Trapezia and Tetralia (Decapoda, Brachyura, Xanthidae) as Obligate 
Ectoparasites of Pocilloporid and Acroporid Corals 1, 2 
Jens W. Knudsen 1 2 3 
The occurrence of marine invertebrates in 
the branches of living and dead corals has long 
been recognized. Two crab genera, Trapezia 
and Tetralia, of the family Xanthidae are deter- 
mined by Garth (1964) as being obligate com- 
mensals of the coral families Pocilloporidae and 
Acroporidae, respectively. Crane (1947) lists 
species of the genus Trapezia as being found 
only in pocilloporid corals along the west coast 
of tropical America. Miyake (1939), in listing 
the Brachyura of Micronesia, records Trapezia 
cymodoce as collected from Stylophora, a pocil- 
loporid coral. Garth’s original collecting tech- 
niques used at Eniwetok Atoll, Marshall Islands, 
were refined in his later collecting in July 1959, 
at which time he segregated each collection of 
coral by species to avoid mixing coral commen- 
sals found therein. 
From 10 collections of acroporid coral Garth 
extracted two species of Tetralia (T. glaberrima, 
also taken once on Pocillopora damicornis, and 
T. heterodactyla) , but found no specimens of 
Trapezia. Conversely, from 14 collections of 
pocilloporid corals he obtained five species of 
Trapezia ( cymodoce , f. ferruginea, digitalis, 
danai, rufopunctata) , but obtained only one 
specimen of Tetralia. Because of this rather 
exclusive distribution of the species of Trapezia 
and Tetralia, Garth rightly concludes that these 
are obligate commensals of long standing. Garth 
states (1964:142), "In general, the larger 
forms were found in the more robust Pocil- 
loporidae, the smaller forms in the more delicate 
Acroporidae. Thus, the Trapezia species oc- 
curred in the pocilloporid corals, the Tetralia 
species in the acroporid corals, although Tetralia 
1 Sponsored by NSF Grant G-2412. 
2 Facilities and sponsorship at the Eniwetok Ma- 
rine Biological Laboratory by the Atomic Energy 
Commission. 
3 Pacific Lutheran University, Tacoma, Washington. 
Manuscript received March 21, 1966. 
was found once in Seriatopora, a finely branch- 
ing member of the Pocilloporidae in which the 
spatial relationships found in the Acroporidae 
obtain.” Thus Garth suggests a basis of crab 
size and coral spatial relationships as a possible 
basis of this "obligatory commensalism.” 
The writer spent four months at the Eniwetok 
Marine Biological Laboratory, from February 
through May 1965, in order to work on crab 
ecology and to determine the possible basis for 
this seemingly ironclad crab-coral relationship. 
Several possible theories seemed worth inves- 
tigating in order to ascertain the factors upon 
which this commensal-host relationship is main- 
tained: (1) the crab-size coral-space relation- 
ship suggested by Garth; (2) that some ocean- 
ographic condition (water temperature, 
currents, wave action, etc.) is coincidentally 
required by both the crab genus and its respec- 
tive host coral, therefore making the relation- 
ship one of convenience; (3) that the crab 
genera may prove to be filter-feeders utilizing 
the same food required by their host corals, thus 
making the relationship one of simple con- 
venience; (4) that the host corals provide some 
special form of protection, in addition to simple 
hiding places, which exclusively attract the crab 
genera ; ( 5 ) the possibility that more collections 
would reveal that both genera of crabs would be 
found almost equally on the Pocilloporidae and 
Acroporidae. 
These five suppositions served to initiate field 
research. The first three (space-size, oceanog- 
raphy, food requirements) could be the key to 
the coral-crab relationship either with live or 
dead corals of the proper families, providing 
such corals were not overgrown with algae. 
However, supposition number four (special 
protection) could function only with live corals. 
Garth’s term of obligate commensal was defined 
for our work as a situation where the crabs 
in question are obligated to live with their host 
corals in order to receive some benefit which 
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