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Biological Invasions by Marine Jellyfish 247
14.5 The Role of Life-Histories
Studies into the ecology of invasions by hydromedusae and scyphomedusae
are hampered by their complex bipartite life-history: a typically cryptic sessile,
asexually reproducing polypoid stage is followed by a pelagic, sexually
reproducing medusa stage. Depending upon food availability and other environmental
variables, the polypoid stages of scyphozoans can asexually produce
large numbers of planktonic young medusae, or remain dormant for
extended periods, perhaps even years or decades. Such a life-history trait is
not only important as a potential vector (Sect. 14.7), but makes monitoring for
invasions all the more difficult, since polyps are typically inconspicuous
members of a larger fouling community. In addition, similarly to the P. punctata
invasion of the Gulf of Mexico in 2000, little warning would likely precede
the sudden onset of an invasive jellyfish bloom.
Lotan et al. (1992) conducted a study involving the culture and growth of
the asexual polyp stage of Rhopilema nomadica. They determined that the
period from planula settlement, through polyp generation, to the release of
new medusae was on the order of 3–4 months. This would suggest that several
cohorts of medusae could be produced from newly recruited planula larvae
2–3 times per year. Likewise, Phyllorhiza punctata also has a bipartite life-history
involving both medusa and polyp. Rippingale and Kelly (1995) provided
laboratory measured growth rates of polyps, and they report fully grown
polyps after only 2–3 days post-settlement of planula larvae. This may be of
little value to the Gulf of Mexico invasive population, as this P. punctata population
is entirely male (Graham et al. 2003; Bolton and Graham 2004). [Interestingly,
a purported invasive population of Cassiopea andromeda sampled
along fish farm canals on Oahu (Hawaii) by Hofmann and Hadfield (2002)
was also entirely male, though the ecological implications of this observation
were not discussed.]
While the ctenophore Mnemiopsis is holoplanktonic (i.e., no sessile stage),
it has its own life-history peculiarity. Mnemiopsis is a simultaneous hermaphrodite
capable of an extremely high degree of fecundity (Mayer 1912; Reeve et
al. 1989). This ctenophore is capable of reproduction within days of hatching
(Martindale 1987), producing thousands of eggs with very little energy investment
(Reeve et al. 1989). As a result, Mnemiopsis is able to very rapidly
increase its population size in response to higher food concentrations
(Feigenbaum and Kelly 1984; Kremer 1994). Accordingly, an initial invading
population, say contained within ballast water, could be very small (theoretically,
only one), but still lead to a successful invasion.