Aggregations of Dactylopleustes (Amphipoda: Pleustidae) on diseased areas of the host sea-urchin, Strongylocentrotus int
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OCEANARIUM
Aggregations of Dactylopleustes (Amphipoda: Pleustidae) on diseased areas of the host sea-urchin, Strongylocentrotus intermedius Masafumi Kodama 1
&
Jun Hayakawa 1 & Tomohiko Kawamura 1
Received: 4 May 2020 / Revised: 9 July 2020 / Accepted: 13 July 2020 # Senckenberg Gesellschaft für Naturforschung 2020
Fig. 1 a, b1, b2 Dactylopleustes yoshimurai aggregated on the diseased area of urchin (oral view); c ovigerous female of Dactylopleustes yoshimurai collected from the diseased urchin. They have characteristic white color-pattern on the antennae and pereon, possibly camouflage color-pattern on the urchin’s surface
Communicated by L. Menzel 1
* Masafumi Kodama [email protected]
International Coastal Research Center (ICRC), Atmosphere and Ocean Research Institute, The University of Tokyo, 1-19-8 Akahama, Otsuchi, Iwate 028-1102, Japan
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The representatives of the genus Dactylopleustes Karaman & Barnard, 1979 are known as obligate symbionts of urchins and thought to evolutionally adapt to inhabiting urchin’s surface (Bousfield and Hendrycks 1995). However, despite their strong symbiotic relationship, the ecology of Dactylopleustes and the interaction between Dactylopleustes and its host urchins are almost unknown (Vader 1978). During a survey of invertebrates in rocky reefs in Otsuchi Bay, Japan (39° 21′ 45″ N 141° 57′ 35″ E, 2– 3 m deep, 14 Jan 2020), we randomly examined fifty individuals of short-spined urchin Strongylocentrotus intermedius, of which four diseased individuals were found with a number of Dactylopleustes amphipods. In the field observation, the Dactylopleustes individuals including adults and juveniles were surprisingly found to aggregate on the diseased area of the urchin (Fig. 1a). They aggregated only on the diseased tissues but not on the fully skeletonized areas (Fig. 1b). In contrast, such distinct aggregation was never found in the healthy urchins. The amphipods congregating on the diseased urchin were subsequently collected and identified as Dactylopleustes yoshimurai Tomikawa, Hendrycks, & Mawatari, 2004 (Fig. 1c). These observations suggest the aggregated Dactylopleustes feed on the diseased tissues of urchin whereas any food no longer exists on the skeletonized areas, implying they use the urchin not just as habitat but also as prey. Tomikawa et al. (2004) implied two possibilities for their feeding habits based on their mouthparts morphology: (1) small organic food items around the spines, (2) urchin test, such as pieces of skin or diseased surface tissue. Our finding supports the latter hypothesis at least in diseased urchins. Moreover, this suggests the following two possibilities: (1) Dactylopleustes cause the disease, and thus, have negative affect on the urchin; (2) Dactylopleustes feed on and remove the diseased tissue, and thus, have potential positive affect on the urchin. Dactylopleustes were thought to inhabit urchins by grasping the spines using their specialized pereopods (Tomikawa et al. 2004). However, since no spines left on the d
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