Host-use and population demographics of the ascidian-dwelling amphipod Leucothoe spinicarpa : indication for extended parental care and advanced social behaviour

Abstract

Leucothoe spinicarpa (Abildgaard, 1789) is an endobiotic amphipod, commonly found in sponges, bivalves and ascidians. I examined the occurrence and population demographics of L. spinicarpa in different species of solitary and stolonal ascidians from two lagoonal embayments on the Florida Atlantic Coast. Between 77 and 95% of the solitary ascidians Ascidia nigra (Savigny, 1816), Microcosmos exasperatus Heller, 1878 and Styela plicata (Leseuer, 1825) were hosts to L. spinicarpa. The high rate of occupancy in large solitary ascidians by L. spinicarpa in September 1997 suggests that competition for these microhabitats can occur. The average number of amphipods ascidian-1 varied between 1.89±0.48 s.e. in S. plicata and 5.60±1.24 s.e. in M. exasperatus both from the Indian River Lagoon. The maximum number of amphipods ascidian-1 was 30 in A. nigra from Lake Worth. Prevalence of L. spinicarpa in stolonal ascidians is substantially lower than in solitary ones. Many zooids contained no amphipods, but at one site, 9.4% of all zooids of Clavelina oblonga Herdman, 1880 were host to L. spinicarpa. At this location, usually single amphipods were found in a zooid, however, one zooid of C. oblonga contained three amphipods and another zooid contained four. The size range of L. spinicarpa found in solitary ascidians differs considerably from that of amphipods collected in stolonal species. All sizes of amphipods occurred in solitary ascidians, while individuals found in stolonal ascidians were usually <5 mm and immature. Some solitary ascidians contained an adult male and female pair and a clutch of similar-sized juveniles. These observations show that juveniles remain in their parents' dwelling after hatching from the female's brood pouch. It is concluded that after leaving the parental dwelling, many subadult L. spinicarpa seek shelter in the zooids of stolonal ascidians before they relocate to larger, solitary ascidians when reaching maturity. The demographics of amphipod groups suggest that some offspring reach maturity in their parents' dwelling and inherit the parental ascidian after the death of their parents. Thus, in this ascidian-dwelling amphipod extended parental care in a stable microhabitat may have led to the evolution of advanced social behaviour.