Reproductive biology of Arthritica crassiformis and A. bifurca, two commensal bivalve molluscs (Leptonacea) Pa u l C h a n l e y * and M atoira C h a n l e y * Fisheries Research Division. Ministry of Agriiulture and Fisheries. P.O. Box 19062. Wellingion. -Vt'ir Zealand Arthritica crassiformis Powell and A. bijurca (Webster) are functional hermaphrodites. Specimens of both species were isolated and maintained in individual plastic contniners until they produced two broods of larvae, A. crassiformis produced an average of 7.75 larvae per day, with brood sizes of 245-3120 larvae and 11-21 days between broods. A. bifurca produced an average of 8.56 larvae per day, with brood sizes of 5-5016 larvae and 8i to 20 days between broods. Larger clams of both species produced more larvae than did smaller clams. Larvae were incubated for about I week and were usually released at one time although releases continued for as much as 3 days. A, crassiformis larvae averaged 150 /im in length at release. The hinge increased from 80 to 105 Aim in length and was without discernible hinge teelh. A low, rounded umbo appeared at lengths of 175-200 and larvae set at 235-270 f»m. A. bifurca larvae averagecl 124 in length at reJease. The hinge line increased from 76 to 89 ^m and was without teelh. A low. rounded umbo appeared at 150-169 ^<m and larvae set at 243-275 nm. K ey w o rd s: Commensal bivalves, Arthritica spp.. Reproductive biology, Brooding patlerns. Larval stages, Larvai morphology, Development, Settlement. INTRODUCTION periostracum bordering of the shell. Several may be Many leptonacean (Erycinacea) bivalve molluscs found on each host, with the densest concentrations are small clams which live in association with other on the ventral surface just posterior to the pedal invertebrates (Boss 1965). Some species are herma­ gape (Morton 1975). A. crassiformis incubates sev­ phroditic or capable of self-fertilisation (Oldfield eral hundred larvae 109-150 /im in length (Ponder 1964, Franz 1973). Gage (1968) reported “sperm­ 1965, Booth 1979). Booth found larvae in 10% of atids, enclosed in elongate sac, opening into the adults collected in January and May 1972 in the Bay supra-briinchial space in the mantle cavity" in of Islands and described these larvae. Mysella cuneata; whether this represents a true A. bijurca (Fig. 2) is of a similar size to A. crassi­ hermaphroditic condition or a modified male is un­ formis and is widely distributed in New Zealand known. Extreme modification, probably parasitic in w'aters. It may live commensally around the outer nature, has been reported by Jeruier & McCrary surface of the head end of the tube w'orm Pectinaria (1968) in the male of Montacuta percompressa that australis (Wear 1966) or it may be free^iving is found within the mantle cavity of the female. The (Ponder 1965, Booth 1979). Booth found that 15­ male of M. floridana has also been found living with­ 35% of adult A. bifurca collected throughout the in the mantle cavity of the female (Dr C. Jenner, year were incubating larvae, but this dropped to pers. comm.). 5-10% during the spring months: incubatcd larvae In most leptonacean species that have been studied were 110-130/xm long. He described these larvae and the lai-vae are incubated briefly, to the straight-hinge tentatively identified the late stage larvae of this stage, and then undergo a relatively long planktonic species from plankton samples. The late stage larval existence (Chanley & Chanley 1970). hinge lacked a true provinculum but did have Arthritica crassiformis is a small white clam (Fig. “feebly developed serrations”. I) that lives commensally with the rock-boring The present study was part of a programme to nholadid Barnea similis in soft rock near the low tide rear larvae from known parents of as many New 'ne throughout the North Island, New Zealand. A. Zealand species of bivalve molluscs as possible and "assiformis reaches a maximum length of 5.4 mm to describe all developmental stages. While main­ "onder 1965), but specimens of 4 mm or less are taining brood slocks of A. crassiformis and A. ore usual. It is most frequently attached to B. bifurca, observations were made on frequency of riilis by byssus threads, especially to the thicker spawning, incubation period, and fecundity. I Jeived 19 July 1979; revision received 2 November 1979 f heries Research Division Publication 388 - esent address: Proyecto Hatchery, Fundacion Chile, Avda. Santa Maria 06500. Santiago, Chile MATERIALS AND METHODS ment were fixed in a sea water solution of 5% for­ malin, 5% sugar, and 15% Tris, and preserved in Specimens of A. crassiformis Powell were collected a sea water solution of 5% formalin, 10% propylene from rock-boring phoiads and their burrows in soft glycol, and 1% propylene phenoxitol, buffered to rock at Piimmerton, on the Pauatahanui Inlet, on 27 pH 8 with sodium glycerophosphate (Turner 1976). October 1977. Most were associated with Borneo Larval measurements were made with a filar similis, but contrary to the reports of Ponder (1965) micrometer. and Morton (1973) a few were found associated with another pholadid, Pholadidea spathulata. A. bijurca (Webster) were collected from the beach at Petone, Wellington Harbour, on 10 Nov­ T e r m in o lo g y ember 1977. They were associated wilJi the marine The following is an expanded version of the ter­ tube-worm Pectinaria austrolis. Both species were minology used by Chanley & Andrews (1971) and is brought to the Fisheries Research Division Shellfish illustrated in Fig, 3. Hatchery at Mahanga Bay, Wellington, and kept Total length. The maximum anteroposterior separate from their hosts. Between 50 and 100 of dimension (usually roughly parallel to the dorsal each were kept at 15.5-I9.0“c, in shallow polyethy­ margin of the shell). lene dishes containing sea water filtered at I p-m. In Tot.al height. The maximum dorsoventral dimen­ addition, 14 A. crassiformis and 12 A. bijurco were sion (roughly perpendicular to the length). isolated and individually maintained in clear 100-ml plastic jars at I8-20°c. The water was changed and D e p t h . The maximum left-right dimension. the larvae were removed and counted daily. Adults Hinge line length. The maximum straight dis­ were fed enough cultured unicellular algae to dis­ tance of the dorsal margin. In intact larvae this is colour the water after every water change; Pavlova measurable only before development of the umbo. lulheri, Isochrysis galbana. Telraselmis suecica. and The term is used to describe the measurable straight Skeletonema costalum were used to feed both lar\<a2 dorsal margin, whether it be prodissoconch I or a and adults. Clams were preserved for histological combination of prodissoconchs I and II. study at various intervals after release of larvae. Larvae were collected and cultured separately at Shoulder height. The maximum distance to the 18-20°c in 1-L beakers at densities of up to 15 per dorsal margin from a line extending from the ml. Larvae were cultured in 1 /tm filtered sea water extreme anterior margin to the extreme posterior which was changed daily by pouring it through margin. In straight-hinge larvae this will usually be a 75-Mm nylon screen. After the water was changed, to or near one end of the hinge line. In umbo larvae larvae were fed sufficient cultured algae to discolour this axis will extend to the maximum dorsal extension the water. Samples of larvae at all stages of develop­ of the umbo. k'ig. 1. .^dult Arlhriliiti craasilormis. Natural size Fig. 2. Adult Arlhrilica bifurca. Natural size 3.0­ 3.0-4.5 mm. 4.0 mm. HInge'tlne length Fig. 3. Terminology used in dcscribins lar­ vae of Arihrilica crassi- forwis and A. bijurca. Larval outline used in the illuslrnlioii is that of Choroniyliliis chorus. Shoulder length C. ]n straight-hinge larvae the Length anterior end. The distance from the an­ maximum straight line distance from the extreme an­ terior extremity of the shell to a line drawn from terior margin to the centre of rJie hinge line (anterior the middle of the umbo or straight-hinge line to shoulder) End the maximum straight line distance the extreme ventral margin of the shell. from the extreme posterior margin to the centre of Length posterior end. The total length less the the hinge line (posterior shoulder). In umbo larv'ae length of the anterior end. the maximum straight line distance from the maxi- mt-m dorsal extension of the umbo to the extreme Umbo length. The distance between the anterior anterior and posterior margins respectively. and posterior points where the umbo extends above the remainder of the dorsal margin of the shell. SHOta.DER LENGTH E. This dimension (measur­ This dimension is measurable in the ‘knobby’ type of able only in straight-hinge larvae) is the maximum umbo where the separation of umbo from t.he dorsal straight line length from the extreme anterior margin margin is clear, but is usually not measurable for to the anterior end of the hinge line (anterior other umbo types. Occasionally it can be measured shoulder) ard from the extreme posterior margin when a minor protuberance extends above the lo the posterior end of the hinge line (posterior dorsal margin with umbones otherwise shaped as shoulder). 'round', angular', or ‘broadly round'. U mbo. The swollen dorsal protuberance of the Arlhritica bifurca. C/ams began releasing larvae shell. (See Chanley & Andrews (1971) for umbo within 1 day of being placed in isolation. AU 12 had shapes.) re.leased larvae by the 14th day of isolation, by which time some of the clams had begun to release their second batch of larvae (Table 2).