A Field Guide to:



[oysters, mussels, clams]

Eastern oyster, Crassostrea virginica

Description: Crassostrea virginica, the eastern oyster, has a rough grey or white exterior that may be cemented to rocks or other shells. The interior is white with a large purple or darkly colored muscle scar.  Spat is often a dark gray-purple color, with only one valve visible.

Distribution/Habitat: Eastern oysters are found along the Gulf of Mexico and Atlantic coasts, from Mexico to Canada. Oyster larvae attach themselves to submerged objects like rocks, logs, man-made structures or the shells of other oysters and then develop into spat, juvenile oysters.

Natural History: Oysters can build large, complex reefs that provide us with many ecosystem services. Oysters feed by filtering phytoplankton, microscopic plant-like organisms, out of the water. One oyster can filter up to 50 gallons of water in a single day. Through filtration, they remove micro-organisms, nutrients, and sediment particles from the water. Filtration improves water quality and clarity.      

Oysters create physical habitat by creating hard surfaces for other animals to live. Without oyster reefs, Galveston would have few natural hard structures since the bay is mostly covered in soft-sediment. These reefs provide a unique habitat for a wide variety of organisms like fish, barnacles, crabs, and other bivalves. Waves also break on them, reducing erosion and protecting the coastline in storms.  

In addition to their ecological significance, oysters are a commercially important fishery.

Hooked mussel, Ischadium recurvum

Description: The hooked mussel is brown or dark grey with prominent ribs radiating from the beak (the oldest and usually smallest point on a shell, close to the hinge). The interior of the shell is shiny, purple, pink or brown. This mussel gets its name from its triangular shape that curves or hooks to one side.

Distribution/Habitat: It is native throughout the Gulf of Mexico, north to the Chesapeake Bay. Hooked mussels attach to submerged objects like rocks, logs, oyster shells or man-made structures using byssal threads.

Natural History: Hooked mussels can form large groups on oyster reefs, and help create a matrix of habitat for other animals. They also are a food source for small fish and crabs. Like most bivalves, the hooked mussel feeds by filtering microscopic organisms from the water, their filtration can improve water quality in the bay.

Dark false mussel, Mytilopsis leucophaeata

Description: Mytilopsis leucophaeata, also known as the dark false mussel or Conrad’s false mussel, are small bivalves that are dark brown as adults and may be zebra-striped as juveniles. Their siphons can be golden/orange, with black speckles. They are related to, and look similar to the invasive Zebra mussel, but these native bivalves are important to a healthy bay ecosystem.

Distribution/Habitat: Mytilopsis leucophaeata is native from Mexico to Massachusetts, but invasive elsewhere in the world. These mussels are euryhaline, meaning they can live in a range of different salinities, and attach to submerged objects like rocks, logs, oyster shells or man-made structures using byssal threads, filaments a bivalve can excrete to cling to surfaces. On oyster reefs they are often found tucked into crevices, and beneath Ischadium recurvum.

Natural History: Mytilopsis leucophaeata can grow in dense aggregations, but populations can be ephemeral and unpredictable. Like most bivalves, M. leucophaeata feeds by filtering microscopic organisms from the water. Through filter feeding, they help clean the water. They are also a food source for small fish and crabs.


[marine segmented worms]

Serpulid worms, Ficopomatus miamiensis, F. enigmaticus and Hydroides dianthus

Description: Serpulid worms form a calcium carbonate tube, have feathery appendages, and have a specialized operculum, an appendage used to seal the tube shut. The operculum shape is a key feature used to identify each species. Ficopomatus miamiensis has a smooth round operculum, F. enigmaticus has black spikes on the operculum, and H. dianthus has a 2-tiered crown shaped operculum.

Distribution/Habitat: Serpulid worms can tolerate a wide range of temperature and salinities. Serpulids grow on hard surfaces such as oysters, pilings, and rocks.

  1. miamiensis is native in the Gulf of Mexico; H. dianthus is considered native in North America; F. enigmaticus is not considered native in the Gulf of Mexico, it is found globally, and its native range is debated.

Natural History: Serpulid worms feed by using their feathery appendages to filter particles out of the water. Populations of serpulids can be dense enough to form reefs.

Sabellid worms, e.g. Parasabella microphthalma

Description:  Sabellid worms, or feather duster worms, are a tube worm similar to Serpulids. As their name suggests, they have feathery appendages, which are used for filter feeding. They differ from Serpulids in that they lack a specialized operculum, and their tubes are not calcified, but are soft.

Distribution/Habitat: A species commonly found on Galveston Bay oyster reefs is Parasabella micropthalma. This species is native to Galveston Bay.

Spionid worms, Polydora sp.

Description: Spionid worms also live in soft tubes, however they lack the feathery appendages of Sabellids. Instead, they have 2 long appendages, called palps. The palps are lined with cilia that carry food particles to their mouth. The worms are cream-pink in color.

Distribution/Habitat: Spionids are marine worms. The species in Galveston Bay inhabit hard substrates and muddy substrates.

Natural History: Spionids feed using  their two long palps. The spionid worms found on the oyster reefs in Galveston Bay are in the genus Polydora. This genus includes the shell-boring “mud worm”, Polydora websteri. This species can be detrimental to oyster health, and reduce the value of commercial oysters. As of now, we have not documented any concerning amount of these worms on the reefs in Galveston Bay.

Nereid worms, e.g. Alitta succinea

Description:  Nereid worms are mobile predators. A common species on the reef is Alitta succinea, the pile worm. These worms can reach up to 6 inches long.

Natural History: Alitta succinea is notable for the swarms it produces when spawning. This phenomenon happens with the lunar cycle, and the worms metamorphose, modifying their appendages for swimming.

As predators on the oyster reef, Nereid worms feed on many of the sessile animals that colonize the surface of the oysters. These worms are also an important food source for birds, fish and crabs.


[hydroids, anemones, jellies, corals]

Hydroids, Obelia sp. and Bougainvillia sp.

Description: Hydroids are colonial animals with polyp and medusa life stages like corals and jellies. Polyps often occur on stalks and are the attached life stage that live on oyster reefs, while the medusae swim freely.  The polyps are small, tentacled buds, growing on stalks. Species in the genera Bougainvillia and Obelia have been documented on oyster reefs in Galveston Bay.  Bougainvillia sp. can grow quite large, with thick, golden stalks. Obelia sp. are a clear and small

Distribution/Habitat: The polyp stages of hydroids are found on hard substrates around Galveston Bay, including on oyster reefs. 

Natural History: Hydroids often have two life stages, polyp, and medusa. Polyp and medusae are predatory and feed on microscopic organisms in the water.   Medusa stages are often jelly-like and free swimming.

Orange striped green anemone, Diadumene lineata

Description: D. lineata is a small anemone, typically <1.2”. It has many colorations ranging from a green or brown anemone with vertical green, orange, yellow or white stripes.

Distribution/Habitat: Non-native in the Gulf of Mexico. Originally from Japan, it is now widespread across the Pacific and Atlantic oceans. It likely was introduced to new locations through the transport of larvae in ballast water of boats. It can live in a range of salinity and temperature conditions.

Natural History: D. lineata can asexually reproduce through a process called longitudinal fission, where one anemone splits into two. It can quickly increase its population this way.


[colonial, “moss animals”]

Encrusting bryozoan, Conopeum sp.

Description: Encrusting bryozoans in the genus Conopeum, have calcified walls between zooids (individuals). Zooids are often rectangular to oval in shape. Each zooid has a set of feeding tentacles. Zooids may have orange or brown coloration from their diet, from diatoms or other organisms growing on their surface, or have coloration on their  operculum, or “door”.

Distribution/Habitat: Conopeum sp. are found across a broad range of salinities, and occur on hard substrates, including oyster reefs in Galveston Bay.

Natural History:  Bryozoans filter feed from the surrounding water, using their tentacle rings. They are a food source for small animals like snails, nudibranchs, fish, and crabs.  Colonies grow outwards from a singular individual, called the ancestrula.

Encrusting bryozoan, Hippoporina indica

Description: Hippoporina indica is an encrusting bryozoan, with calcified walls between zooids (individuals). Zooids are rounded-tear drop shaped. Zooids will appear pink when they are full of eggs.

Distribution/Habitat: It is non-native in the Gulf of Mexico. Considered native to India and China. It grows on hard substrates including oysters, docks, and buoys.

Natural History: Although H. indica is not native in the Gulf of Mexico, negative impacts of its presence have not been documented. Colonies grow outwards from a singular individual, called the ancestrula.

Gelatinous bryozoan, Alcyonidium sp.

Description: Bryozoans in the genus Alcyonidium form an uncalcified gelatinous sheet across the surface of the substrate. The tentacle rings emerge out of small nubs on the surface of the bryozoan.

Distribution/Habitat: Alcyonidium sp. are found on hard substrates around Galveston Bay.

Natural History: Like other bryozoans, Alcyonidium sp. feeds from the surrounding water using its feeding tentacles.

Soft bryozoan, Amathia imbricata

Description: Amathia imbricata is colonial, like other bryozoans, however it lacks the calcification seen in the encrusting bryozoans, Conopeum sp. and Hippoporina indica. Individuals are soft brown tubes, connected by a stolon, and each individual has tentacles for feeding. Without magnification, it can be difficult to see, but is part of the soft brown matrix growing on the surface of hard substrates.

Distribution/Habitat: A. imbricata can be found growing on  hard substrates around Galveston Bay, across a range of salinities. Available surface area may be a limiting factor in their abundance, for example, they may be out competed for space by encrusting bryozoans (Conopeum sp.).

Natural History: They are food for small grazers like nudibranchs and flatworms.


Kamptozoans, Bartensia sp.

Description: Kamptozoans, also called Entoprocts, are small, colonial animals. They are stalked, with a tentacle ringed cup on the end of the stalk. Without magnification, these are hard to see, but are often part of the soft, brown matrix growing on the surface of hard substrates.

Distribution/Habitat: Kamptozoans can be found growing on hard substrates around Galveston Bay, across a range of salinities. Available surface area may be a limiting factor in their abundance, for example, they may be out competed for space by encrusting bryozoans (Conopeum sp.).

Natural History: Kamptozoans feed by collecting food with the cilia on the ring of tentacles around their mouth.  They are food for small grazers like nudibranchs and flatworms.


[sea squirts, sea grapes]

Sea grape, Molgula manhattensis

Description: Tunicates are soft bodied marine invertebrates. The native species of tunicate is Molgula manhattensis, or the sea grape. This is a relatively small tunicate, generally less 1 inch in size.

Distribution/Habitat: Native to the Gulf of Mexico. It grows on hard substrates in the bay, and is typically restricted to the higher salinity waters in the southern portions of the bay.

Natural History: It filter feeds by drawing water in through its incurrent siphon and filtering out the particles. The filtered water is expelled through its excurrent siphon.


Encrusting sponges

Description: Yellow, brown, and orange encrusting sponges are often observed on Galveston Bay oyster reefs.

Distribution/Habitat: Sponges are found on hard substrates throughout Galveston Bay, observed more frequently in higher salinity sites.

Natural History: Sponges are filter feeders. They often have chemical defenses or spines (spicules) to deter predators.

Boring sponge, Cliona sp.

Description: This sponge looks like small yellow-ish lumps protruding from the surface of an oyster shell.

Distribution/Habitat: This particular sponge lives on oyster shells, where it will bore channels into the shell and reside within them.  

Natural History: As the name implies, the boring sponge will chemically etch tunnels and holes into oyster shells, and reside within these channels. If you have ever found a shell covered in small holes, it was likely from this sponge. Colonization by Cliona sp. may have negative effects on oysters, however research on this has been inconclusive.


Barnacles, Amphibalanus sp.

Description: Barnacles are sessile crustaceans (like shrimp) that build plated calcium carbonate domes. At least three species in the genus Amphibalanus occur within Galveston Bay, A. ebernus, A. improvisis, and A. subalbidus, but dissection is required for species identification. The outside of barnacles will often be colonized by bryozoans, kamptozoans, spionid worms, or sponges.

Distribution/Habitat: Barnacles are found in marine environments on hard substrates.

Natural History: Barnacles have planktonic larvae, which settle onto hard substrates. Once they find a location, they glue themselves down and transform to their adult form in that spot, where they will live out their lives. Barnacles live within calcareous plates, cemented to the substrate, and extend feathery legs out the opening to feed. Barnacles are a food source for fish, crabs, and flatworms.



Description: Amphipods are small shrimp-like crustaceans with laterally compressed body shapes. Most are very small and clear, white, or gray in color.

Distribution/Habitat: This diverse group can be found in most aquatic and marine environments. On oyster reefs they can be highly abundant.

Natural History: These small crustaceans are abundant invertebrates in marine ecosystems. On the oyster reef and other hard substrate communities, many build soft tubes, but are mobile in their search for food.  Most are detritivores or scavengers.  Amphipods are an important food source for fish.


[mud crabs, stone crabs, blue crabs]

Flat-back mud crab, Eurypanopeus depressus

Description:  The flat-back mud crab is a small crab, no more than a half inch in width, with a mottled dark brown carapace, and unequal claw sizes. Claws have a white tip. Spines are present on the edge of the carapace.

Distribution/Habitat: Mud crabs are found throughout Galveston Bay, including on oyster reefs.

Natural History: These mud crabs are omnivorous and can feed on algae, detritus, and many of the small animals that live on the oyster reef. Mud crabs can be parasitized by a type of barnacle (Loxothylacus panopaei), that grows within their abdomen and functionally castrates males.

Oystershell mud crab, Panopeus simpsoni

Description:  Panopeus simpsoni is similar to E. depressus, but can be larger in size, up to 2-1/5 inches in carapace width. The distinguishing feature is the presence of a large tooth on the movable (top) major (larger) claw.

Distribution/Habitat: Mud crabs are found throughout Galveston Bay, including on oyster reefs.

Natural History: These mud crabs are omnivorous and can feed on algae, detritus, and many of the small animals that live on the oyster reef.

Gulf stone crab, Menippe adina

Description: The gulf stone crab has a dark brown to purple carapace, with darker coloration on the tips of the claws. They are larger than E. depressus and P. simpsoni, growing up to 6” in length, and can be distinguished from these by the presence of 2 teeth on the major claw (one on the top and one on the bottom).

Distribution/Habitat: Stone crabs are found throughout Galveston Bay, including on oyster reefs.

Natural History:  Stone crabs are predatory, and their strong claws can even break open oysters! Their claws will regenerate, which makes stone crab claws a sustainable seafood.

Blue crab, Callinectes sapidus

Description: Blue crabs are a swimming crab, with the rear pair of legs modified for swimming. The carapace is wide with a sharp point on each side, called a lateral spine. The claws are skinny and sharply pointed. The claws and legs have a bluish tint. However, juveniles, which are commonly found on the oyster reef, may lack the blue coloration.

Distribution/Habitat: Blue crabs are widespread throughout the East Coast and Gulf of Mexico. Juveniles are especially found within the oyster reef due to the abundance of food.

Natural History:  Blue crabs are omnivores, and will eat almost anything, but the oyster reef can provide them a feast of bivalves and worms! They are eaten by some fish, and are an important commercial fishery.


Green porcelain crab, Petrolisthes armatus

Description:  Porcelain crabs are not true crabs, rather, they are more closely related to hermit crabs, mole crabs, and squat lobsters. They look like true crabs in form, but are very flat, and have a flexible abdomen. They are fairly small (<1” carapace width), have large claws, and two long antennae.

Distribution/Habitat: P. armatus has a wide range, and its native range is uncertain. It is found throughout the Gulf of Mexico in intertidal habitats, like oyster reefs.

Natural History: P. armatus is quick to drop legs and claws when threatened. It can regenerate these appendages. The flattened shape allows them to fit into small crevices, such as the spaces between mussels and oysters.  Porcelain crabs are filter feeders, and collect food particles from the surrounding water.


[snapping shrimp and grass shrimp]

Bigclaw snapping shrimp, Alpheus heterochaelis

Description: As indicated by the name, this shrimp has one disproportionately large claw.

Distribution/Habitat: This species is found in shallow water throughout the Gulf of Mexico.

Natural History: Snapping shrimp, also called pistol shrimp, use their large claw to produce a cavitation bubble to stun prey. This also creates a snapping sound that can be heard on the reef. There is evidence that the big claw snapping shrimp will live in burrows of mud crabs. 

Grass shrimp, Palaemonetes pugio

and P. vulgaris

Description:  Grass shrimp are small and clear. These two species are very similar, and both have teeth on their rostrum. However, P. vulgaris has a tooth near the tip of the rostrum, and P. pugio (the daggerblade grass shrimp) lacks this tooth.

Distribution/Habitat: Grass shrimp are common to estuarine waters in the Gulf of Mexico, often associated with aquatic vegetation, they are also found near the oyster reef.

Natural History:  These shrimp are an important food source for carnivorous fish.


[snails, nudibranchs]

Oyster mosquito snail, Boonea impressa

Description:  Boonea impressa is a tiny white snail, growing to only 3/10 of an inch. The shell has distinct whorls, and spiral ridges present.

Distribution/Habitat: Boonea impressa is an ectoparasite on eastern oysters. It can be found along the Atlantic Coast down through the Gulf of Mexico.

Natural History: As the name “oyster mosquito” may imply, B. impressa feeds on the body fluids of oysters. They can be found in large abundances on oysters.


Description: Nudibranchs are soft-bodied gastropod mollusks. The nudibranchs observed on Galveston Bay oyster reefs tend to be small or juvenile aeolids (unidentified species), with two sensory structures at the head (rhinopores), two oral tentacles, and dorsal cerata (appendages along their body that sequester defenses and aid in respiration).

Distribution/Habitat: Nudibranchs are widely distributed marine predators. They have been observed in Galveston Bay mostly at higher salinity locations.

Natural History:  Nudibranchs are carnivorous and feed on the small organisms that grow on the surface of the oyster shells, such as bryozoans and hydroids.


[gobies, toadfish]

Gulf toadfish, Opsanus beta

Description: The Gulf toadfish is a small mottled grey and brown fish. It is well camouflaged along the oyster reef. It has a somewhat flattened body, suitable for life on the bottom. They have a wide mouth with small sharp teeth.

Distribution/Habitat:  Native to the Gulf of Mexico benthos.

Natural History: Gulf toadfish are predatory, often ambushing prey from below. They can make sounds with their swim bladder, and a common sound is similar to that of a toad, giving the fish its common name. 

Gobies, e.g. Gobiosoma bosc

Description: Gobies are small fish, with fused pelvic fins that form a sucker. This allows them to stick to rocks or other surfaces.

Distribution/Habitat: Gobies are a diverse group of fish, with members found around the world in various, often shallow, marine habitats. Locally, gobies have been found among the oyster reefs in Galveston Bay.

Natural History:  Gobies eat small invertebrates, such as marine worms and amphipods. Most live in burrows, but some have been documented using empty oyster shells to lay their eggs in.