Download 2.2 Biology of Strombus gigas

Strombus Gigas Shell
2.2 Biology of Strombus gigas
Strombus gigas are large, soft-bodied, marine shelled gastropod
molluscs. They have a thin layer of tissues between the body and the shell, a
mantle, that creates a hard external spiral-shaped shell up to 30 cm in length
from calcium carbonate extracted from the seawater and sediments. This
outer shell develops the distinctive pink coloured flared lip that easily
identifies the species and is why the shell also has a horny periostracum
coating to deter predators.
The body is divided into the head, the visceral mass, and the foot.
posterior
anterior
\
Fig 1.1 Adult female conch without her shell (FWRI, 2006)
The conch head has a pair of tentacles tipped with light-sensitive eyestalks
and a long proboscis radula that has thousands of tiny denticle protrusions for
feeding. The foot, at the posterior, is a pointed, sickle-shaped, hardened
operculum tip used to propel forward in a unique type of hopping locomotion
commonly referred to as “strombid leap propulsion” that enables escape from
predators by breaking up their scent trail (FWRI, 2006). They have a siphonal
canal with an indentation near the anterior end called a stromboid notch.
(Hyman 1967, Abbott 1974).
2.2.1 Ecology of Strombus
gigas
The Queen Conch inhabits the neotropical Atlantic waters of Bermuda,
southern Floridian and Mexican coasts of Central America, in the Gulf of
Mexico, Caribbean Sea region, and off the South America coasts of
Venezuela and Brazil. Strombus
gigas are herbivorous, grazing primarily
on algae, grasses, and floating organic debris and are consequently usually
found in warm, shallow clear subtidal water of oceanic or near-oceanic
salinities settled on sandy substrates, in rocky habitats, on coral reefs or coral
rubble sea floors amongst seagrass and algae (McCArthy,
2007). Strombus
gigas can be found in discrete aggregations up to
hundreds or thousands of individuals, actively selecting these preferable
habitats, with juveniles being most selective due to their dependancy on
habitat requirements such as shallow seagrass meadows (Stoner, 1997).
Adult S.gigas are typically found in at depths less than 100 meters
concentrated in water 10- 30 meters deep due to the photosynthetic light
requirements of algae and plant growth (Stoner 1997). Predators of the
Queen Conch are known to be around 130 marine species, E.g. various
species of mollusc, lobster, turtles, crabs, sharks, rays, snappers and Nassau
Grouper, (Coulston, Culp and Stoner 1999; Randall, 1964; Jory and Iversen,
1983) which is why they bury into the sand to hide as unprotected/unburied
conch are less likely to survive (Coulston 1987). Conchs burying behaviours
show wide variations, possibly related to environmental conditions of water
temp - conch increase burying in cooler winter period (Appledoorn 1985) and wind/sea conditions - conch are more active at high tide as a response to
increased predator activity in the upper intertidal zone (). The increased
amount of attached organisms on the shell of older conch suggests a
decrease in long-term burying activity with increases in conch size (Iverson et
al, 1986).
2.2.2 Conch Reproduction
In the wild, adult queen conch maintain a 1:1 sex ratio in an undisturbed
population, and sexual maturity for males and females occurs by
approximately between 3.5 and 5 years, usually when the flared lip is greater
than approximately 0.5 cm thick (Appeldoorn 1994a; Appeldoorn, 1988b;
Berg and Olsen, 1989; Chiquillo et
al. 1997). Queen conch are dioecious
(McCarthy 2007), and spawning occurs when the male, situated behind the
female, inserts a verge into the female's siphonal notch (Randall 1964). After
receiving sperm from the male during initial copulation, the female retains it
for several weeks prior to spawning (D'Asaro 1965), releasing it whilst laying
eggs in order to fertilize them (Randall 1964). The seasonal reproductive
period increases copulating in the warmer months from mid-March to
November, during both day and night (James & Wood) as a linear function of
bottom water temperature the summer months (Stoner et al. 1992). Water
quality, food supply, a 12-hour photoperiod, and temperature (Stoner 1992;
Shawl 2004) acting as limiting factors all having an affect on individual female
reproduction decreasing egg masses Appeldoorn (1993). Females lay
demersal egg masses in long continuous strands up to 50 to 75 feet long
containing 185,000 to 460,000 eggs in one strand (Sterrer 1986), deposited in
requirement sand substrate (Shawl 2004) at an average rate of 1.5m hr-
1 completing in less than a day (Randall 1964). Spawning can occur six to
eight times during an egg-laying season, which varies depending on
geographic location (Stoner?), but lasts typically 6 - 8 months usually
between March and October. (TABLE ?)
2.2.3 Life Cycle of Queen Conch
The life cycle of queen conch begins by embryonic development that
proceeds rapidly, temperature dependently, after the fertilization of spawning,
reaching the gastrula stage after sixteen hours. The pelagic larvae emerge
from the eggs within 72 hours (McCarthy, )- 5/6 days after spawning (D'Asaro,
1965) also temperature dependant and positively influenced by increased
supply of phytoplankton (Stoner, 1997). By around 12 days they are lobed,
free-swimming veligers, found in open water up to 100 meters deep, localised
in the upper ocean layers above the thermocline, where they drift over 18-40
days in the currents of the upper layers feeding on the plankton (Posada and
Appeldoorn, 1994; Stoner, 1997). During this period there is great potential for
long distance transport and larval exchange by surface currents to deeper
water areas (Iversen, et. al 1990) occuring up to 900 km (Davis
et al., 1993).
Larvae then descend, 17 to 22 days after hatching, settling into the adult
benthic habitats, when induced by settling cues such as type of substrate and
location. (Boettcher and Targett 1996). Larvae then require a specific
environmental stimulus within the substratum and sediment (Davis and
Stoner, 1994) to induce metamorphose response such as the presence of
specific algae foods Laurencia
poitei and the epiphyte Fosliella spp.
found on Thalassia testudinum (Davis). Metamorphosis is usually within
five days of settlement, unique in developmental history as the competence
period is shorter than the precompetence period, instead of equal to or longer
than the precompetence period - they are competent for only 6 days at 28 to
30°C, losing this ability if not induced to metamorphosise (Davis and Stoner,
1994). Short-term competence is ordinarily associated with metamorphosis to
a broad spectrum of cues explaining conch response to a variety of benthic
cues found in juvenile conch seagrass habitats. The larvae reach
metamorphosis between 25 and 29 days turning lobes into feet while the
proboscis develops to about 0.2 cm in length (Sterrer 1992) developing a
small transparent shell within 24 hours called a protoconch, the start of the
adult shell (Antigua Barbuda Environment Division 2006). Again development
shows environmental variation for example larvae of March, May, April and
September have slower development than the larvae of June, July and
August. The survival at settlement averaged 30±5.18% with highest survival
June and July with 38±6.30%, lowest March (22±7.22%) and September
(20±7.02%) (Brito-Manzano & Aldana Aranda 2004).
2.2.3.2 Juvenile Strombus
gigas
Young Queen Conch (<one year) settle in areas of soft sand and remain
buried as they are particularly prone high mortality (63%) from predation
(Alcolado, 1976) and if unburied, conch 1.3-3.7cm long show complete
mortality (Iverson). Very few small conch have been found in nature unburied
(Ray & Stoner, 1995) suggesting that conch may be buried almost
continuously until shell lengths of 5-10cm, when juveniles emerge and
become epibenthic (), some periodically reburying (Appeldoorn and Ballantine
1983; CFMC 1999) possibly to avoid winter storms. In shallow areas, Sandt
and Stoner (1993) documented a habitat shift at the time of emergence, from
the area of settlement into nearby seagrass beds. Juveniles then tend to
aggregate 0.2-2 ind./m2, up to 100,000 individuals over large areas (>100
ha) requiring specifically a shallow depth with high tidal circulation where
algae production is sufficient and moderate or dense seagrass coverage
(Stoner and Waite, 1990; Stoner et
al., 1996) to reduce mortality from
predation (Stoner and Ray, 1993), which is why (Stoner, 1997) deems the
most crucial productive nursery habitats must be proteced for population
stability are determined by a complex untransferable interaction of physical
oceanographic features, seagrass and algae communities and larval
recruitment.
2.2.3.3 Conch Morphology
Conch shell growth is deterministic; from approx. 3 years conch does not
continue to increase in shell length, growing only by thickening of the shell,
particularly the flared lip that it starts producing. At sexual maturity lip flare
growth initiatiates (McCarthy) which is at approx. 3 years (Berg 1976) and
lasting approx. 7-10 months (Glazer and Berg, 1992) both growth directions
occuring simultaneously till adult shell length is reached (Appledoorn 1998).
Shell lengths are most accurate to date juveniles - estimates for mean shell
length range from approx. 10.8cm for a 1-year old animal, 17cm for a 2-years
old animal, and 20.5cm for +3years (Berg, 1976). In adults shell lip thickness
increase has been used to estimate adult conch growth from maturation in
years (Appeldoorn, 1988a, 1990), but only relatively, as the deterministic
growth affects estimates of juvenile growth and therefore accurate aging, and
mortality (CFMC/CFRAMP, 1999). Additionally hindering effective aging, the
shell length of adult Strombus
gigas can decrease by bioerosion of the
shell on substrate types and interior volume of the shell can shrink with age
inducing significantly smaller body size (CFMC/CFRAMP, 1999).
Extreme spatial variation occurs in shell size of different Queen Conch
populations, depending on the site habitat quality, food availability and quality
and water depth (Martin-Mora et
al., 1995, the presence of predators and
increased depth all thought to thought to slow juvenile and adult conch
morphometric growth (Appledoorn). Growth rate is positively correlated to
final shell length, indicated by slower growing conch tending to reach smaller
final shell lengths and greater age at maturation (Alcolado, 1976), increased
predation can cause weaker, thicker or denser/heavier shells with shorter
spines (Delgado et al. (2002), and increasing depth causing tighter coiling of
the shell resulting in a wider shell, thicker shells, and fewer, longer spines
(Alcolado, 1976, both quoted in McCarthy, 200).
2.2.5 Migrations
Two migrations occur in conch, travelling most actively at night up to 100
yards per day (). a). A long-lived ontogenetic migration movement of larger
juveniles leaving nursery areas moving into deeper water in the direction of
the seasonally synchronous tidal currents accumulating increasing conch
density with the passage of the migration and serving as a density-dependent
or habitat-dependant dispersal mechanism for juvenile conchs from centres of
recruitment (Stoner et al. 1988). b). A summer migration of adults inshore to
shallower water grass beds for spawning as temperatures start to increase in
March (Stoner and Standt 1992; Weil and Laughlin 1984; Coulston 1987) and
return offshore to sand or algae habitat and deeper water in
October/November. Also with age, they have been observed to move to
deeper water (Stoner, ).
2.2.6 Natural Mortality of Strombus
gigas
The Queen Conch is a relatively slow-growing long-lived species, reaching a
maximum longevity of average of 26 years old, generally between 20 to 30
years, although in deeper water this can be extended to 40 years (NOAA).
Appeldoorn (1988b) derived a relationship between age and natural mortality
which exponentially decreases until the conch reaches sexual maturity
(Appeldoorn, 1988a). Mortality along with most other morphometric and
maturity data also varies seasonally, due to habitat, predation and food
limitation (Stoner and Glazer, 1998) but natural mortality of S. gigas has not
been accurately quantified due to bioerosion of the shell by substrate, and it
is thought that aging any S. gigas specimen greater than 10 years old should
be considered is unreliable, and therefore the complete lifespan of queen
conch is unknown.
2.3 The Biological/Ecological Importance of Strombus
gigas
Strombus gigas is an important member of marine benthic communities,
as a hebrivory mollusc, decreasing significantly the standing crop of biomass
of dead or detritus remains of senescent seagrass blades, seagrass
epiphytes, macrodetritus and macroalgae, without reducing living seagrass
biomass. (Stoner 1995) showed S.
gigas decreased the abundance of
bottom-dwelling algae such as Batophora
oerstedi, performing a visual
‘cleaning' of the sediment surface to white, as opposed to the normal light
brown colour, clearing filamentous algae and small detrital particles. Conch
faecal pellets left may also provide an input of regenerated nutrients from
these excretions for other marine life. S.
gigas are crucial to the structure of
the benthic and macrofauna communities in seagrass meadows (Randall,
1965), their grazing having an positive effect in regulating the abundance of
seagrass detritus and algal blooms (Stoner et
al., 1995). If similar to other
important marine herbivory grazers such as Diadema,
the S.gigas grazing potentially increasing stimulate rates of primary
production of algae, macrophytes, seagrasses and the role of below ground
nutrient reserves (Valentine). In addition, grazing on epiphytes and detritus
could influence other components of the benthic community such as
amphipods and other smaller Mollusca invertebrates, which are dependent
upon detritus for food or cover, reduced in numbers by S.
gigas grazing. S.
gigas must therefore play a major role in the trophic flux of this tropical
seagrass community and over-exploitation may cause significant ecological
changes, increasing small grazers or rapid accumulation of organic matter in
the sediments and trophic cascade changes that may reduce productivity and
limit recruitment of S.
gigas and all other species (Klumpp, et al 1992.).
2.4 Future Outlook and Conservation - Conserving Reproductive Stocks
Having ascertained as above, that conch are important to the ecosystem, the
CITES ruling was a cause for concern, although mainly for the fisheries
economy rather than ecological importance. With the well-documented
decline of Strombus
gigas that led to the CITES ruling, research
programs were developed designed to monitor the recovery of the conch
stock and to determine how best to rehabilitate the depleted population.
Attempts at researching methods to halt the decline and preserve the species
have been focusing on both preserving the current stocks of
native Strombus
gigas specimens and maintaining stocks by ensuring
reproduction or transplanting hatchery reared juveniles into the wild.
Increasing and preserving the natural global stock resulted in lengthy
research into conch reproduction to assess how to increase the reproduction
of the species and maintain the species as a successful fisheries economy.
Research into potential mariculture hatcheries led to a focused account
of Strombus
gigas reproduction, but to maintain any mariculture a strong
healthy stock of native conch will need to be conserved. Two methods to
protect the high densities of native adult queen conch are at the forefront of
conservation of the fisheries economy: depth refugia and marine reserves
(Stoner?).