by Bryan Duong Milstead (Virginia)

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Introduction

The blue crab, Atlantic blue crab, or Chesapeake blue crab (Callinectes sapidus), is a type of crustacean with a population of more than 323 million as of 2023. They are native to the waters of the Western Atlantic Ocean, most notably along the eastern coasts of Virginia and Maryland. In addition, these crabs can be found along the Gulf of Mexico from Florida to Texas. Dubbed the “most iconic” Chesapeake Bay species by the Chesapeake Bay Foundation, blue crabs are known for their distinctive cerulean color. By supporting the Bay’s commercial fishing industry, they not only hold cultural but also economic significance. In Maryland alone, commercial landings of blue crab have surpassed $45 million in annual dockside value, which is based on the species “at the dock” prior to any processing and/or added products.

Blue crabs possess a complex taxonomic classification, which provides insight into the amazing evolutionary history of these creatures. This research paper will touch upon other species that are related to Callinectes sapidus, such as the lesser blue crab (C. similis), and broader members of the crustacean family. Said members include shrimp (Caridea) and lobster (Nephropidae). Deep analysis of interspecific morphological, physiological, behavioral, and genetic traits will be made in an effort to best evaluate the evolutionary implications of the blue crab.

The goal of this paper is to effectively inform the scientific community of the taxonomy, history, and biological relationships of the blue crab. Furthermore, it is hoped that these studies will encourage other students to become more interested in biology, especially pertaining to the Chesapeake Bay.

What is Taxonomy?

Before defining the taxonomy of the blue crab, we must understand what this actually means. Taxonomy is the practice of classifying organisms based on a wide range of scientific factors as well as shared characteristics. Organisms are grouped into taxa (plural: taxon), forming a taxonomic hierarchy. The universally accepted ranks of this hierarchy are domain, kingdom, phylum, class, order, family, genus, and species, in order from highest (most broad) to lowest (most specified). As species are being discovered at a more frequent rate, thanks to advances in technology and scientific knowledge, taxonomy’s job is to establish order for these evolutionary-based studies.

Taxonomy also provides an organized naming system for life forms. Within this scientific practice is a concept known as binomial nomenclature, where species are given names consisting of two words. Binomial nomenclature utilizes Latin, in which the first and second parts of the name are the genus and species, respectively. When typed, scientific names are italicized, whereas with writing, they are underlined. As Dr. Stephen Jay Gould, an American evolutionary biologist, once said, “Classifications are theories about the basis of natural order, not dull catalogs compiled only to avoid chaos.” It is crucial to acknowledge that taxonomy is not just an organized naming system, but rather a better way of understanding why organisms have evolved into the creatures we know today.

Taxonomic Classification of the Blue Crab

Historical etymology: Callinectes sapidus (from the Greek calli-, meaning “beautiful”; nectes, meaning “swimmer”; and lastly, Latin sapidus, meaning “savory”).

The blue crab (Callinectes sapidus) belongs to a distinct scientific classification. For the purposes of this research paper, various characteristics regarding physiology, genetics, behavioral sciences, and morphology will be discussed within all taxonomic levels of the blue crab. This is to ensure that there is clarity throughout the whole research paper, leaving no room for confusion.

With that being said, here is a comprehensive taxonomic classification of Callinectes sapidus:

(#1): Domain: the highest taxonomic rank of all organisms. There are three domains within taxonomy, including Eukarya, Archaea, and Bacteria. Blue crabs belong to the Eukarya domain and are denoted as “eukaryotes.” This indicates that the Callinectes sapidus species are organisms (in particular, animals) whose cells have membrane-bound nuclei. In addition, the genetic material, or DNA, of the cell is contained within the nucleus. Furthermore, blue crabs’ cells have numerous membrane-bound organelles, which are “little organs” that assist in carrying out cellular functions. The term “organelle” is derived from the Modern Latin organella, a diminutive from the Latin organum, “instrument.” Think of organelles as these awesome instruments that make up a biological orchestra, each serving a different purpose that contributes to the success of the whole. One example of an organelle is the mitochondria, discovered by physiologist Albert von Kölliker in 1857. Also known as the “powerhouse of the cell”, they play a critical role in providing energy to the cell and maintaining cellular metabolism. Mitochondria store their produced chemical energy in a molecule called adenosine triphosphate (ATP).

In the below Figure 1 is a depiction of an animal cell. Notice the general sphere shape and how the cell possesses a plethora of organelles for optimal cellular function. Other vital organelles that aren’t the mitochondria include ribosomes, which synthesize proteins; the Golgi apparatus, which processes and packages proteins for secretion or other destinations; and lysosomes, which contain digestive enzymes that degrade cellular waste.

(#2): Kingdom: the second highest taxonomic rank, just below domain. Out of the six kingdoms within taxonomy, including Animalia (Animals), Plantae (Plants), Fungi, Protista (Protists), Archaea (Archaebacteria), and Bacteria (Eubacteria), the Callinectes Sapidus would fall under Animalia. Think of the word “animated,” meaning “full of life and breath; lively,” to describe this kingdom.

As animals, blue crabs are ingestive heterotrophs, indicating that they acquire nutrients from consuming other organisms. Possessing a highly specialized digestive system and the physical adaptation of sharp chelae (claws) allows the blue crab to effectively obtain energy. The chelae are used to catch and tear apart the food, which subsequently travels to the esophagus (between the mouth and stomach). From there, food enters the stomach, positioned behind the eyes of the Callinectes sapidus. Digestible matter will pass to the digestive gland, and the anus will excrete undigested material. Although not pictured in the diagram below, the anus would be somewhere adjacent to the hindgut, on the ventral part of the crab.

Blue crabs are also carnivores and scavengers. The average diet of blue crabs consists of 20-40% mollusks, 10-26% arthropods, 5-12% fishes, and 1-7% polychaetes. They primarily feed on benthic bivalves, some of which are listed in Figure 3. Blue crabs may even participate in cannibalism in order to stay alive.

Secondly, the ability for movement (locomotion) and sexual reproduction supports the idea that the Callinectes sapidus is an animal species. In the Chesapeake Bay, blue crabs mate and spawn from spring to fall. Females migrate to the mouth of the Bay to spawn and can produce between 750,000 – 3,200,000 eggs per brood.

Lastly, on a microscopic level, blue crabs are multicellular organisms, meaning that they have more than one cell. Multicellular organisms have a genetic advantage due to their increased size. They also have longer lifespans, as they are not affected by the death of a singular cell.

(#3): Phylum: the third highest taxonomic rank, just below kingdom. Blue crabs are a part of the phylum Arthropoda, derived from Modern Latin, literally meaning “those with jointed feet.” Arthropods can be found all across the globe in a wide variety of environments, including the deep sea, arid deserts, and even our verdant gardens. Over 800,000 species of arthropods have been identified, but scientists estimate that there may be tens of millions of species in this phylum, many of them yet to be discovered.

Organisms in this phylum possess the following characteristics:

• Invertebrates, meaning that they lack a spine, backbone, or vertebral column. Over 90 percent of all living animal species are invertebrates, the first appearing 570 million years ago as soft, simple, ocean-dwelling organisms. Over an astronomically long span of time, specialization has morphed the Callinectes sapidus into the blue-clawed, carapace-bearing beings of today.
• An exoskeleton, which is the skeleton on the exterior of an animal. Callinectes sapidus have cuticle exoskeletons that contain chitin, a biochemical, fibrous polysaccharide that makes crab shells durable, hard, and effective against predation.
• The ability to molt, which is a multi-stage process of shedding an exoskeleton to reveal a new one. When a blue crab is ready to molt, it seeks refuge in a safe place to avoid any predators that could attack during the transformation. Then, the blue crab will ingest water through its mouth, and the back of the shell cracks. From there, it will gradually emerge from the old shell in a soft condition. In a blue crab’s lifetime, it can molt around 18-20 times (female) or 21-23 times (male) to reach its full size.

(#4): Class: the fourth highest taxonomic rank, just below Phylum. Blue crabs belong to the class Malacostraca. The etymology of the word “Malacostraca” is from the Greek μαλακός (malakós) meaning “soft”, and όστρακον (óstrakon) meaning “shell.” Molting, mentioned in the phylum category, is an integral part of the Malacostracans’ lives, hence the “soft” and “shell” etymology. Other organisms within the Malacostraca class include lobsters (of family Nephropidae) and shrimp (of infraorder Caridea). Blue crabs, lobsters, and shrimp differ profoundly in size, shape, and other morphological factors. Despite this, they are evolutionarily related in a multitude of ways.

See Figure 5 for the similarities and differences between these three organisms. Their physical characteristics reveal clues as to how they evolved, with their shared taxonomic levels indicating that they emerged from a common ancestor.

Shrimp, blue crabs, and lobster also all have bilateral symmetry, meaning that the right and left sides of these organisms are mirror images of each other. There are some characteristics only present within organisms belonging to the Malacostraca class, including a fixed body plan of head, thorax, and abdomen with a certain number of segments. In addition, Malacostracans have a five-segmented head, an eight-segmented thorax, and a six-segmented abdomen. Appendages can be found protruding from the abdomens.

(#5) Order: the fifth highest taxonomic rank, just below Class. Callinectes sapidus falls under the order of Decapoda, which literally means “ten-footed” (from the Greek δέκα (deca-) meaning “ten” and ποδός (-pod) meaning “foot”). As the name suggests, organisms within this order have ten appendages that are considered legs. However, they are not limited to having ten appendages.

Decapoda is an order specifically within the class Malacostraca, and includes the previously mentioned blue crabs, lobsters, and shrimp, along with crayfish and prawns.

(#6) Family: the sixth highest taxonomic rank, just below Order. Portunidae is the family that blue crabs belong to and is characterized by having a flattened pair of legs (swimmerets) used for swimming. Swimmerets moderately resemble a set of paddles used to propel a small boat. This physical adaptation, along with strong chelae, allows blue crabs to capture prey in a swift manner. Moreover, portunids have the ability to streamline across the water because of their broad carapace (shell).

(#7) Genus: the second lowest taxonomic rank, above species and below family. The genus forms the beginning part of the binomial nomenclature (two-part name) of the blue crab, Callinectes. Callinectes differs from other crabs within the Portunidae family due to their lack of an internal cartilaginous spine on the carpus (the middle segment of the claws/appendages). It appeared before the great ice ages of the Pleistocene, when immense glaciers formed across the Earth, and the Holocene, which is our current geological epoch. The Susquehanna River Valley began flooding roughly 10,000 years ago as a result of these glaciers melting, ultimately forming the Chesapeake Bay. Blue crabs were then able to adapt to the variety of estuarine habitats that the Bay provides.

A species within the Callinectes genus (other than C. sapidus) is the Callinectes similis (lesser blue crab). This species of crab is found in the Western Atlantic Ocean (along the east coast of the United States), the Caribbean Sea, and the Gulf of Mexico. They are known for their greenish-brown coloration, especially on the carapace, as well as being agile swimmers. A more informal title of the Callinectes similis would be the “dwarf crab,” as they are considerably smaller than other species within the genus.

There is also the Callinectes ornatus (ornate blue crab), a species of crab found in the Western Atlantic Ocean (along the east coast of the United States) and Caribbean coastlines. They are known for their reddish-brown coloration, with quasi-blue appendages. Although very similar in appearance to the Callinectes sapidus, the presence of six frontal teeth on the carapace compared to the Sapidus’ four creates notable distinction.

Notice how organisms that fall under the Callinectes genus bear strong resemblances. This is because the blue crab, lesser blue crab, and ornate blue crab have multiple taxonomic levels in common (kingdom going all the way down to genus), indicating how evolutionarily related they are. While the main Bauplan, or body plan (morphological features that members of a taxonomic group share) stays the same, size and color are a few factors that differ between these species.

(#8) Species: the final (and lowest) taxonomic ranking, just below genus. This taxa is highly specified and applies to a group of organisms who can reproduce viable offspring. In other words, their young will successfully live and reproduce. The species name “sapidus” forms the second part of the blue crab’s binomial nomenclature, Callinectes sapidus.

As established earlier under the “historical etymology” section of page 2, “sapidus” is Latin for “savory.” Perhaps this is because of the blue crab’s sweet and rich flavor when cooked. Callinectes sapidus, from a gastronomic perspective, is known for having a less fishy taste compared to other seafood. Historically, the blue crab was an important source of food for Native Americans and English settlers in the Chesapeake Bay area.

Callinectes sapidus’ distinctive cerulean color results from a number of pigments within their shells, including alpha-crustacyanin mixed with astaxanthin. Alpha-crustacyanin is an assembly of eight beta-crustacyanin protein dimers, or macromolecular complexes formed by 2 protein monomers. This pigment is directly responsible for producing the blue color of blue crabs, and when mixed with the carotenoid astaxanthin, it creates a blue-green hue.

Behavior of the Blue Crab (C. sapidus)

The behavior of the blue crab species is known to be aggressive when threatened, except for when they have recently molted and must enter hiding due to being in a state of vulnerability. These cerulean creatures also participate in courtship rituals, or a set of behaviors that are displayed to attract a mate. A male Callinectes sapidus crab will “dance” to get a female’s attention. He does this by extending his walking legs and stretching out his claws. Then, he will fan pheromones contained in his urine towards female with his swimmerets (swimming paddles).

The female blue crab will either show her interest by rocking and waving her claws or display disinterest by simply not approaching the male. If she accepts his courtship ritual, she will allow the male to clasp and carry her in what is known as a “precopulatory” embrace. These actions initiate copulation (reproduction) between the two crabs, which will happen during the molting process of the female, the only time she will be able to mate.

This way of mating is quite tame compared to other relatives of the blue crab, such as the lobster (Nephropidae). Male lobsters participate in brutal fights to impress a female lobster, sometimes even yanking off their opponent’s limbs. Like blue crabs, lobsters (both male and female) will release pheromones in their urine to signal interest in mating with one another.

Evolutionary Implications of the Blue Crab (C. sapidus)

The evolutionary relationships within the Callinectes genus are not entirely clear. Scientists and evolutionary biologists have concluded that Callinectes evolved from a Portunid group that dwelled specifically in estuarine environments. Estuaries, of course, are vastly different from other bodies of water. The salty ocean mixes with freshwater, resulting in brackish water that has a moderate salinity level. Home to many marshes, wetlands, and beaches, the Chesapeake Bay happens to be the largest estuary in the United States.

Other than their environment, natural selection and genetic variation have been key contributors to shaping (metaphorically and literally) the Callinectes sapidus species. Over millions of years, these evolutionary mechanisms acted on an ancestral version of the blue crabs. These ancient organisms originated in the Eukarya domain as multicellular beings, then became more and more specified with every taxonomic level (ex. Arthropoda, Malacostraca). Natural selection allowed the ones with more favorable traits to survive and reproduce. From there, they would pass on that genetic information to their offspring, providing the building blocks for physical traits and body plans. It is important to understand that natural selection is an extremely lengthy process— often hundreds of millions of years.

In the Callinectes sapidus’ case, they have many specialized characteristics that allow them to survive. As mentioned earlier, blue crabs have “swimmerets”, or paddle-like appendages that help them swim and catch prey swiftly. Other than physical adaptations, blue crabs also have a variety of behavioral responses. An example of this is their ability to move away from oxygen-depleted waters or shift into a low-activity state in order to prevent hypoxia (a lack of oxygen to sustain bodily functions).

Ecological Implications of the Blue Crab (C. sapidus)

Blue crabs play an important role as both predator and prey for the Chesapeake Bay’s food chain. Juvenile and adult blue crabs serve as food for fish, birds, and even other blue crabs. As predators, they consume many bottom-dwelling organisms, such as mussels, oysters, dead fish, and anything else available, depending on the accessibility within a region. They play a key role in regulating the periwinkle snail population of the marshes by feeding on them. In turn, this keeps the marsh habitat stable, as periwinkles feed on its grass. Mutualism also occurs: the grass gives the crabs safety during key events like reproduction and hiding from predators.

For humans, blue crabs support large recreational fisheries within the Bay and are a nutritious (and delicious) food source. There is an estimated one-third of the United States’ blue crab catch coming from the Chesapeake Bay. However, overfishing, pollution, and other human activity can cause annual fluctuations in the blue crab population. Nutrient runoff is a notorious form of pollution in which too many nutrients (such as nitrogen and phosphorus) cause excessive growth of algae in water. An abundance of algae soaks up all of the underwater oxygen, effectively stealing it from other organisms. To combat this, we must better manage nutrient runoff reaching the Bay from farms/lawns, practice sustainable fishing, and continue monitoring population trends.

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Bryan Duong Milstead is an Asian American high school student based in the Shenandoah Valley. He was a national winner of the 2022 NASA “Power to Explore” essay challenge and has had two journalistic articles published on the “Virginia Association of Journalism Teachers and Advisers” (VAJTA) website. In the future, he aspires to pursue a career in engineering or biotechnology, while still maintaining a passion for literature.