length of the trunk (pharynx, digestive gland, and gonad) in
four Mediterranean specimens: 21 eggs at 800 μm, 94 at 920 μm, 118 at 1000 μm,
and 187 at 1100 μm. Greater fecundity occurs in colder waters, to >500 eggs. Eggs of
O. dioica and many other species are about 100 μm in diameter, an ideal food size for
copepods. Experiments by Sommer et al. (2003), with different concentrations of
copepods in mesocosms suspended in a Norwegian fjord, suggest that appendicularian
abundance may be controlled by copepod predation on their eggs, a notion supported