Petrolisthes eriomerus
Encyclopedia
Petrolisthes eriomerus is a species
of marine porcelain crab
found in the eastern Pacific Ocean. It is commonly known as the flattop crab. It is a flattened, rounded animal, with a carapace
up to 20 mm (0.78740157480315 in) across. It is a filter feeder, and also sweeps food from rocks.
s and hermit crab
s. This small porcelain crab has a carapace
up to 20 millimetre (0.78740157480315 in) wide with a rounded outline. The body, limbs and claws are flattened dorsally. There are a pair of long antennae
just beside the eyes. The broad carpi
of the claw-bearing chelipeds have parallel sides and are twice as long as they are wide. These features distinguishes it from the otherwise similar Petrolisthes cinctipes. The chelipeds are armed with claws usually of similar size which are half as long again as the length of the carpus and disproportionally large. The second and fourth pereiopods end in a pointed dactyl or moveable finger and the fifth is rudimentary and tucked under the abdomen. There are uropods attached to the abdomen which is long and is folded under the thorax. The limbs are covered in tufts of seta
e which are most noticeable when the animal is underwater. This porcelain crab is mainly reddish-brown or blueish-grey but the palps, some of the mouthparts and the knuckles of the cheliped dactyls are blue.
to Alaska
. It prefers areas with strong currents and can be found under rocks, especially those embedded in sand or shingle, among seaweed
and in beds of mussel
s on both exposed and sheltered coasts.
subsisting largely on diatom
s which it gathers with the setae on its mouthparts. It also uses the tufts of setae on its chelipeds to sweep deposits off the surfaces of rocks for consumption.
Females usually have two broods per year. The larvae
are free living and form part of the zooplankton
. The zoeal larval stage has a characteristic long rostrum
. Males, females and a few juveniles may congregate in small groups, and in these, only the dominant males are likely to mate.
Like other porcelain crabs, P. eriomerus sometimes casts off limbs as a diversion when trying to escape from a predator
, a process known as autotomy
. In this species however, the claws of the cast limb continue to grip vigorously after separation. The limb is regrown by the porcelain crab over the period of several moults
.
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of marine porcelain crab
Porcelain crab
Porcelain crabs are decapod crustaceans in the widespread family Porcellanidae, which superficially resemble true crabs. They are typically less than wide, and have flattened bodies as an adaptation for living in rock crevices...
found in the eastern Pacific Ocean. It is commonly known as the flattop crab. It is a flattened, rounded animal, with a carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
up to 20 mm (0.78740157480315 in) across. It is a filter feeder, and also sweeps food from rocks.
Description
Porcelain crabs differs from true crabs in having four rather than five pereiopods or walking limbs normally visible, and are more closely related to king crabKing crab
King crabs, also called stone crabs, are a superfamily of crab-like decapod crustaceans chiefly found in cold seas. Because of their large size and the taste of their meat, many species are widely caught and sold as food, the most common being the red king crab, Paralithodes camtschaticus.King...
s and hermit crab
Hermit crab
Hermit crabs are decapod crustaceans of the superfamily Paguroidea. Most of the 1100 species possess an asymmetrical abdomen which is concealed in an empty gastropod shell that is carried around by the hermit crab.-Description:...
s. This small porcelain crab has a carapace
Carapace
A carapace is a dorsal section of the exoskeleton or shell in a number of animal groups, including arthropods such as crustaceans and arachnids, as well as vertebrates such as turtles and tortoises. In turtles and tortoises, the underside is called the plastron.-Crustaceans:In crustaceans, the...
up to 20 millimetre (0.78740157480315 in) wide with a rounded outline. The body, limbs and claws are flattened dorsally. There are a pair of long antennae
Antenna (biology)
Antennae in biology have historically been paired appendages used for sensing in arthropods. More recently, the term has also been applied to cilium structures present in most cell types of eukaryotes....
just beside the eyes. The broad carpi
Arthropod leg
The arthropod leg is a form of jointed appendage of arthropods, usually used for walking. Many of the terms used for arthropod leg segments are of Latin origin, and may be confused with terms for bones: coxa , trochanter , femur, tibia, tarsus, ischium, metatarsus, carpus, dactylus ,...
of the claw-bearing chelipeds have parallel sides and are twice as long as they are wide. These features distinguishes it from the otherwise similar Petrolisthes cinctipes. The chelipeds are armed with claws usually of similar size which are half as long again as the length of the carpus and disproportionally large. The second and fourth pereiopods end in a pointed dactyl or moveable finger and the fifth is rudimentary and tucked under the abdomen. There are uropods attached to the abdomen which is long and is folded under the thorax. The limbs are covered in tufts of seta
Seta
Seta is a biological term derived from the Latin word for "bristle". It refers to a number of different bristle- or hair-like structures on living organisms.-Animal setae:In zoology, most "setae" occur in invertebrates....
e which are most noticeable when the animal is underwater. This porcelain crab is mainly reddish-brown or blueish-grey but the palps, some of the mouthparts and the knuckles of the cheliped dactyls are blue.
Distribution and habitat
This crab is found on the shore and at depths of up to 90 metres (295.3 ft) on the western coast of North America from CaliforniaCalifornia
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...
to Alaska
Alaska
Alaska is the largest state in the United States by area. It is situated in the northwest extremity of the North American continent, with Canada to the east, the Arctic Ocean to the north, and the Pacific Ocean to the west and south, with Russia further west across the Bering Strait...
. It prefers areas with strong currents and can be found under rocks, especially those embedded in sand or shingle, among seaweed
Seaweed
Seaweed is a loose, colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...
and in beds of mussel
Mussel
The common name mussel is used for members of several families of clams or bivalvia mollusca, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which are often more or less rounded or oval.The...
s on both exposed and sheltered coasts.
Biology
P. eriomerus is a filter feederFilter feeder
Filter feeders are animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feeding are clams, krill, sponges, baleen whales, and many fish and some sharks. Some birds,...
subsisting largely on diatom
Diatom
Diatoms are a major group of algae, and are one of the most common types of phytoplankton. Most diatoms are unicellular, although they can exist as colonies in the shape of filaments or ribbons , fans , zigzags , or stellate colonies . Diatoms are producers within the food chain...
s which it gathers with the setae on its mouthparts. It also uses the tufts of setae on its chelipeds to sweep deposits off the surfaces of rocks for consumption.
Females usually have two broods per year. The larvae
Crustacean larvae
Crustaceans may pass through a number of larval and immature stages between hatching from their eggs and reaching their adult form. Each of the stages is separated by a moult, in which the hard exoskeleton is shed to allow the animal to grow...
are free living and form part of the zooplankton
Zooplankton
Zooplankton are heterotrophic plankton. Plankton are organisms drifting in oceans, seas, and bodies of fresh water. The word "zooplankton" is derived from the Greek zoon , meaning "animal", and , meaning "wanderer" or "drifter"...
. The zoeal larval stage has a characteristic long rostrum
Rostrum (anatomy)
The term rostrum is used for a number of unrelated structures in different groups of animals:*In crustaceans, the rostrum is the forward extension of the carapace in front of the eyes....
. Males, females and a few juveniles may congregate in small groups, and in these, only the dominant males are likely to mate.
Like other porcelain crabs, P. eriomerus sometimes casts off limbs as a diversion when trying to escape from a predator
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
, a process known as autotomy
Autotomy
Autotomy or self amputation is the act whereby an animal severs one or more of its own appendages, usually as a self-defense mechanism designed to elude a predator's grasp...
. In this species however, the claws of the cast limb continue to grip vigorously after separation. The limb is regrown by the porcelain crab over the period of several moults
Ecdysis
Ecdysis is the moulting of the cuticula in many invertebrates. This process of moulting is the defining feature of the clade Ecdysozoa, comprising the arthropods, nematodes, velvet worms, horsehair worms, rotifers, tardigrades and Cephalorhyncha...
.