Notes on Development of the Limpet Acmaea asmi, a Commensal on Tegula funebralis (Mollusca; Gastropoda) by Diane E. Wickizer Stanford University, Hopkins Marine Station, Pacific Grove, California Tarttg. Adults of the limpet Acmaea asmi (Middendorff, 1849) live as commensals on the shells of the intertidal gastropod Tegula funebralis (A. Adams, 1854). The larvae, however, pass through a pelagic phase; the longer they remain pelagid the greater the chance they will be carried away from the shore and hence away from the host population. The densest adult populations of A. asmi occur in relatively /12-7 Hdtd donel high, and somewhat isolated pools, where neither wave action nor surge will as easily carry larvae away from T. funebralis (Eikenberry and Wickizer, 1964). Legie suggests the possibittty oromparativelg beief o swimming period and a tendency of the larvae to settle ogresiblyy and directly on the shells of T. funebralis, I therefore attempted to follow larval development and to determine if this were the case. a4 as mighl in ms afod ataateno, 27. D.E. Wickizer Spawning of Adults and Rearing of Larvae All larvae used for embryological work were obtained by natural spawning in the laboratory during the period from April 13 to May 24, 1963. A. asmi were removed ga from T. funebralis collected on rocks by-the Hopkins Marine Station, Pacific Grove, California. They were placed in 5-inch finger bowls of filtered sea water, changed daily, and kept at 14°0. A. asmi removed from their hosts immediately began to clump together and to climb up one another's shells. Eggs and sperm are shed fee inthe water. The males generally spawn first, although isolated females did spawn occasionally. Various stimuli were tried to elicit spawning. The most successful was light shock. Bowls containing several A. asmi each were completely covered with inverted black dishes at night so that light was excluded. Between 8 a.m. and 10 a.m. the following morning, the dishes were removed and on eight occasions one or more animals spawned immediately or within an hour. In uncovered dishes exposed to normal diurnal light conditions animals generally spawned at dawn if at all. Although light fol- lowing a period of darkness appears to elicit spawning, A. asmi will sometimes spawn without it, for twice A. asmi spawned while in the dark. Although larvae were raised in bowls at a constant 208 D.E. Wickizer temperature (some at 10°6, some at 12°0, some at 14°0 ) and filtered sea water was changed daily, larvae began to die at 48 hours. Attempts were made to prolong life beyond two days by providing food including filamentous diatoms ground in a Waring blendor, diatoms from a high tide pool, and pure cultures of both Platymonas and Stichococcus. msrl ppastedter? Although larvae were noted ta Stichococcus, the food did not increase larval life span. Aeration of the water likewise appeared to have no beneficial affects. The larvae began dying shortly after torsion. Between 45 and 60 hours the larval vel detached and the larvae disintegrated. Ciliate protozoa were nearly always found in the shells of these animals. The only larvae which lived for as long as 6-7 days were those in three bowls raised at 12°0, without a daily change of water. Larval Development Figure 1 tabulates the timing of events observed during larval development and figures 2-6 illustrate selected stages. Phase I—Development to the troophore stage. The unfertilized egg is pale yellow eer and measures eilis C D.E. Wickizer 0.126-0.130 mm in diameter. A clear fertilization mem- brane is formed. Cleavage follows the gastropod spiral pattern. To the 4-cell stage, the blastomeres are equal; at 8-cells micromeres are differentiated. Other details observed conform generally to the findings of Boutan Virgign (1899) on Acmaea Phase II—Trochophore. The prototroch, consisting of a ring of large cells bearing a single girdle of cilia actively rotates the larvae which at first remain near the bottom. Shorter cilia cover the surface anterior to the prototroch. The apical tuft bears two long flagella measuring approximatelyO.12 mm in length which appear to have a sensory function, as the larvae back off whenever these touch a surface. Similar structures were noted in Patella vulgata by F.G.W. Smith (1935). The trochophore is very yolky, and no internal organization could be seen. Phase III—-Pre-torsional Veliger. Modifications at this stage include the evagination of the shell gland, development of the foot rudiment and expansion of the velum. The apical tuft with its sensory flagella remains. Longer cilia develop on the sides of the velum. By 33 hours the velum has become a double row of cells with two sets of cilia alternately placed. Two 5. D.E. Wickiz er eye spots develop subapically. Veliger larvae are free swimming throughout this period, but become negatively phototactic at about 18 hours and tend to remain on the bottoms of bowls for brief periods from this point on. Phase IV—Torsion and Post-Torsional Larvae. The muscular phase of torsion begins between 30 and 35 hours and is relatively rapid. Boutan (1899) states that in Acmaea Viaginte t the process takes only 2-3 minutes. The shell and visceral hump rotate well over 90 degrees with reference to the head and foot, allowing the animal to retract its head partly into the mantle cavity. The long apical flagella are lost at this time. The remaining phase of torsion takes place gradually, but by 55-60 hours torsion appears completed, and the head and velum can be withdrawn completely into the shell. Other changes occurring during late torsion or immediately thereafter include enlargement of the foot and further development of the operculum, development of additional muscles, thickening of the mantle edge, and differentiation of the gut. The larval shell at this stage is an unusual structure with two small lateral flanges projecting medially from the lateral margins of the shell just inside the aperture. Metamorphosis and settling possibly occur between 6 and 7 days after fertilization. As stated, in a few cases whole animals still with velum intact were observed O 6. D.E. Wickizer at 6-7 days. These animals appeared to be shelless; yet no shells were observed in any of the bowls. Perhaps reabsorption of the shell occurs. When an adult T. funebralis was placed with these larvae a definite attraction towards it was noted, and several larvae began to swim over the surface of the animal, appearing to settle there. An opportunity to repeat the observations was not available. Summary 1. Eggs and sperm are shed into the water, and spawning seems to be stimulated by exposure to light following darkness. 2. The larvae develop rapidly; torsion begins between 30 and 35 hours and is completed by 55-60 hours. 3. A. asmi larvae appear to be attracted to T. funebralis at 6-7 days and probably settle about this time. 212 Figures D.E. Wickizer . . . Fig. 1. Developmental stages, size, and behavior of A. asm i larvae. Dashed line denotes uncertainty or variation between individuals. Fig. 2-6. Developmental stages. Fig. 2. Trochophore larvae, 6-7 days after fertilization. Fig. 3. Pre- torsional veliger, 28-30 hours after fertilization, from dorsal side. Fig. 4. Pre-torsional veliger, 28-30 hours after fertilization, from right side. Fig. 5. Veliger with muscular phase of torsion completed, 48-50 hours after fertilization, from left side. Fig. 6. Post-torsional veliger, 55-60 hours after fertilization, from right side. All figures except number 6 were drawn from life. The larvae shown in figures 2,3,4, and 5 were relaxed for one hour in two parts sea water and one part distilled water saturated with chlorobutanol. The larva in figure 6 was fixed in osmic acid. Apical flagellum, af; foot, f; mantle cavity, mo; mantle margin, mm; operculum, o; velum, v; visceral mass, vm; velum retractor muscles, vrm; prototroch, ph; retractor muscle, r; shell, sh; mouth mh; yolk, y. Wickizer Literature Cited Boutan, L. 1899. La cause principale de l'assymetrie des Mollusques gasteropodes. Arch. Zool. Exp. (2), 7: 203-213, 2 48-2/. Eikenberry, Art., Jr. and D. E. Wickizer. 1963. Ecology of Acmaea asmi in relation to Tegula funebralis. The Veliger. Smith, F. G. W. 1935. Development of Patella vulgata. Phil. Trans. Roy. Soc. London, Series B--Biol. Sci. 520 (225): 95-125. 2 45 S . e.