HAPALOCARCINUS, THE GALL-FORMING CRAB, WITH SOME 

 NOTES ON THE RELATED GENUS CRYPTOCHIRUS. 



BY F. A. POTTS, M. A. 



HAPALOCARCINUS MARSUPIALIS STIMPSON. 

 INTRODUCTION.* 



Hapalocarcinus is a genus of Brachyrhynch crabs, the individuals 

 of which are very small in size and profoundly modified, owing to 

 the fact that they pass the greater part of their lives confined in small 

 cavities in coral colonies. At an early age the female crab settles 

 between two adjacent branches, usually terminal, and so influences 

 their further growth that they broaden and, later, unite to form the 

 so-called gall, a lenticular or spherical structure about the size of a 

 hazel nut. Within this is the living chamber of the crab, which com- 

 municates with the outside water by a series of apertures. The animal 

 thus becomes sedentary, never leaving its gall and producing enor- 

 mous broods of larvae. 



Hapalocarcinus marsupialis was first described by Stimpson between 

 1856 and 1859 from specimens "found clinging to the branches of living 

 madrepores in the harbour of Hilo, Hawaii." It was thus thought at 

 first to be a free-living animal, but Verrill, in 1867 (6), pointed out 

 that certain " deformities on corals," compared by Ehrenberg to the 

 galls formed by the influence of insects on plants, are in fact caused 

 by Stimpson' s Hapalocarcinus. He describes the phenomenon in the 

 following passage: 



"Another peculiar mode of parasitism I have observed in a singular crusta- 

 cean (Hapalocarcinus marsupialis, Stimpson) from the Sandwich Islands. 

 This creature lodges itself among the slender branches of a coral (Pocillopora 

 ccespitosa, Dana) and causes, probably by its incessant motions, the branches 

 to grow up and often interlock above, leaving openings between them suitable 

 for the uses of the parasite but usually too small to allow of egress. Most 

 specimens of the corals of this species sustain one or more and often numerous 

 examples of these curious enlarged bulbs among the branches." 



In subsequent papers (7, 8) he makes further observations on the 

 occurrence of "galls," without adding much to the above description. 



Karl Semper became well acquainted with Hapalocarcinus (and the 

 related form Cryptochirus} during his researches in the East Indies. 

 In his well-known book on "The Natural Conditions of Existence as 

 they Affect Animal Life" (published in English, 1881) he illustrates 



*Hia duties as au officer in the English Army made it impossible for Mr. Potts to correct 

 the proof of this paper. A. G. M. 



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