THE BEHAVIORAL IMPACT OF A TROPICAL BACKREEF URCHIN

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THE BEHAVIORAL IMPACT OF A TROPICAL BACKREEF
URCHIN ON SNAIL GROWTH, SURVIVAL, AND SEAGRASS
PRODUCTIVITY   Heather M. Coleman Department of
Organismal Biology, Ecology and Evolution, UCLA
Few studies have examined the repercussions of
the coral to algal domination phase change in
Caribbean backreef communities. I studied the
impact of the sea urchin Tripneustes ventricosus
on the snail Cerithium litteratum in terms of the
urchins covering behavior, as well as the
cascading impact of the removal of an epiphytic
grazer of seagrass in Discovery Bay, Jamaica. I
hypothesized that the urchins displacement of
the snail limits its access to food, causing a
decline in weight gain, while at the same time
increasing its susceptibility to predation. I
predicted this removal would also allow epiphytes
to increasingly overgrow seagrass. Snails were
isolated in pens with or without urchins, and
were exposed to a predator while located on
urchins and on substrate. Urchin predators were
introduced as well to examine whether this
covering behavior might function as a protective
measure. Results indicated that snails gain
significantly less weight while located on and
around urchins, compared to isolated
conspecifics. Furthermore, snails on urchins
were consumed less frequently than those
positioned on the substrate. Finally, urchins
appear to exhibit a defensive response, in that
they pick up and hold more snails in the presence
of a predator than in its absence. While snails
consumed a large percentage of each blades
epiphytes, in situ seagrass productivity was not
altered upon their removal. While these results
illustrate a complex symbiosis, the repercussions
of snail immobilization do not impact
productivity in a manner similar to the algal
displacement of coral.
Introduction   Two of the most commonly occurring
mobile invertebrates in the backreef of Discovery
Bay, Jamaica are the snail Cerithium litteratum
and the sea urchin Tripneustes ventricosus. Both
of these organisms have historically been
described as seagrass residents in Jamaica and
other parts of the Caribbean. However, far more
urchins can be seen in the fleshy macroalgal zone
of the Discovery Bay reefcrest than in seagrass
beds. Furthermore, these snails are virtually
nonexistent in backreef sea grass beds at
present, and instead can be found primarily on
the tests of urchins. Upon observing Cerithium
and Tripneustes interactions, it is clear that
the urchins initiate this association. They
extend their tube feet to nearby snails and bring
them onto their tests, where the snails are held
amongst the spines. While covering in urchins is
by no means an unusual occurrence, it is rare for
a non-algal organism to be extensively displaced
by an urchin. The question of whether the snails
are adversely affected in the process will be
addressed in this study. Based on previous
work stating that Cerithium and Tripneustes were
found almost exclusively in seagrass beds, and
the contrasting situation at present, I
hypothesized that these two species prefer to eat
the previously unavailable macroalgae. Since the
large-scale bloom in Jamaica twenty years ago,
the backreef has experienced a substantial
increase in fleshy macroalgal cover as well. It
appears that the urchins and snails have largely
abandoned seagrass beds for the algal-rich reef
crest area. Since the snails eat seagrass
epiphytes, and are probably one of the main blade
cleaning species under normal circumstances, the
removal of this snail could cause a substantial
decrease in productivity and overall health of
the bed, an important resource in Jamaica and the
Caribbean.
Methods   360 snails were collected, marked,
weighed and measured, while 18 concrete based,
circular 1 m2 pens were constructed. 9 control
pens contained only 20 snails each, while the
other 9 held 20 snails along with 5 urchins each.
The identification number of each snail that was
located on an urchin during a daily visit to the
pens was recorded. Upon recollection of the
snails after six weeks, the measurements were
repeated. ANCOVA was used to detect
discrepancies in weight change between control
and treatment snails. Similarly, ANCOVA was used
to determine the degree of weight change in
snails observed on urchins in varying
frequencies.   5 urchins holding 6 snails each
were placed in a tank containing a snail
predator, the balloonfish (Diodon holocanthus).
6 snails were stationed on the other side of the
tank. The fish was observed for 1 hour and the
number of snails eaten from each side was
recorded. 8 trials were completed with a
different fish for each. A chi-square test was
used to determine the deviation in the number of
snails eaten in each group from an equal
representation. 4 urchins were placed in each
of 8 wet tables. 50 snails were added to each
table before half of the urchins were exposed to
effluent from a predator, the flame helmet
(Cassis flammea). The number of snails held by
each urchin was recorded at 5 time
intervals. Repeated measures (MANOVA) was
completed to compare the number of snails held by
urchins in treatment and control conditions over
time.   10 .25m2 wire mesh cages were positioned
over seagrass plots in the backreef. 5 of the
cages contained no snails, while 30 snails were
added to the other 5. Each blade within the
plots was marked with a staple just above the
substrate level. After two weeks, blades were
cut at substrate level and just below the staple.
This new growth was dried and weighed. T-Tests
were used to compare the productivity in seagrass
in the presence and absence of snail grazers.
Results and Discussion   In other bays where no
fleshy macroalgae has invaded the backreef,
urchins and snails inhabit separate areas of the
seagrass bed. In light of this observation, a
recent habitat change is what appears to have
drawn these two species together in Discovery
Bay.
Snails exposed to urchins gained significantly
less weight than controls (Fig. 2, P0.0166), and
those observed on urchins more frequently gained
significantly less weight than those rarely or
never seen on urchins (Fig. 3, P0.0065). These
results imply that urchins effectively restrict
snails access to food through their covering
behavior, eliminating the possibility of a
mutualistic relationship.
The predatory balloonfish chose snails on the
substrate more frequently than those held on
urchins (Fig. 4, P0.0039). While the prey
should visually stand out on a purple background,
its envelopment of spines must outweigh this and
serve to deter predation. Thus, one can infer
that urchins may offer snails some degree of
protection from their predators.  
The increase in snail cover in tanks receiving
effluent cues (Fig. 6, P0.0011) could imply that
the response is an attempt at camouflage from
urchin predators. This result supports my
hypothesis that urchins attempt to camouflage
themselves by covering.   There was no
significant difference in the productivity of
seagrass in regard to the presence or absence of
snails. Thus, we can conclude that the backreef
areas that have undergone such a phase change are
probably unaffected in terms of primary
productivity, unlike their forereef
counterparts.  
In situ observations support the laboratory
result that more snails are held on urchins
during the day than at night (Fig. 5, P0.0006).
This could imply that urchins cover their tests
as a shading mechanism, or to avoid predators
through camouflage, or both.  
Acknowledgements Undergraduate Research Center
(UCLA), Discovery Bay Marine Lab, Sal Genovese,
Peter Edmunds, John Bruno, Mark Steele, Robert
Carpenter, Dalton Smylie, Amanda Shearin, Sarah
Lee, Genevieve Bernatchez, Jahsen Levy,
Debbie-Ann Ramsay, Kayan Campbell