14 U.S. NATIONAL MUSEUM BULLETIN 2 58 



Of those Amphipoda associated obligatorily with sponges or 

 tunicates, Polycheria osborni is best known because of its occurrence 

 in small burrows in the tests of Amaroucium spp. Presumably the 

 tunicates provide only refuge and not food for the amphipod, because 

 the burrows apparently are formed by displacement and not con- 

 sumption of the host tissues. The amphipod lies on its back in the 

 burrow, circulates water with its pleopods, and apparently ingests 

 filtered materials such as diatoms. On the other hand, Anamixis 

 linsleyi bears extensively modified piercing-and-sucking mouthparts 

 and presumably feeds on the tissues of its host, whatever that may 

 be, tunicate or sponge. Leucothoides pacifica, Leucothoe alata, and L. 

 spinicarpa generally are found with sponges. They may be inquilines, 

 although their mouthparts are not as strongly modified for piercing 

 and sucking as are those of Anamixis. The tiny lysianassids, Ocosingo 

 borlus and Fresnillo fimbriatus , have piercing-and-sucking mouthparts 

 and strongly reduced or modified urosomal appendages. These modi- 

 fications indicate that they are sedentary inquilines. Lysianassa 

 pariter and L. dissimilis are obligate associates of sponges and tuni- 

 cates; this is unusual because some of their congeners are known to 

 be deposit feeders and grazers (e.g. stomachs of L. macromerus have 

 been found to be filled with fusiform diatoms; alimentary tracts of 

 L. oculata and L. holmesi have been found to be filled with silt). Both 

 of these spongicolous lysianassids have normal biting and chewing 

 mouthparts. Presumably they have survived niche exclusion (by 

 L. macromerus for instance) by living and feeding within sessile hosts. 

 Stomachs of the few available specimens of L. pariter and L. dissimilis 

 have been empty. 



The supposition regarding niche exclusion may be true of Elasmopus 

 holgurus, a gammarid rarely found outside of its association with 

 sponges and tunicates. Its more "successful" relative, E. rapax, 

 occupies a nestling niche in the Phyllospadix-pelvetud zone. Netamelita 

 cortada is also in this category — a ch ewer-biter associated with sessile 

 organisms. 



Often a rich source of small crustaceans, almost exclusively 

 amphipods, are the sandy, reef-like masses of the polychaete, Phrag- 

 matopoma sp. Calcareous masses of Dodecacaria sp. are rather poorly 

 inhabited by nestling Crustacea. Some quantification was possible 

 in samples of the phragmatopomid mass at Pt. Dume (table 15) 

 where 8 species representing 1380 individuals per square m. were 

 collected. The amphipodan composition of these masses is not con- 

 sistent because another sample taken at Corona del Mar (station 6) 

 was dominated by Ericthonius brasiliensis, Eurysthevs thompsoni, and 

 Elasmopus holgurus, whereas the mass at Pt. Dume contained no 



