Marine Invertebrates of Bermuda West Indian Top Shell (Cittarium pica) By Tim Sartwell and James B. Wood and Abel Valdivia (Eds) Taxonomy  Abstract  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links

Taxononmy


Phylum: Mollusca
  Class: Gastropoda
    Subclass: Prosobranchia
      Order: Archaeogastropoda
        Family: Trochidae
          Genus species: Cittarium pica


The class gastropoda of the molluscan phylum consists of the snails, slugs and limpets. They are usually torted, often secrete shells and have a strong muscular foot. Although most slowly crawl there are a few which are active swimmers. The class prosobranchia contain gills inside the mantle cavity and have a torted body inside a coiled shell. The order archeogastropoda lack a proboscis or siphon, often have a nacreous layer inside its shell and have numerous teeth on each row on the radula. The trochidae family consists of orgainism with broad, conical shells, with a thin spiraled operculum. Cittarium pica is the genus species name of the West Indian Topshell.

Abstract


Cittarium pica is a trochid gastropod. It has a large, conical, dextral shell that is black and white in color. Its common name is the West Indian TopShell but there are other common names for it depending on which Caribbean island you are on. C. pica grows to a size range of 90-110 mm, but some giants have been recorded up to 130 mm. The West Indian TopShell inhabits rocky shores through out the greater West Indies, with the northern limit being Bermuda and recorded as far south as Venezuela. They are herbivores/ detritivores that feed on a variety of algae. They have a variety of predators that prey upon the slow moving snail throughout the rocky shore. This predator pressure could be why the larger snails are most often found on the high wave energy of exposed shores. Because they were once extinct from Bermuda there are now many conservation efforts and population surveys under way to monitor the reintroduction of this species in Bermuda. The West Indian TopShell is an important food source and bait for fisherman throughout its range.

Habitat


Cittarium pica is well documented throughout the West Indies. They are present as far north as Bermuda, throughout the Caribbean and Central America and have been documented as far south as Venezuela (Meyer and Coates, 2005). C. pica inhabits rocky shores hiding in crevices and small holes. Larger C. pica have been known to inhabit more exposed areas, where there is a high wave energy, while the small juvenile snails are more commonly found in protected areas, such as sheltered bays (Toller and Gordon 2005). Debrot (1990) observed higher mortality in the more exposed sites as well as higher number of predators and suggests that the low population densities at the calmer sites is due to poor recruitment. There is zonation observed with the topshell community as well. Smaller snails (shell width, SW, <10 mm) are found in the upper tidal zone, medium snails (SW 10-70 mm) are found to move with the tides and the larger snails are present in the lower tidal zone (Debrot 1990). This zonation has been suggested to be attributed to predator avoidance and/or food availability (Debrot, 1990).

Ecology


Reproduction and Life Cycle

C. pica fertilization occurs externally in the water column. The male topshells have been observed to spawn individually and in aggregates while females were usually seen together while spawning. The male C. pica release their sperm in a slow steady stream or all at once in a huge cloud. The females release their eggs in a slow steady stream (Bell, 1992). The eggs are slightly negatively buoyant and once settled on the bottom the eggs can be easily resuspended with light water movement. Collective spawning of both the males and females lasted between 3-5 hours, while individual spawning durations have not been directly observed (Bell, 1992). The spawned eggs of C. pica are approximately 170-175 microns in diameter, nearly spherical and are a melon green color (Bell, 1992). Clevage is rapid with the first clevage occurring approximately 20 minutes following fertilization. Because the C. pica is a protostome a spiral clevage pattern is observed. The trochophore larvae hatch from its egg between 9 and 11 hours after fertilization. Trochophore larvae are actively swimming, ciliated larvae. The next larval form for C. pica was the veliger, the second molluscan larval stage that includes a velum. It is at this stage in development that torsion occurs (Bell, 1992). After 23-26 hours following fertilization the larval shell is complete with one whorl developed and is approximately 270-280 microns at its greatest width (Bell, 1992). At this particular point in development the larva can completely withdraw into its shell and it posses a well-developed operculum. After 60 hours the larva are observed to be crawling rather than swimming and about 3 days following fertilization the larvae can attach to a substratum with their sticky muscular foot. Bell (1992) calculated the mean growth rate for juvenile C. pica to be approximately 10 microns per day. It has been suggested that the reproductive season ranges from July to October in C. pica from the Bahamas, but it should be noted that this season could vary from region to region (See Current Research). Recruitment of 1-2 mm juvenile topshells occurs in January, which suggests spawning around October (Bell, 1992). Sexual maturity in C. pica occurs in animals with a shell length greater than 45 mm (Coates et al, 2003). Most reproductive C. pica are older than three years (Coates et al., 2003). C. pica has its greatest amount of movement during the night and most snails move a total distance of less than 1 m per day (Debrot, 1990). The actual life span of C. pica is not known but other trochid molluscs live from 3-30 years (Robertson, 2003).

Predation

C. pica has many natural predators. Although it displays no escape response to Purpura patula, another Caribbean mollusc, they have been observed to be feeding on C. pica by Randall (1964) in the Virgin Islands. The escape response of C. pica is as simple as releasing its strong hold of the rock and falling into the water below (Robertson, 2003). Randall (1964) also documented empty C. pica shells in front of the dens of Octopus vulgaris. There are many organisms among the rocky shore habitat the prey on C. pica including spiny lobster, bonefish, porcupinefish, puddingwife wrasse and rock hind (Robertson 2003). The oyster-catcher bird, Haemotopus ostralegus, has had C. pica identified in its stomach as well (Randall, 1964). It appears that humans are one of the most deleterious predators/ harvesters of C. pica. C. pica has a high risk of being over harvested to extinction because it is so economically valuable (Meyer and Coates, 2005).

Prey

C. pica is a herbivore-detritivore (Robertson, 2003). Randall (1964) documented the gut contents of 40 C. pica specimens with size ranges between 25-77 mm. She found blue-green algae, green algae, red algae, brown algae, diatoms, organic detritus as well as sand, spicules and calcareous debris. What makes the topshell very interesting is the unique morphology of their radula. The number of lateral teeth on the radula has increased compared to other trochid gastropods (Robertson, 2003).

Symbiosis

The topshell’s shell is often covered with symbiotic green filamentous algae or incrusting calcareous algae (Robertson, 2003). The blue green algae Plectonema terebrans has been documented to erode away the white striations of the topshell, causing the black areas to be higher in relief. The reason as to why only the white areas are targeted is yet unknown (Robertson, 2003). Limpets can be seen attached to the topshell as well as the Pinnotheres barbatus crab witch lives inside the mantle cavity (Randall, 1964). An important use of the C. pica shell after the animal has died is by hermit crabs. The large land hermit crab Coenobita clypeatus needs the larger C. pica shells because of their massive size. The C. pica shell is so heavy though that the crab causes external modifications to the shell because it drags it across the substrate (Walker, 1994) Because C. pica has been extinct in Bermuda there has not been any new shells for the hermit crab population. Thus the hermit crab population has declined to almost extinction in Bermuda because they had to depend on fossil or subfossil shells to be released from the sediment by natural disturbances such as hurricanes (Coates et al, 2003).

Recent Research


There have been many attempts to reintroduce C. pica in Bermuda, as early as 1901 and as recently as 1982 (Coates et al, 2003). All previous attempts were thought to be unsuccessful until 1989. Since 2000 there have been extensive population surveys to learn more about its abundance, distribution and population structure and dynamics (Coates et al., 2003). Coates et al (2003) had made some more interesting discoveries as well. They found that juvenile recruitment is greatest from June to September, suggesting that spawning takes place after February. Also, that individuals <45 mm are not reproductive and that they most likely become reproductive in the third year of their life. Additional spawning data is being generated to help determine if limited harvesting is possible. Coates et al (2003) warns that the reintroduction of C.pica could negatively impact the rocky shore because it could take the form of an invasive population because it has been extinct here. E. Meyer and K. Coates (2005) made three more conclusions based on the population surveys and captive rearing experiments in Bermuda. First, C. pica are probably not synchronous spawners; second, there are no prominent cyclical aspects to gamete release; and third, the topshell probably has a larger habitat range than scientists once thought (Meyer and Coates, 2005). Toller and Gordon (2005) also did population surveys of C. pica in the United States Virgin Islands.

Commercial Importance


C. pica is the third most commercially important invertebrate in the Caribbean behind the spiny lobster, Panulirus argus, and the queen conch, Strombus gigas. Humans collect C. pica as a food source, bait and for their beautiful shell. One reason C. pica is harvested so often is because of the accessibility of it’s intertidal habitat. Toller and Gordon (2005) report that price per pound for whole animals in their shell is about $10 (US) in the United States Virgin Islands. Granted there are size restrictions and laws regulating collection, C. pica is still collected throughout its range.

Bermuda Laws


C. pica is protected under the Fisheries Act of 1972 (Wood and Jackson, 2005). Under the Fisheries (Protected Species) Order 1976, it is illegal to remove C. pica dead or alive from anywhere on the island. Although protected, illegal harvesting is still common.

Personal Interest


My interest in Cittarium pica began the day when my Marine Ecology class took a field trip to a rocky shore near Fort St. Catherine in Bermuda. We completed population counts for the Conservation Dept here in Bermuda. It was very interesting to see how large these snails got and I was immediately impressed. Once I heard that they were once extinct in Bermuda and are flourishing once again, I decide to explore more about this animal and learn as much as I could.

References

Bell, L.J. 1992. Reproduction and larval development of the West Indian Topshell, Cittarium pica(Trochidae), in the Bahamas. Bulletin of Marine Science 51: 250 - 266.

Coates, K., C. Jantzen-Marson, J. Madeiros, and E. Meyer. Dec 2003. West Indian topshell, Cittarium pica, reintroduction project in Bermuda: Current status. Integrative and Cooparative Biology 43(6): 862.

Debrot, A.O. 1990. Temporal aspects of population dynamics and dispersal behaviour of the West Indian Topshell, Cittarium pica (L.), at selected sites in the Exuma Cays, Bahamas. Bulletin of Marine Science 47: 431-447.

Meyer, E. and K. Coates. Jan 2005. An exploration of gamete maturity, growth and movement studies of the Cittarium pica (West Indian Topshell) in Bermuda. The Tentacle (13): 20-21

Randall, H.E. 1964. A study of the growth and other aspects of the biology of the West Indian Topshell Cittarium pica (Linnaeus). Bulletin of Marine Science of the Gulf of Mexico and the Caribbean 14: 424-443.

Robertson, R. 2003. The edible West Indian “whelk” Cittarium pica (Gastropoda: Trochidae): Natural history with new observations. Proceedings of the Academy of Natural Sciences of Philadelphia 153: 27-47.

Schmidt, S.; Wolff, M. Vargas, J. 2002. Population Ecology and Fishery of Cittarium pica (Gastropoda: Trochidae) on the Caribbean coast of Costa Rica. Rev. Biol. Trop 50: 1079-90.

Toller, W. and Gordon, S. (2005) A Population Survey of The West Indian Topshell or Whelk (Cittarium pica) in the U.S. Virgin Islands. SEAMAP-C: USVI Whelk Survey Final Report: 2005.

Walker, S.E. 1994. Biological remanie: gastropod fossils used by the living terrestrial hermit crab, Coenobita clypeatus, on Bermuda. PALAIOS 9: 403-412.

Wood, James and Jackson , Kelsie. “Bermuda” Caribbean Marine Biodiversity: the Known and Unknown. DEStech Publications: Pennsylvaina, U.S.A., 2005.



Links

Western Atlantic Mollusk Database
Bermuda Biological Station
Pictures of C. pica