A case of associated occurrence of the crab Lauridromia inter- media (Laurie, 1906) (Crustacea: Decapoda: Dromiidae) and the actinian Nemanthus annamensis Carlgren, 1943 (Anthozoa: Actiniaria: Nemanthidae) M.S.S. Lavaleye & J.C den Hartog Lavaleye, M.S.S. & J.C. den Hartog. A case of associated occurrence of the crab Lauridromia intermedia (Laurie, 1906) (Crustacea: Decapoda: Dromiidae) and the actinian Nemanthus annamensis Carlgren, 1943 (Anthozoa: Actiniaria: Nemanthidae). Zool. Med. Leiden 69 (11), 31.vii.1995: 121-130, figs 1-5, tabs 1-2.— ISSN 0024-0672. M.S.S. Lavaleye, Nederlands Instituut voor Onderzoek der Zee (NIOZ), Postbus 59, 1790 AB Den Burg, The Netherlands. J.C. den Hartog, Nationaal Natuurhistorisch Museum (NNM), Postbus 9517, 2300 RA Leiden, The Netherlands. Key words: Crustacea Decapoda Dromiidae; Lauridromia intermedia; Actiniaria Nemanthidae; Neman- thus annamensis; association; first record; Kenya; review. Members of the crab family Dromiidae tend to cover their carapace mostly with sponges or colonial tunicates. More rarely are used other objects or organisms such as solitary Tunicata, Zoantharia (= encrusting anemones), valves of Bivalvia, etc.; a review of these is presented. A new association of a dromiid crab Lauridromia intermedia (Laurie, 1906) and an actinian Nemanthus annamensis Carlgren, 1943, is described and discussed. Lauridromia intermedia is recorded for the first time from the continental shelf of East Africa. Introduction (table 1) It is a well known fact that many representatives of the crab family Dromiidae carry a camouflage cover on top of their carapace. This cover, consisting generally of sponges or compound ascidians, is clutched by specially adapted fourth and fifth per- eiopods. Species of some genera (Hypoconcha Guérin-Méneville, 1854; Conchocoetes Stimpson, 1858) use an empty bivalve shell. Less common are camouflage covers of other origin, such as solitary ascidians, seaweeds, barnacles, anthozoans and artifi cial material (see table 1). Among anthozoans, zoanthids and soft corals (Xenia) have occasionally been reported to serve as cover. So far, documented records of dromiid crabs carrying Actiniaria have not been published. Kerstitch (1989: 96), indeed, men tioned that the crab Dromidia (= Cryptodromiopsis) larraburei "...usually carries a sponge on its back, but occasionally will be found with a sea anemone or tunicate.", but he did not explicitly state this sea anemone to be an actinian. However, in a letter dated 5.X.1994, Mr Kerstitch kindly provided the following information: "I have only seen this dromiid crab occasionally with Calliactis polypus but most of the time asso ciated with various sponges. In captivity I have observed D. larraburei actually remove small C. polypus from the gastropod shell of [the hermi tcrab] Dardanus sinis- tripes and 'plant' them on its carapace." The present paper deals with a record of a single specimen of the dromiid crab 122 Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med Leiden 69 (1995) Lauridromia intermedia (Laurie, 1906) carrying a specimen of the actinian Nemanthus annamensis Carlgren, 1943, dredged off the coast of Kenya during the Netherlands Indian Ocean Expedition, 1992-1993. Table 1. Summary of genera and species of Dromiidae with recorded cover-objects and organisms other than sponges or compound ascidians, and the most relevant references. Added are some unpublished records from the RMNH Decapoda collection. Crab nomenclature updated according to McLay (1993). Associated organism Crab species Reference or object Valve of Bivalvia Unidentified Conchocoetes spec. McLay, 1993:175 Unidentified Conchocoetes artificiosus Morton & Morton, 1983:95; Barnard, 1950:309 Unidentified Hypoconcha arcuata Arnold, 1901: 264 Unidentified Hypoconcha lowei Brusca, 1980: 319 Pectinidae Hypoconcha lowei Kerstitch, 1989: 97 Cardiidae Hypoconcha panamensis RMNH D 23540 Trachycardium, Anadara Hypoconcha parasitica RMNH D 12147 Glycymeridae, Veneridae cf. Hypoconcha spec. Mauritania 1978, personal observ. and photos Solitary Tunicata Ascidia mentula Dromia personata Carlisle, 1953:142 Ascidiella aspersa Dromia personata Carlisle, 1953:143 Microcosmus sulcatus Dromia personata Carlisle, 1953:142 Phallusia mammillata Dromia personata Carlisle, 1953:143 Unidentified Cryptodromiopsis unidentata Sakai, 1936:14 Ascidia sydneyensis Cryptodromia hilgendorfi McLay, 1983: 24, 27 Anthozoa Zoantharia Cryptodromiopsis antillensis Kaplan, 1982:162 Zoanthid polyps Cryptodromiopsis antillensis Chace et al., 1986: 340 Zoanthus spec. Cryptodromia fallax RMNH D. 13203 Palythoa nelliae Cryptodromiopsis unidentata Barnard, 1950: 324 Octocorallia (soft coral) Cryptodromiopsis unidentata Ortmann, 1894: 34 Xenia spec. Cryptodromiopsis unidentata Lewinsohn, 1984:107 Sea anemone /Calliactis polypus Cryptodromiopsis larraburei Kerstitch, 1989: 96/ in litt. 5.X.1994 Other organisms or objects Seaweed Dromia personata George & George, 1979: 79 Seaweed (Padina) Cryptodromia fallax Stimpson, 1907:176 Barnacle (Balanus crenatus) Dromia personata Zirpolo, 1926 Artificial (paper/plastelin) Dromia personata Dembowska, 1926:164-166 Artificial Dromia personata Fenizia, 1935 Anything from algal fronds Cryptodromiopsis larraburei Brusca, 1980: 319 to pieces of newspaper Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden69 (1995) 123 Description of the newly discovered association (figs 1,4,5) Material.— NIOP-expedition 1992-1993, RV "Tyro", leg A2, sta. 511, 29.xi.1992, beam-trawl 1, off the Kenyan coast, 3° 09.37 S 40° 13.86' E, depth 53 m; 1 male specimen of Lauridromia intermedia (Laurie, 1906) (RMNH D 46132), carapace width 20 mm, carrying an actinian, Nemanthus annamensis Carlgren, 1943 (RMNH Coel. 18779). In spite of the impact of the dredging activities and the accumulation in the net of ca 0.5 m3 of benthic material (mostly fish, crabs and gastropods), followed by roughly shaking the catch from the end of the net onto the deck, the crab had not lost its anemone cover (fig. 1). This indicates that it had a solid grip on its relatively slip pery partner. No other species or specimens of Actiniaria or dromiid crabs were found among the catch. Before being preserved the crab lived for over a week in a small container, keep ing the anemone on its back. In dromiid crabs the legs of the fourth and fifth pairs are oriented dorsally or sub-dorsally, often in such a way that the fifth legs are above and in front of the Fig. 1. Dorsal view of Lauridromia intermedia carrying the actinian Nemanthus annamensis. Dredged off the coast of Kenya. Carapace width 20 mm. Drawing after a photograph. 124 Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden69 (1995) fourth legs. In Lauridromia intermedia the ends of the fifth legs have three sharp thorns, two fixed and one movable, acting as nippers (fig. 5); the ends of the legs of the fourth legs, too, have a cheliped-like structure (fig. 4). In our specimen the four cheliped-like structures of these legs held the anemone firmly by piercing its limbus, the base of the anemone being stretched over the carapace by action of the legs. Apparently undisturbed, the anemone was most of the time fully expanded, show ing considerable variation in body form. After the crab died the anemone attached itself to the wall of the container. Descriptive notes on te crab Lauridromia intermedia (Laurie, 1906) (figs 1-5) The crab, a male specimen with a carapace width of 20 mm, agrees very well with the description of Lauridromia intermedia given by McLay (1993: 147-148). There are, Figs 2,3. Lauridromia intermedia, outer and inner aspect of right cheliped; dense hair cover removed. Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden69 (1995) 125 however, some small differences, notably in the number of tubercles on the mer iof the chelipeds (figs 2-3). These differences are listed below (McLay's data added in square brackets). The signs (R) and (L) refer to the right and left cheliped, respec tively. The superior margin of the merus bears 7 larger tubercles on the right cheli ped and 6 on the left cheliped [McLay: 4 to 5]. The outer inferior border bears 10 (R) and 11 (L) tubercles [McLay: 7 to 8], and the inner inferior border 8 (one bi- lobed) (R) and 10 (L )[McLay: 9 to 10] small tubercles. Furthermore the lowest one of 3 small tubercles [McLay: 2] on the distal border of the inner face of the carpus of the chelipeds deviates by being bilobed [McLay: simple]. The upper border of the propodus of the right cheliped has 3 tu Figs 4, 5. Lauridromia intermedia, dorsal view of distal, cheliped-like parts of fourth (fig. 4) and bercles [McLay: 2], The base of the dacty fifth pereiopods (fig. 5) and their position rela lus does not have a prominent subacute tive to each other (C—R = caudal-rostral axis); tubercle. The fingers of the chelipeds are dense hair cover removed. armed with 8 (9 if the very blunt tooth on the inside of the tip is included) teeth [McLay: 7]; the first 4 teeth are large, the rest small. The propodial part of the nipper has 5 large and 4 small tubercles. The dactyli of the second and third pereiopods are shorter than the propodi [McLay: equally long]. In all, the differences are rather small and are here considered to lie within the range of variation of the species. The differences may be due to the small size of the individual, well under the mean carapace width of 26.2 mm for males given by McLay (1993: 148). Furthermore pereiopods of crabs may easily be lost by autotomy (e.g. following attack by a predator) and will regenerate after one or more successive moultings. As a result the character of the newly grown legs may deviate from the original condition. Added to this the fact that in our specimen the right and left chel iped differ from each other in the number of tubercles ,it may be obvious that chel iped characters should be used with reserve. The differences may also be linked to the specimen representing an extra-distributional record, the first from the continen tal shelf of East Africa. So far, this wide-spread Indo-West Pacific species had not been recorded further westward than the Seychelles and Madagascar (McLay, 1993). Descriptive notes on the anemone Nemanthus annamensis Carlgren, 1943 (fig. 1, table 2) Morphology.— Body rather firm but variable in shape. Base (after attachment to 126 Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden69 (1995) wall of container) irregular in outline, ca 30 x 20 mm across. Column low and spreading, except for the slightly expanding distal part carrying the oral disc. Mouth a slit with 2 siphonoglyphs. Tentacles ca 120-130, entacmaceous and apparently irregularly arranged, the inner ones of variable size but much better developed than the outer ones. Colour.— Column and tentacles entirely opaquely white. Oral disc semi-trans parent, showing insertions of mesenteries. Anatomy— Not studied. Only three so-called acontioids were found (and re moved for study). Cnidom.— See table 2. The data presented in the table are in general agreement with specimens from the Seychelles (RMNH Coel. 18768, 18769) and the Maldives (RMNH Coel. 18797). Table 2. Survey of the cnidom of Nemanthus annamensis (RMNH Coel. 18779), the actinian partner of Lauridromia intermedia.. N = number of nematocysts measured. Frequency indicated by: = sporad ic, - - = rare, - = uncommon, + = rather common, ++ = common. Organ Nematocyst Mean and range (in parentheses) N Frequency type of length and width of nematocyst capsules in urn Pedal disc Spirulae 15.4(8.0-21.4) x 2.8(2.0-4.2) 60 ++ Penicilli B2 16.9x3.3 1 Column Spirulae 18.0(12.5-20.5) x 2.9(2.0-3.6) 50 ++ Penicilli B2 24.8(22.3 - 27.6) x 4.1(3.6 - 4.5) 10 Tentacle tip Spirocysts ca. 22-46x2.9-5.6 ++ Spirulae 10.0(8.9-12.5) x 2.4(2.2-2.7) 20 --/- Spirulae 17.0(13.4 - 20.0) x 2.5(2.2 - 2.7) 12 Spirulae 28.8(23.1-32.0) x 3.8(3.3-4.5) 30 +/++ Penicilli B2 19.0(15.1 - 21.4) x 3.7(3.3 - 4.0) 25 -/+ Stomodaeum Spirulae 24.7(18.7-26.7) x 3.1(2.7-3.3) 30 ++ Penicilli B2 22.2(19.6-24.9) x 4.0(3.6-4.5) 20 + Filaments Spirulae 10.3(8.5 -12.9) x 2.7(2.4 - 3.1) 20 -/+ Penicilli BÍ 18.3(16.9 -19.6) x 3.6(3.1 - 4.0) 25 ++ Acontioids Spirulae 10.7(8.9-12.5) x 2.6(2.2-2.9) 20 - Penicilli B2 ca. 17.8-21.4x3.6-4.2 3 Discussion.— The monotypic family Nemanthidae and the genus Nemanthus were proposed by Carlgren (1940: 212-214) to include forms of which the mesenteries may form thread-like structures referred to as acontioids, which, unlike true acontia, are not heavily laden with nematocysts but instead with numerous glandular cells and few nematocysts only. The three species of the genus Nemanthus described so far, viz. N. nitidus (Wassilieff, 1908) (Sagami Bay and Bonin Islands, Japan), N. californicus Carlgren, 1940 (California, Sea of Cortez) and N. annamensis Carlgren, 1943 (Gulf of Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden 69 (1995) 127 Tonkin) are unidentifiable on the basis of the available descriptions. The morphologi cal and anatomical differences given by Carlgren (1940: 212-217, figs. 1-5; 1943: 36-38: fig. 26) are in degree rather than substantial, and therefore possibly just individual variation. Hence, we are much inclined to regard these three nominal species as a single biological species with a wide Indo-Pacific distribution. Carlgren's data on the cnidom of the three species are also reasonably in agreement, both as regards the types present and the length-ranges of the main categories. On the other hand, the width-ranges do not quite match, but these differences might prove fictitious (due to the condition of the material examined and/or inaccurate measurements) .By way of precaution and pending a re-examination of the types, we here refrain from synon- ymizing the three species recognized by Carlgren. In the context of the present note it also would seem undesirable to propose any nomenclatural changes. For geo graphical reasons we identify the Kenya specimen with Nemanthus annamensis. Coin cidental^?), the details of the cnidom of the Kenya specimen are also best in agree ment with the data given by Carlgren for N. annamensis (Carlgren, 1943: 37). General discussion and conclusions The function of carrying foreign objects by dromiid crabs is generally explained as camouflage against predators (Gerstaecker & Ortmann, 1881-1901: 1217, 1256; Morton, 1989: 22; Zann, 1980). Prédation experiments in aquaria with Octopus on covered and naked crabs are strongly in support of this explanation (Polimanti, 1911). A distasteful or repellent character (due to the toxic character of secondary metabolites) of the cover material may provide an extra protective function (Yonge, 1966; Branch & Branch, 1981). In most cases the cover consists of sponges or ascid ians. These are not generally foraged upon by potential crab predators such as fish and cephalopods. A third protective function of the cover can be that of a decoy (Carlisle, 1953): when attacked, a crab can instantly let loose its camouflage and leave the predator alone with it. This trick, possibly of some importance in the case of fast moving crabs such as species of Dorippidae, is not likely to produce much effect in the case of the rather slow dromiid crabs. An actinian used as cover would not only seem to provide good camouflage to the crab, but also form an effective means of defence on account of the potentially harmful nematocysts. Hence, it is not surprising that there are many examples to illustrate that decapod crustaceans readily associate with sea anemones. The common Mediterranean-Lusitanean Anemonia sulcata (Pennant, 1777) offers shelter to a variety of crab species which tend to hide against its column under cover of the long tentacles (personal observations). It is obvious that this form of associa tion is more obligate in some species than in others, and a number of species found with A. sulcata are probably just occasional visitors, seemingly accepting the shelter of the anemone just as they would accept camouflage by algae, or the relative safety under a stone or in a crevice. Be that as it may, they clearly do not avoid the actinian as shelter and apparently suffer no harm from its nematocysts. Among the species found associated with Anemonia sulcata also figures a dromiid crab, viz. Dromia mar- morea Forest, 1974 (personal record, 1 specimen with sponge cover, Canary Islands, 1986; RMNH D 41673; identification C.H.J.M. Fransen). 128 Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden 69 (1995) Notably in tropical waters, a host of shrimp species are obligate actinian symbi- onts that live among the tentacles of their hosts (mostly medium-large to large zoo- xanthellate species belonging to the families Stichodactylidae, Thalassianthidae and Actinodendronidae). The spider crab Cyclocoeloma tuberculata (Miers, 1880) covers its carapace with corallimorpharian anemones of the genus Discosoma Rüppell & Leuck- art, 1828 (den Hartog, 1988). Boxer crabs (Lybia ssp.) carry small actinians in their claws which are used against potential predators. Also well known is the protection of hermitcrabs against predators like Octopus by symbiotic actinians. Many more examples could be listed, but a survey of crab/sea anemone associa tions is beyond the scope of the present note. However, in view of the above, it strikes that associated occurrence of dromiid crabs and Actiniaria has not been recorded more often. Speculating about the reasons, this may not be due primarily to intrinsic depreciation by the crab, but possibly rather to the behaviour of the anemo nes. With the exception of burrowing species, most Actiniaria tend to attach actively to underlying substrata by means of an often flexible pedal disc. This habit may not be compatible with the usual habit of dromiid crabs to hold passive, unattached cover subjects and might be a reason why Actiniaria are not favoured as camouflage. Be that as it may, the association here described shows that this particular crab speci men, presumably in the absence of more suitable associates ,could nonetheless cope with the anemone. Members of the family Dorippidae and in particular of the subfamily Dorippi- nae, which, in accordance with dromiid crabs, cover themselves with unattached organisms, generally have more specific, and often more fancy associations. They are found associated with e.g. sea urchins (personal observations, Amboina, December 1990), jellyfish of the genus Cassiopea (observation B.W. Hoeksema, N. Sulawesi, October 1995), holothurians, solitary ascidians, dead or living crabs, fish heads, etc. (zur Strassen, 1918: 692-693). At least one species of this family, the SE Asiatic Dorip- poides facchino (Herbst, 1785) is practically always found associated with the actinian Cancrisocia expansa (Stimpson, 1855) (see e.g. Holthuis & Manning, 1990: 58-63). However, in this association the base of the anemone is invariably carried along with its substratum, viz. a valve or valve-fragment of a bivalve mollusc, so that the crab does not face the supposed problem of handling a soft, flexible body of variable shape. Moreover, the base of the anemone secretes a rather thick plate of a velvety substance, which partly takes over the function of the shell once the growing anemo ne exceeds the outline of the shell (Delsman, 1925:175). Dromiid crabs generally seem to have a preference for sponges and compound ascidians. In a recent monograph of the Dromiidae of New Caledonia and the Philip pines, 14 species are mentioned that have been found with sponge covers and 15 spe cies carrying compound ascidians (McLay, 1993). Nine of these species are recorded to associate with both, indicating a low specific preference at best. Dembowska (1926: 163) already demonstrated that the eastern Atlantic Dromia personata will take a variety of organisms and materials for cover, including artificial matter. A certain preference, however, was shown for sponges. Crabs will drop an artificial cover when a sponge cover is available. Lavaleye & den Hartog. Associated occurrence of crab and actinian. ZooL Med. Leiden 69 (1995) 129 So far, Lauridromia intermedia had only been recorded uncamouflaged or with a sponge cover (McLay, 1993; RMNH D 35228 from Seychelles). However, data on the species are scarse, and it therefore is difficult to say whether associated occurrence with Actiniaria is accidental or not. We are inclined to consider the case here described as an exception. In the first place, no substantial fragments of sponges or compound ascidians were present in the trawl catch yielding the crab. Moreover, members of the genus Nemanthus normally live attached to gorgonians, their bases surrounding the branches, and hence are sometimes referred to by the vernacular name "gorgonian wrappers" (Kerstitch, 1989: 24). It is puzzling, therefore, how the crab obtained the anemone. The trawl catch came from a sandy-muddy bottom and did neither contain any gorgonians nor any suitable hard substratum for these. Besides, the colour of the anemone was unusually pale, almost ghostly white, the usual variegation all but faded, suggesting less favourable conditions of living. Rep resentatives of Nemanthus normally are yellowish to orange, as a rule more or less distinctly variegated with dark patches. Acknowledgements We thank Mr Alex Kerstitch, Tucson, USA, for unpublished information on the occasional association of Cryptodromiopsis larraburei and Calliactis polypus. References Arnold, A.F., 1901 (Reprint 1968). The sea-beach at ebb-tide. 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