278 BULLETIN OF MARINE SCIENCE, VOL. 29, NO 2, 1979 BULLETIN OF MARINE SCIENCE, 29(2): 278-282, 1979 DEVELOPMENT OF OSSICLES IN JUVENILES OF THE SEA STAR ECH/NASTER SE!\'TUS Scott E. Siddall Asteroid development may be either direct, in whi:h there are no free-swim- ming stages and the eggs may be brooded, or indirect, involving a free-swimming larva which derives its nutritional requirements from feeding (planktotrophic) or from yolky reserves (Iecithotrophic). Barker (1978) Lsted many of the detailed investigations of asteroid development. General discussions of asteroid skeletal morphology may be found in Hyman (1955), Moore (1966), and Blake (1973). Kinne (1977) reviewed several echinoderm culture procedures but centered at- tention on echinoids prior to settlement. Sea stars of the genus Echinaster (Order Spinulosida, Family Echinasteridae) exhibit indirect development and have lec- ithotrophic larvae. Early studies of Echinaster by Lo~ner (1913) and Nachtsheim (1914) (E, sepositus) and Mortensen (1938) (E. purpure'ls) describe developmental patterns. Kempf (1966) describes Echinaster echinophorLls larvae which do not pass through a free-swimming stage. Atwood (1973) associated the presence or absence of a free-swimming stage with morphologically dissimilar adults and two different egg types. The early life history of E. sentus, a shallow water Caribbean species, was not mentioned by Agassiz (1877) in his detailed description of the adult skeletal structures. Reaumur (1732) was probably the first to describe the asteroid skeleton. Since the work of Fewkes (1888) on the development of ossi::les in late brachiolaria and juveniles of Asterias, only adult skeletal morphology has been studied. The purpose of this study is to observe the pattern of larval development and describe ossicle morphology in newly settled E. sentlls. MATERIALS AND METHOD~, Adult E. sen/lis (R = 10 cm) were collected on two occasions from several rocky, subtidal grass flats of the middle and lower Florida Keys. Specimens were held fc r up to 5 months in closed culture systems (a maximum of 10 adults/400 I; 26°C at 34%0) before spawn ng took place naturally. Through- out this conditioning period, a wide variety of foods were available to the adults including macerated bivalves, fish flesh and heavy mats of benthic algae. Larvae and juveniles were cultured in the same I 10.5 MM Figure I. Modified brachiolaria of E, sen/liS, 3 days old. SHORT PAPER AND NOTE 279 Figure 2. (A) Aboral view of juvenile E . .1'1'1111I.\', 10 days old; R = 0.62 mm; (B) Oral view of juvenile E. .1'1'1111/.1', 10 days old; R = 0.8] mm; (C) Detail of ray of juvenile E. SI'III11.1', oral view; width across marginals is 0.5 mm; (D) Aboral view of juvenile E . .1'1'1111I.1', ]0 days old; R = 0.75 mm. amb- ambulacral, b-basal, ib-interbrachial, mar-marginal, mad-madreporite, p--periostome, pp--first podial pore, ter-terminal. closed systems in which the adults were fed and maintained. Eggs and modified brachiolaria were observed and measured under a dissecting microscope. Settled juveniles (1.8-2.0 mm ray tip to tip) were prepared for scanning electron microscope (SEM) examination as follows: (I) Juveniles were relaxed in 1-2% magnesium chloride-seawater solutions then killed in cold, buffered 95'7c alcohol. (2) After removal from the alcohol, specimens were gently blotted dry and placed on an SEM mounting stud covered with a 3-mm layer of rapid setting epoxy glue ("5-Minute Epoxy" brand). Both oral and aboral views were obtained. (3) When the epoxy was firmly set, the entire sample stud was submerged in a slowly circulating, buffered (pH = 9.0) 0.5-1.0% sodium hypochlorite solution. This operation was carried out under a dissecting microscope. (4) When sufficient organic material had been removed from the juvenile skeleton (30 sec to 5 min), the stud was very gently flushed with distilled water then allowed to air-dry. 280 BULLETIN OF MARINE SCIENCE, VOL. 29, 1>0. 2,1979 Table I. Rates of early development in three species of E('hinllster Time E. l'chinophorus E. sepositlls E .. H'ntll.\ (days) (Alwood 1973) (Lohner, 1913; Naehlsheim, 19141 (present sludy~ 2 modified brachiolaria modified brachiolaria 2Vz larvae settle to substrate 3 bulges of 2 pairs of podia modified bipinnaria 5 hydrocoellobes form appear 4 hydropore opens 2 pairs of podia appear 5 water ring and radial canal is larvae settle to substrate radial canal apparent distinct 6 podia differentiated bulges of 2 pairs of podia rudiments of terminal appear ossicles appear; podia are functional 7 podia are functional; larval larvae move on their own: attachment released; bright orange color preoral lobe folds 8 third pair of podia appears oral and ahoral spines appear 9 third pair of podia appears 10 fourth pair of podia appears; podia are functional spmes appear 12 feeding begins (mouth open?) 14 mouth open third pair of podia appear;,; preoral lobe resorbed (5) The studs were coated in a vacuum evaporator with cal bon followed by a gold-palladium layer before SEM examination. RESULTS AND DISCUSSION E. sentlls adults spawned in late spring (May-June:) as ambient water temper- atures were raised to 30°C. The adults released betv'een 200-500 bright orange, yolky eggs (0.7-0.8 mm) which adhered to the sides of the aquariums. The two distinct egg types as reported by Atwood (1973) were not observed. Eggs of E. sentus closely resemble his Type 2. Early cleavages were not observed, though Atwood (1973) describes E. echinophorus as having radial, holoblastic equal cleavage. No free-swimming larval stages were observed; the modified brachio- laria became attached to the substrate by the sucker depicted in Figure I. Table I summarizes the timing of the ontogenetic events 0 Jserved for three species uf Echinaster. Barker (1978) used antibiotics in culture and felt thc.t planktotrophic larvae may require circulating water, presumably for feeding. In the present study, strong seawater aeration maintained near-saturation oxyger. levels but antibiotics were not used. Most mortalities occurred in the early stag~s amounting to 70% by the fifth day. Heavy benthic diatom layers supported g 'owth of the juveniles after the 12th day when the mouth and digestive tract apparently were functional. Growth rates were somewhat greater than those obtained by Kempf (1966) or Atwood (1973). At 3 weeks, R (center of mouth to t p of ray) averaged 1.4 mm. Once settled, morphogenetic changes proceeded ra Jidly especially with respect to the development of ossicles. Figure 2 presents 1he SEM photomicrographs REVIEWS AND COMMENTS SHORT PAPER AND NOTE 281 Figure 3. (A) Oral view of mouth armature spines of mouth frame of E. sen/lis; width of field is 0.25 mm; (B) Terminal spine, ]00 f.J.m long. obtained from typical specimens. Skeletal material which depended solely on the removed tissues for attachment was also removed. The apical system (five plates surrounding a central plate) was removed from the aboral mounts. Rudiments of the terminal ossicles of each ray appeared in the brachiolaria by the sixth day. Basals developed soon after the terminals, the basal closest to the madreporite being the largest. No skeletal joining was observed between terminals and basals or between basals and basals. By the time the preoral lobe of the brachiolaria was completely folded over (ninth day), three pairs of functional podia were present. This is reflected in the three pairs of podial pores of Figure 2C. In Atwood's E. echinophorus study, three pairs of podia appeared on the eighth day. Kempf (1966) noted the third pair of podia "toward the tenth day." Gemmill (1914) reported three pairs of podia present at settlement for Asteria.I' ruhen.\' while Barker (1978) observed only two pairs in metamorphosed 5th'haster au.\'- tralis and Coscinasterias calamaria. Though Mortensen (1938) refers to rudimentary ossicles in E. purpureus, these were not figured in his paper. The SEM photomicrographs of Figures 2 and 3 may be compared to Fewkes' (1888) detailed renderings of juvenile Asterias ossicles. Figure 3A shows two mouth armature spines intact in their original position on the mouth frame (oral view). Figure 3B shows a terminal spine 100 p'm long. In spite of similar culture temperatures, there are significant differences in rates of development between the Asteriidae studied by Barker (1978) and Gemmill (1914) and the Echinasteridae examined by Atwood (1973) and the present study. Time to metamorphosis in the asterids ranged from 33-45 days while E. echi- nophorus and E. sentus metamorphosed in 7-10 days. The longer time to settle- ment in the Asteriidae promotes dispersion of the planktonic larvae. More rapid 282 BULLETINOFMARINESCIENCE,VOL.29,NO.2, 1979 development of the attached and vulnerable Echinasteridae larvae minimizes nat- ural mortalities by predation. Zoogeographic patterns therefore may be related to rates of development in the two families. LITERATURE CITED Agassiz, A. 1877. North American starfishes. Mem. Mus. Compar. Zool., Harvard Univ. 5: 1-136. Atwood, D. G. 1973. Larval development in the asteroid Echinaster echinophorus. BioI. Bull. 144: 1-11. Barker, M. F. 1978. Descriptions of the larvae of Stichaster australis (Verrill) and Coscinasterias calamaria (Gray) (Echinodermata; Asteroidea) from New Zealand, obtained from laboratory culture. BioI. Bull. 154: 32-46. Blake, D. B. 1973. Ossicle morphology of some Recent asteroids and description of some West American fossil asteroids. Univ. Calif. Publ. Geol. Sci. 104: 1-59. Fewkes, J. W. 1888. On the development of the calcareous plates of Asterias. Bull. Mus. Compo Zool. Harvard Univ. 17: 1-56. Gemmill, J. F. 1914. The development and certain points in the adult structure of the starfish Asterias rubens L.