Alpheus aequus Kim and Abele, 1988

Alpheus aequus
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Original DescriptionAlpheus aequus Kim and Abele, 1988: 55, fig. 23. 
ClassificationArthropoda: Crustacea: Malacostraca: Decapoda: Alpheidae 
Type LocalityPlaya Blancas, Costa Rica (symbol on the map). 
Type MaterialHolotype: USNM 229917; in ethanol, in good condition. 
Common NameEastern Pacific peanut-worm snapping shrimp. 
Taxonomic StatusValid. 
Geographic DistributionTropical eastern Pacific [EP]: Costa Rica, Panama and Galapagos (see map). 
FrequencyRare, partly because of highly cryptic infaunal life style (see below). The only localities from where A. aequus is currently known are Playa Blancas in Costa Rica, Santa Fe (Barrington) Island in the Galapagos archipelago (Kim and Abele, 1988), and Coibita in the Coiba Marine National Park in Panama (Anker, Hurt and Knowlton, 2007b.). 
HabitatRocky shores, on sand-shale or sand-rock bottoms. 
Depth RangeIntertidal to about 5 m. 
Life History and BehaviorLives probably in pairs, in or close to burrows of large peanut worms, Ochetostoma edax (Echiura, Thalassematidae). 
Related SpeciesAlpheus naos Anker, Hurt and Knowlton, 2007b [EP] A. christofferseni Anker, Hurt and Knowlton, 2007b [WA]; A. ribeiroae Anker and Dworschak, 2004 [EA]; A. barbatus Coutière, 1897 [IWP]. Alpheus aequus is difficult to distinguish morphologically from A. naos and A. christofferseni. The most reliable difference between A. aequus and A. christofferseni appears to be the absence (in A. aequus) or presence (A. christofferseni) of a distal tooth on the ventromesial margin of the merus of the minor cheliped. Alpheus aequus may be more easily separated from the sympatric A. naos by the shorter, more ovate major chela, more gaping fingers of the minor chela, and a distinctly broader merus of the third pereiopod; from the eastern Atlantic A. ribeiroae by the stouter major chela and the much more developed setal brushes on the fingers of the minor cheliped; and from the Indo-West Pacific A. barbatus by the proportions of the major chela, and the more elongate scaphocerite (see comparison photos). 
SizeBody length to about 22 mm. 
Color PatternSee photo
Molecular BarcodeGenBank number EU084881
RemarksAll species of the A. barbatus complex may be associated with echiurans. 
Major ReferencesKim and Abele, 1988 [original description and illustrations]; Anker, Hurt and Knowlton, 2007b. [new record, association with echiurans, comparison with other species]. 

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