Macrostrombus costatus

Lore & Background

The milk conch derives its common name from the milky white interior coloration of the aperture and the light cream exterior of its shell, in contrast to other conch species such as Aliger gigas or Strombus pugilis, which have various shades of pink and red tones in the aperture and various patterns or colors on the external surface. Its eyes have rings that can range in colors from red, orange, yellow, to black. The shell of an adult can be distinguished by the high spire and flaring lip. An average shell measures around 20 cm in length, with a maximum recorded shell length of 23.1 cm, making it a smaller conch compared to the queen conch, Aliger gigas, which can measure up to 31 cm.

Macrostrombus costatus prefers shallow-water back reef habitats such as algal banks, seagrass beds, and patch reefs, which provide shelter, protection, and food. Its habitat environment changes with geographical location: in Paraíba, Brazil, it has been seen at one meter under surface water in areas of sand between patch reefs; in Puerto Rico, it is mostly found in algal mats; and in Panama, there is an overwhelming abundance in seagrass meadows where turtle and manatee grass dominates. It can inhabit depths varying between three meters to twenty meters, with a minimum recorded depth of 2 m and a maximum of 55 m.

Milk conchs have external sex organs: males have a verge, females an egg groove. They are oviparous and begin to spawn in early November throughout the winter and spring seasons, ending in May. They aggregate in groups to spawn in locations of open sand and deeper waters. The life cycle begins with development inside eggs as planktonic larvae called veligers, which feed on phytoplankton. After approximately 26–30 days, the pelagic veliger reaches metamorphosis to a benthic snail, marked by development of the proboscis, loss of velar lobes, and outward migration of the eyes. Juvenile milk conchs have a conical shell with tight whorls and a green-colored proboscis, and begin grazing on algae; crustaceans such as crabs and lobsters are their main predators at this stage.

Reader's Guide

Macrostrombus costatus is fished for its meat and ornamental shell along the tropical Atlantic regions, and has commercial importance, especially in the Caribbean. However, the total catch volume has not been estimated, and it is believed that the species is overfished, which may lead to a major decline in populations. In Panama, there are no current regulations on the harvesting of milk conch. The conservation status of the species is unknown due to lack of research. Despite its importance as a food source for local inhabitants and its role in the ecosystem of seagrass beds and algal banks, the milk conch remains less studied than the more widely known queen conch. Its presence across a broad geographic range—from North Carolina to East Brazil, including the Caribbean Sea, Gulf of Mexico, and Lesser Antilles—underscores its ecological significance in tropical Atlantic waters. The lack of harvest regulations and unknown population trends highlight the need for further research to inform management of this species.

Did You Know?

• The milk conch's common name comes from the milky white interior of its aperture and light cream exterior.
• Its eyes have rings that can be red, orange, yellow, or black.
• Juvenile milk conchs have a green-colored proboscis and are preyed upon by crabs and lobsters.
• The maximum recorded shell length is 23.1 cm, smaller than the queen conch which can reach 31 cm.