Pinnixa chaetopterana

Stimpson, 1860

tube pea crab

Pinnixa chaetopterana is a small, soft-bodied pea crab that lives symbiotically within the tubes of polychaete worms, primarily Chaetopterus variopedatus. It is a facultative commensal, occasionally free-living or associating with alternative such as Amphitrite ornata. The exhibits strong host fidelity and can distinguish conspecifics from heterospecifics. It ranges from Massachusetts to Uruguay along the western Atlantic, including Brazil and the Gulf of Mexico.

Pronunciation

How to pronounce Pinnixa chaetopterana: //ˈpɪnɪksə kaɪˌtɒptəˈreɪnə//

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Identification

are distinguished from primarily by association and subtle morphological characters including details of the , ambulatory legs, male , and male first . The is most reliably identified by its facultative commensal relationship with Chaetopterus variopedatus, in contrast to the obligate commensal Polyonyx gibbesi which shares this host. The two crab species are mutually intolerant and will fight if placed together. Larvae are identifiable by the caltrop-shaped carapace with specific spine proportions and the distinctive abdominal somite knobs.

Appearance

are tiny, soft-bodied crabs with a that is distinctly wider than long—a characteristic of the Pinnixa. Adults of different Pinnixa are difficult to distinguish from one another. Larvae are markedly different from adults: they have a caltrop-shaped carapace with , rostral, and lateral spines; the dorsal spine is less than 1.5 times the length of the rostral spine. are reduced to a spinous process and a single seta. The second and third abdominal somites bear dorso-lateral knobs, while the fifth somite has lateral wing-like knobs projecting over the , which has a notch.

Habitat

Marine coastal waters at depths up to 60 meters. Lives within the parchment-like tubes of polychaete worms, primarily Chaetopterus variopedatus. Also occasionally found free-living or in association with Amphitrite ornata tubes.

Distribution

Western Atlantic Ocean: Massachusetts to Uruguay along the eastern seaboard of North and South America, including Florida, North Carolina, the Gulf of Mexico, and Brazil.

Host Associations

Chaetopterus variopedatus - facultative commensalPrimary ; crabs position themselves to the worm and move to remain behind it when the worm turns. Likely feed on plankton from water pumped by the host and gain protection from .
Amphitrite ornata - facultative commensalAlternative , used occasionally.
Carcinonemertes pinnotheridophila - Nemertean worm found in branchial chambers; female nemerteans attach sacs to crab pleopods alongside crab eggs.

Life Cycle

Five zoeal stages spent drifting in zooplankton before settlement. involves attachment to pleopods; in parasitized females, nemertean and crab eggs attach, develop, and hatch synchronously.

Behavior

Exhibits strong fidelity to Chaetopterus variopedatus and can distinguish conspecifics from heterospecifics. When paired in a host tube, crabs maintain position to the worm, repositioning when the worm turns. Individuals may bite holes in the tube wall to enlarge apertures for entry and exit. Intolerant of Polyonyx gibbesi; the two fight and may tear off each other's limbs if confined together.

Ecological Role

Facultative commensal of tube-dwelling polychaetes. Likely derives food from plankton in -ventilated water and gains from . Serves as host for the parasitic nemertean Carcinonemertes pinnotheridophila.

Similar Taxa

Polyonyx gibbesiObligate commensal of Chaetopterus variopedatus; distinguished by being obligate rather than facultative, and by mutual intolerance with P. chaetopterana when co-occurring.
Pinnixa banzuSimilar and ; distinguished by , ambulatory leg, male , and male first characters, and by geographic range (Japan vs. western Atlantic).
Pinnixa occidentalisSimilar ; distinguished by subtle characters of and male , and by geographic range (northwest North America).
Pinnixa rathbuniSimilar ; distinguished by , ambulatory legs, male , and male first characters; previous Chaetopterus associations attributed to this may refer to P. banzu.