BIOS
Marine Invertebrates of Bermuda

Milk Conch (Strombus costatus)

By Julie E. Barker
and
James B. Wood (Ed)


Abstract Taxonomy  Habitat  Ecology  Recent Research  Commercial Importance  Bermuda Laws  Personal Interest  References  Links 


Strombus costatus

Abstract


Strombus costatus, the milk conch, is a gastropod that can be found in Caribbean habitats. Due to its diet of algae, it is mainly found in shallow habitats: seagrass meadows, sand beds, and algal flats. S. costatus' life cycle involves metamorphosis from a pelagic veliger to a growing benthic snail in a period of only 30 days. During the summer, these snails gather in large groups to spawn. Unfortunately, this species is highly sought after commercially, and its population numbers are beginning to reflect this demand. As a result, scientists are working to discover alternative ways to meet the commercial demand. Bermuda helps in the effort by enforcing laws protecting this species.

Taxonomy


Phylum: Mollusca
  Class: Gastropoda
    Order: Neotaenioglossa
      Family: Strombidae

Strombus costatus is easily identified by its shell which can be about 200 mm in length (Sterrer 1986). Adult, S. costatus shells have a flaring lip (Sterrer 1986). The exterior of the shell has a straw coloration with a milk white aperture (Sterrer 1986). Their eyes are large and have rings that are orange, yellow, red, or black (Sterrer 1986).

Habitat


Strombus costatus can be found throughout the Caribbean and in Bermuda in a variety of habitats including shallow seagrass meadows, sand beds, and algal flats (Tewfuk and Guzman 2003). Sand types can range from silt to coral rubble, depressions, mounds, and some patches of detritus (Brownell 1977). The depths at which they are found can vary from 3 m to 20 m (Brownell 1977). Overall, the habitats are shallow enough for light to penetrate through the water column, which allows the surrounding plant life to photosynthesize (Appeldoorn 1985). The type of environment in which the milk conch is found varies with its location. In Puerto Rico, milk conchs were observed inhabiting algal mats (Berg 1975). Comparatively, in Venezuela, milk conch can be found in seagrass beds (Brownell 1977). Milk conch can also be found in waters off the coast of Panama in densities of 1.27 conch ha-1 and in Bermuda in densities of 2.6 conch ha-1 (Tewfik and Guzman 2003). In Bermuda, S. costatus is found in algae beds of inshore waters ranging in depth from 5-20 m (Sterrer 1986).

Ecology


When Strombus costatus are juveniles, hermit crabs are their greatest threat (Brownell 1977). As adults, it is thought that lobsters are one of their main predators (Berg 1975). In Puerto Rico, S. costatus shells were not present among the shells surrounding octopus lairs (Berg 1975). However, in the U.S. Virgin Islands S. costatus is known to be part of the diet of Octopus vulgaris (Boletzky and Hanlon 1983). Fish and rays were the most commonly observed predators of the gastropod in Puerto Rico (Berg 1975). An escape reaction of S. costatus to such predators is to “flip” backwards (Berg 1975).

Milk conchs are herbivores. In Puerto Rico, S. costatus was most commonly observed in a dense algal mat (Berg 1975). They were found moving and feeding throughout the day and night (Berg 1975). S. costatus also grazes on algae that can be found on Thalassia blades (Brownell 1977).

The larval stage of S. costatus is a ciliated swimming organism called a veliger (Brownell 1977). The veliger changes from pelagic to primarily benthic after 15-18 days of life (Brownell 1977). Next, the veligers begin to crawl and continue to feed on plankton (Brownell 1977). After 26-30 days the veliger undergoes metamorphosis which involves the development of the proboscis, outward migration of the eyes, and disappearance of velar lobes (Brownell 1977). In a tank setting, S. costatus had the fastest growth rate in the first year compared to all other Strombus species (Brownell 1977). In situ, queen conchs (Strombus gigas) grew at a faster rate than milk conchs (Appeldoorn 1985). Strombid snails in Bermuda have been shown to have restricted shell growth in the warmer periods of the year (Wefer and Killingley 1980).

S. costatus found in Bermuda are described as having motion similar to “leaping” (Berg 1975). The foot-stalk becomes elongated and the shell is lifted and then falls forward (Berg 1975). This type of movement keeps all parts protected by the shell when not in motion (Berg 1975). The operculum of strombid snails is located at the posterior end of the foot, this is utilized in a kicking motion to right themselves when they have been overturned (Berg 1975). S. costatus are less active and burrow deeper in the benthic substrate than other Strombus species (Appeldorn 1985).

S. costatus exhibits courtship behaviors consisting of the male following the female, and then repetitively touching the female by approaching her from behind and placing a front portion of his shell on a back portion of hers (Berg 1975). This process can be repeated a number of times before copulation begins (Berg 1975). Copulation begins when the male extends his verge, the male reproductive structure, under the female’s shell, where internal fertilization occurs (Berg 1975). The verge always remains protected under the shells, no matter the snails position (Berg 1975). Egg masses are laid for up to 8 hours, and contain between 92,000 and 400,000 eggs (Shawl and Davis 2004). The eggs hatch within 100 hours (Berg 1975). The reproductive season for the milk conch in Los Roques, Venezuela is mid-November to May (Brunell 1977). Spawning occurs in large groups (Brunell 1977), in shallow sandy habitats or seagrass beds (Shawl and Davis 2004). The milk conch has successfully spawned in captivity (Shawl and Davis 2004).

When producing eggs, females encapsulate each egg individually (Reed 1995b). These encapsulated eggs then become part of a large strand of encapsulated eggs, which is deposited on sandy surfaces (Reed 1995b). Female milk conchs have the ability to store sperm in the receptaculum seminis, a female reproductive structure, which allows them to be reproductive in the absence of a male (Reed 1995b). The act of copulation does not prevent the female from beginning to spawn, and both processes most commonly occur simultaneously (Reed 1995b).

Male conchs are smaller than female conchs (Berg 1975). Two males are often observed to be mating with one female at the same time (Reed 1995a). However, the verge of only one male can be securely attached (Reed 1995a). The secondary male does have the ability to deposit his sperm over the open portion of the uterine terminus of the female, in hopes that some of his sperm will be used (Reed 1995b).

Recent Research


Research by Shawl and Davis (2004) studied the captive breeding behavior of Strombus costatus and other Strombus species. This work is crucial because of the great commercial importance of this genus. Knowing how to successfully breed conch species, and what affect this has on them, could greatly alleviate the stress that current populations are experiencing due to over fishing (Shawl and Davis 2004). Shawl and Davis (2004) observed successful spawning of all conch species that were in captivity. This is important because previously conch farms were relying on wild reproductive adult conchs for egg masses (Shawl and Davis 2004). Captive conch were raised in a recirculating system with the proper water quality, food supply, and sand substrate to allow for reproductive success, which shows that conch do not have to be brooded near an ocean coast (Shawl and Davis 2004). They also found that tanks can be stocked with a higher density of females than males, and still result in reproductive success for all females present (Shawl and Davis 2004). Overall this study was a crucial step towards producing a program for commercial scale culture of S. costatus and other conch species (Shawl and Davis 2004).

Commercial Importance


The queen conch, Strombus gigas, is highly sought after by fisheries for human consumption. As the number of queen conch started to heavily decline in the 1970s, fisherman sought a new species, Strombus costatus (Tewfik and Guzman 2003, Shawl and Davis 2004). Conch is considered a luxury food item in some areas of the world (Tewfik and Guzman 2003). Whether a delicacy or not, there is a demand for conch meat and conch have been commercially exploited in at least 22 countries (Tewfik and Guzman 2003). Conch are not only sought after for their meat, but for the beauty of their shells. Tourists seek the shells of conch in many Caribbean locations, further encouraging capturing them from the wild for profit (Tewfik and Guzman 2003). Work done by scientists such as Shawl and Davis (2004) discussed above is important for finding ways to alleviate the stress on conch populations.

Bermuda Laws


In the 1978 Fisheries (Protected Species) Order, it states that one can not take Strombus costatus from anywhere within the exclusive economic zone (Laws of Bermuda). Taking any species of Strombus is prohibited (Sterrer 1986).

Personal Interest


Although my interest in sea creatures has changed over the years, my true fascination has always been with organisms in the phylum Mollusca. Snails are especially interesting because of their shells, which many people enjoy collecting. Conch shells are large and highly sought after by tourists. I personally have a milk conch shell that I collected from a beach on my desk at school. I decided that it would be interesting to learn about the creature whose shell I look at everyday.

References

Appeldoorn, R.S. 1985. Growth, mortality, and dispersion of juvenile, laboratory-reared conchs, Strombus gigas and S. costatus released at an offshore site. Bull. Mar. Sci. 37: 785.

Berg, C.J. 1975. Behavior and ecology of conch (superfamily strombacea) on a deep subtidal algal plain. Bull. Mar. Sci. 25: 1975.

Boletzky S.V. and R.T. Hanlon. 1983. A Review of the Laboratory Maintenance, Rearing and Culture of Cephalopod Molluscs. Memoirs of the National Museum of Victoria: Proceedings of the Workshop on the Biology and Resource Potential of Cephalopods, Melbourne, Australia, 9-13 March, 1981, Roper, Clyde F.E., C.C. Lu and F.G. Hochberg, ed. 44:147-187.

Brownell, W.N. 1977. Reproduction, laboratory culture, and growth of Strombus gigas, S. costatus, and S. Pugilus in Los Roques, Venezula. Bull. Mar. Sci. 27: 668.

Laws of Bermuda, Fisheries (Protected Species Order 1978), Bermuda Statutory Instrument BR 8/1978, (1978).

Reed, S.E. 1995a. Reproductive anatomy and biology of the genus Strombus in the Carribean: I. Males. J. Shellfish Res. 14: 325.

Reed, S.E. 1995b. Reproductive anatomy and biology of the genus Strombus in the Carribean: II. Females. J. Shellfish Res. 14: 331.

Shawl, A.L, Davis, M. 2004. Captive breeding behavior of four strombidae conch. J. Shellfish Res. 23 (1): 157.

Sterrer, W. 1986. Marine fauna and flora of Bermuda: a systematic guide to the identification of marine organisms. John Wiley & Sons Inc. Canada. P:419-420.

Tewfik, A., Guzman, H.M. 2003. Shallow-water distribution and population characteristics of Strombus gigas and S. costatus (Gastropoda: Strombidae) in Bocas Del Toro, Panama. J. Shellfish Res. 22 (3): 789.

Wefer, G., Killingley, J.S. 1980. Growth histories of strombid snails from Bermuda recorded in their O-18 and C-13 profiles. Mar. Biol. 60: 129.

Links

Strombus costatus An Elusive Common Species
Queen Conch (Strombus gigas)
Conch