Crassostrea rhizophorae

Crassostrea rhizophorae
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Mollusca
Class:	Bivalvia
Order:	Ostreida
Family:	Ostreidae
Genus:	Crassostrea
Species:	C. rhizophorae
Binomial name
Crassostrea rhizophorae (Guilding, 1828)

Crassostrea rhizophorae, also known as the mangrove cupped oyster, is a species of bivalve in the family Ostreidae. C. rhizophorae is one of the predominant oyster species in the South Atlantic, specifically in Central and South America. It is often found in the vast mangrove ecosystem along the coast of Brazil.

Environment

C. rhizophorae is typically found in the intertidal or shallow subtidal regions of tropical mangroves and other estuarine regions. The optimal vertical range for C. rhizophorae is between 1.0 m and 1.5 m above the 0.0 m level of spring tides. At greater depths, the substrate is too soft for the oysters to settle and the pressure from predators like crabs and fish is too extreme. Above 1.5 m, C. rhizophorae will not settle due to extensive exposure time. Due to the narrow vertical band that C. rhizophorae inhabits, species survive best when securely fixed on rocks, hard substrates, and on mangrove roots, such as the aerial roots of the red mangrove (Rhizophora mangle). Like most oysters, C. rhizophorae tend for form clusters of individuals which may develop into oyster reefs.

The optimal salinity range for C. rhizophorae is approximately 7.2 to 28.8‰, however, it can tolerate significant salinity fluctuations of short duration, which are experienced in Central and South America during the rainy seasons. C. rhizophorae thrives best in temperatures below 30 °C (86 °F). While it is able to withstand fluctuations, very few larvae are found at temperatures exceeding 30 °C.

Characteristics

C. rhizophorae is often called the Caribbean or mangrove oyster due to the environment that it is found in. This species of oysters is an oviparous species, which indicates that they are animals that reproduce by laying their eggs without much embryonic development within the mother. C. rhizophorae, and more generally the genus, Crassostrea, are cup-like, or cupped, oysters, meaning that the shell itself has a cup shape to it.

C. rhizophorae has a promyal chamber and small ostia. The oyster also has a thin, foliaceous, deeply cupped right valve and the upper left valve is small and flat, which enables it to fit into the lower one. The beak is twisted dorsally, and the muscle scar is near the dorsal margin of the shell. The muscle scar is often unpigmented.

Adult C. rhizophorae can reach up to 10 cm in height. However, in their natural environment, their growth is stunted, leading to a maximum height of 5 cm.

Diet

C. rhizophorae tend to consume any microscopic particles that are carried in suspension in the water, regardless of their nutritional value. They consume a great range of organisms belonging to the following groups: Cyanobacteria, Xanthophyta, Bacillariophyta, Dinophyta, Euglenophyta, Chlorophyta, Protozoa, Rotifera, Annelida, Arthropoda, and Mollusca. C. rhizophorae have also been shown to consume fragments of Phytoplankton, Zooplankton, and Phanerogamae and grains of sediment. A study found that Bacillariophyta was the dominant group of consumption by C. rhizophorae at 63% of the food content in the stomach, followed by Chlorophyta at 12% of the food content in the stomach. This study also looked at the percentage of food items in the stomach contents. They categorized certain amounts of food as "full", "almost full", "almost empty", and "empty". 57% of the individuals were categorized as being in the full stage, which suggests the existence of good availability of food for C. rhizophorae in the environment that they are in.

Reproduction and growth

Reproduction

C. rhizophorae have primary bisexual gonads that form associations of cells in the connective tissue anterior to the heart by the time they reach 0.7 cm or 45 days after setting. The gonada has cells for both sexes but this is shown the most with spermatogenesis cells in 90% of animals that are sexually mature before reaching 2.0 cm or 120 days after setting. In older individuals ranging from 6 to 18 months and 4 to 6 cm in size, 83.5% were females so most change happened between 2 and 4 cm in size, yet only 0.5% are hermaphrodictic. The active gonad goes through prematuration and maturation stage before spawning and then after partial spawning, the gonad enters a recuperation stage. During this stage, the gametogenesis starts a new maturation that leads to the complete cytolysis of the gamete and obliteration of the follicles. Most adult oysters ranging from 4 to 6 cm in length become mature without an undifferentiated stage after the spawning or resting stage.

Due to the constant high water temperature, gametogenesis happens twice during the year, March and October. These peaks happen when drastic changes in salinity, rainy periods, but intense rains like 150 mm per week depress spawning. If done in the lab, C. rhizophorae embryonic development can be done in 24 hours at a density of $10^{4}$ to $4\times 10^{4}$ ovocyte per liter when fertilized at concentrations of 500 to 5000 spermatozoans per ovocyte. From this it was determined that the best range of salinities for embryonic development is 25% to 37% and the best temperatures are around 25 but below 30 degrees Celsius.  

Growth

C. rhizophorae can grow in a variety of locations, but grow best in the roots of mangroves. C. rhizophorae tend to grow to 4 to 7 cm in length, and it can take up to 18 months for most members of the species to reach their full size. The maximum size of C. rhizophorae is approximately 7 to 8 cm. Adult C. rhizophorae can reach up to 10 cm in height. However, in their natural environment, their growth is stunted, leading to a maximum height of 5 cm.

C. rhizophorae begin their life as floating larvae, which soon settle onto a solid substrate. Once settled onto their substrate, the growing oysters are known as spat. Spat grow 1 cm a month for the first 3 months and then growth rates slow to an approximate growth of 0.78 cm a month. After reaching 6.5 cm, growth rates drop considerably. C. rhizophorae grow best during the rainy season due to a higher influx of nutrients into estuarine areas.

The size class between 4.1 and 6.0 cm is of most interest for fishers, as oysters of this size tend to yield the most meat. The best time to harvest C. rhizophorae is 2 years after spawning.  

Fishing industry

C. rhizophorae is a vital fishery resource for the Caribbean and South Atlantic. In the early 2000s, as many as 5,600 metric tons of C. rhizophorae were harvested in the Caribbean and South Atlantic. Due to high consumer demands and declines in C. rhizophorae populations due to pollution, C. rhizophorae is now most commonly farmed using artificial reefs known as farming platforms. These platforms are typically made of branches of mangrove trees suspended from racks in the inter- and sub-tidal regions. These allow for farmers to maintain populations of C. rhizophorae that meet consumer demands while preventing overfishing.

The artificial reefs of C. rhizophorae have also acted as nursery environments for many marine and estuarine species in the Caribbean. These artificial reefs also provide a reproductive substrate for fishes and protect them from predation.