MERCER02-14
3
covering. Planula larvae then settle onto hard substrate and eventually produce amature hydroid
colony.
Planula settle and attach to a variety of solid substrates (stones, wood, rope, concrete). After
attachment, growth is vertical to help facilitate feeding, and then becomes horizontal as the
colonies spread (Vervoort 1964, Stone andWebster Engineering Corp. 1992). Colonies become
reproductive 19-27 days after planula settlement (Turpayeva et al. 1976, Stone and Webster
EngineeringCorp. 1992). Evidence suggests that
G. franciscana’s
growth rates are dependent on
food supply and environmental conditions such as water temperature and salinity (Vervoort
1964, Calder 1990). In temperate latitudes, reproduction has been observed from June to
September as long as temperatures remained above 14 °C (Schuchert 2007). Individual colonies
grow from about 1-10 cm and often twist themselves into ropes over 30 cm high (Schuchert
2007, Lippson andLippson 2006).
In the Chesapeake Bay, near CCNPP, epifaunal growth plate studies (Cory 1967, Abbe 1987)
describe a “yearly climax” community with growth beginning in spring, maximum biomass in
summer, a late-summer die-off and little to no growth during the winter months. Hydroids were
shown settling fromApril to Junewith
G. franciscana
actively colonizing in July and extremely
abundant in September (Abbe 1987). There were a few colony clumps still observed over the
winter months. During the summer growth period, colonies measured 50-65 mm in length on
monthly panels and up to 80mm on 3-month panels (Cory 1967).
2.3. Tolerances
2.3.1. Environmental Conditions
Both laboratory and field experiments and observations have shown
G. franciscana
to tolerate
exposure to freshwater and survive in salinities up to 35 ppt (Crowell and Darnell 1995,
Thompson 1993).
G. franciscana
is highly abundant in oligohaline to low-mesohalinewaters and
thrives best at salinities between 5 - 15 ppt (seeTable 1).
Laboratory and field studies were performed at the Surry Nuclear Power Station (see Section 8
for additional information) to test the effects of low dissolved oxygen (DO) and varying salinities
on
G. franciscana
(Stone and Webster Engineering Corp. 1992)
.
Both sets of experiments
showed that low DO resulted in loss of polyps during at all treatment durations (1, 3, 5 or 7
days), but at all concentrations (6.8, 0.5 and 0.1 mgL
-1
O
2
),
G. franciscana
was able to recover
eventually. Recoverywas faster in salinity of 10 ppt than in 20 ppt. Only under anoxic (0mgL
-1
O
2
) conditionswere colonies killedwith no re-growth.
Thompson (1993) found that
G. franciscana
declined in salinities over 30 ppt, maintained itself
in a salinity of 15 ppt and grewwell in 5 ppt. Studies on a similar species (
Bimeria franciscana
)
showed that hydroids could tolerate short periods of exposure to freshwater and survive in
salinities up to 35 ppt (Crowell and Darnell 1955). It is hypothesized that
G. franciscana
may
exhibit a behavioral adaptation to osmotic stress bymaintaining polyps as buds during exposure
to stressful salinities and reducing the surface area exposed to the environment (Thompson
1993).
