<I>Echinometra Lucunter</I> (L.)
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BULLETIN OF MARINE SCIENCE, 28(1): 181-188, 1978 CORAL REEF PAPER INTRASPECIFIC AGONISTIC BEHAVIOR IN THE ROCK-BORING SEA URCHIN ECHINOMETRA LUCUNTER (L.) (ECHINODERMATA: ECHINOIDEA) Herman Griinbaum, Glen Bergman, Donald P. Abbott, and John C. Ogden ABSTRACT The agonistic behavior of the sea urchin Echinometra lucunter (L.) was studied in the algal ridge reefs of Boiler Bay, St. Croix, U.S. Virgin Islands, where it inhabits burrows in the reef surface. Sixty-four encounters were set up by placing an urchin (intruder) at the opening of an occupied burrow (host). In 46 cases, agonistic behavior between the host and intruder resulted. Most of the interactions involved pushing, but biting was fre- quently observed. The encounter usually resulted in the eviction of the intruder from the burrow, but occasionally the host was evicted or the urchins cohabited the burrow. Burrow occupants successfully evicted all intruding urchins of their own or slightly larger size, and regained possession of their burrows after being experimentally replaced by an urchin of equal size. As Echinometra live in dense aggregations in wave-swept areas, this behavior may be significant in defending protected living space and access to a food sup- ply. Many echinoids in the sea today are gre- in a variety of habitats. It is most abundant garious and form homotypic and heterotypic on tidal terraces, beachrock, and other areas aggregations which have been considered to of rocky shore from just below the low tide be the result of individual reactions to the level to depths of a few meters, especially in physical environment or short-term aggrega- regions swept by surge and surf (Ginsburg, tions for reproductive activities, and not the 1953; Kaye, 1959; Lewis, 1960; McPher- result of any social behavior (Reese, 1966). son, 1969; Hunt, 1969; Teytaud, 1971; Ab- Pearse and Arch (1969), however, con- bott et aI., 1974; and earlier authors cited in sider that the large groups of diademid Mortensen, 1943). Here it usually inhabits echinoids common in tropical seas do in- holes and crevices that afford protection volve true social behavior. They present against dislodgment. evidence to support this view consisting of The northeast coast of St. Croix, U.s.V.I., group responses to disturbance, and aggre- in the area of Boiler Bay, has well developed gation behavior involving particular patterns algal ridge reefs and beachrock benches of spine contact. Other authors have exam- which are extensively tunnelled and bur- ined different sorts of echinoderm aggrega- rowed by E. lucunter (Adey, 1975 and Ab- tions and have concluded that some form of bott et aI., 1974 for detailed maps of the social interaction is involved (Dix, 1969; area). Within this area, as elsewhere, most Branham et aI., 1971; Warner, 1971). 80- animals are in individual pits or burrows cial behavior involving agonistic encounters (Fig. 1). Others are arranged along con- has been recently reported for all three or- tinuous crevices, and in areas where urchin ders of sea stars by Wobber (1975) and de- boring has been extensive, adjacent bur- scribed for a few species. rows have run together and a small group Echinometra lucunter (L.), a widely dis- of animals occupies a larger open depres- tributed sea urchin in the tropical and sub- sion. However, even in situations where ad- tropical Atlantic (Mortensen, 1943; Clark, jacent animals are not separated by obvious 1954) is found abundantly in shallow waters physical barriers, they are always out of 181 182 BULLETIN OF MARINE SCIENCE, VOL. 28, NO. I, 1978 Figure 1. Echillometra [ucullter in burrows in one of the algal ridge reefs at Boiler Bay, St. Croix. Note close spacing of burrows and benthic algae at burrow openings. reach of one another. Repeated observa- appeared in a preliminary account (Griin- tions on selected individuals, day and night, baum and Bergman, 1974). show that each animal has its home site, distinct from that of any other individual. It METHODS AND MATERIALS moves short distances, especially at night, From January 11 to 20, 1974, observa- but each animal clearly "homes" in the tions were made on the agonistic behavior of classic sense, and ranges over a narrow area. E. lucunter living in the algal ridges of Boiler One rarely finds two individuals in the same Bay. The ridges, located within 50 m of the burrow, or two with spines, tests, or tube shoreline, afford an excellent site for ob- feet touching; when one does, the situation servation of the urchins in their burrows, as is far from static (Abbott et al., 1974). well as providing protection from the surge Prior to the present study, Griinbaum, characteristic of this bay. Observations were Warner and Ogden (unpubl.) noted the made both day and night by observers rapid disappearance of tagged E. lucunter standing on the bottom near the ridges us- which were released in areas with extensive ing snorkel gear. All burrows chosen for ex- burrows of conspecifics. On closer exam- perimentation were completely submerged ination it was discovered that the tagged during observations. As the ridges are ex- urchins were being forcibly prevented from tensively burrowed, many burrows run to- taking residence in occupied burrows. The gether and thus have multiple entrances, present study documents this behavior and large openings, or complex structures. Ex- assesses its importance in the establishment perimental situations were set up by remov- and stabilization of urchin populations in ing urchins of various sizes from their bur- burrows. Some details of the work have rows and moving them to the entrances of GRONBAUM ET AL.: BEHAVIOR IN THE SEA URCHIN ECHINOMETRA 183 BURROW TOO SMALL TO INTER IN'mUDEA MOVED AWAY ~ NO VISIBU! CONTACT BETWI(N ~ HOST AND INTRUDER NO 'IGHT.. >1 INTRUDER un ~ ," BURROW ~ IPINI tONIACT BITWIIN . HOST AND INTAUDiA INTRUDIR ATTEMptED FUR'tHEA ADVANCE 1 BUT WAS PUSHED OUT ~ 0' BURROWBYHOS'> HOST ADVANCED AND INTRUDER LEfT PUS~~~R~::RINS' ~ BURROW INTRUDER RUIiAnD WIIHOUT 'URTHER .PUSHING BY HOSt INnUDII PLACID A'"IN- [),. 'NTRUDER .HTlRED D TRAHC' r~.H6~~T BURROW \)HOST BUIAOW HOST PUSHED .0 BURROW . fiGHT INVOLVED 6 INTRUDER ADVANCED ~ OPENING, LE" BURROW 46 . PUSHINGIONLY ~ AND PUSHED AGAINST HOST ~ BURROW LARG' 'NOUGH '011 ~ two. IHfRUDER DISPLACED 26 HOST BUT BOTH CONtiNUE TO OCCUpy BURROW FIOHT B RESULTED ':~.~~:~,~~T~:L~~::CTED 80T" HOST AND 'NTAUDER UUD ~'NTRUD'R ACTIVELy.nR'ATID MOUT" (91 FROM HOST, LEFT BURROW PlOHT INVOLVED IITING AS WELL AS PUSHING ONl Y Hon USED HOST ACTIVELY IVlcnD IY MOUTH (7) PUSHING OF INTRUDIR, I UFT IURROW ONl'f INUUI.'IU } USED MOUTH (4) aURROW LARGI ENOUGH fOIl lWOI INTRUDeR I.'IISP1.ACED 1 Hon BUT BC)IH CONTINUE TO 'OCCUpy BURROW Figure 2. Agonistic behavior patterns between host and intruder. Figures and relative width of arrow indicates number of occurrences of each sequence. See text for details. already occupied burrows. The transferred visible contact with the host, (3) aU of the urchin was called the "intruder" and the oc- remaining 46 trials resulted in agonistic be- cupant of the burrow the "host." Sizes of havior. urchins were taken by measuring the largest The agonistic encounters began when the test diameter at the equator with a vernier intruder moved far enough into the burrow caliper (the test is often markedly oval). to make physical contact with the host, which reacted with immediate movement of RESULTS spines and tube feet. Once aware of the in- trusion, the host usually moved directly to- Sixty-four encounters were set up and ward the intruder, keeping spines and tube various sequences of agonistic behavior were feet in constant motion. In nine out of 20 noted (Fig. 2). Following the initial place- encounters (Fig. 2) in which the host gave ment of the intruder, one of three events an initial push to the intruder, the latter ac- occurred: (1) in three trials the intruder was too large for the opening of the burrow, tively backed off and left the burrow; in the and simply moved away, (2) in another 15 other 11 encounters the intruder resisted the trials there were no agonistic responses; in push but was forced all the way out by the 10 of these cases the intruder retreated im- host. While being evicted the intruder often mediately after making physical contact with flattened its spines against the test. In six the host in the burrow; in the other five cases the results were reversed, the intruder cases the intruder retreated prior to any pushing harder than the host; here the host 184 BULLETIN OF MARINE SCIENCE, VOL. 28, NO. I, 1978 Figure 3. Agonistic encounter between two E. lucunter in an aquarium involving the use of teeth. Photograph by C. Kitting. was either evicted altogether (N == 4) or in host (eight cases) or by cohabiting the bur- the case of larger burrows, was displaced to row. Cohabitation was observed in three in- one side and continued to cohabit the bur- stances; in all cases the cohabiting urchins row with the intruder (N == 2). took up positions well-separated from the Twenty trials resulted in the use of the host in the burrow. teeth by either host or intruder or both (Fig. The duration of the encounters varied 2). Here the attacker rotated its body until considerably. Interactions involving only tbe oral surface faced tbe opponent, then pushing lasted 3 to 30 min. When biting was protruded its Aristotle's lantern and pro- involved, the interactions lasted up to 5 h. ceeded to bite spines off of the opponent The greater the size difference between the (Fig. 3). Subsequent pushing, often with urchins, the shorter the encounter. continued biting, led to the eviction of the The correlation between the size of the opponent or to a counterattack.