The Larval Development of the Sand Crab Emerita rathbunae Schmitt (Decapoda, Hippidae) ! MARGARET D. KNIGHT Two SPECIES of the sand crab Emerita have been tures (Rees, 1959) and from the plankton found on the western coasts of N orth and South (Smith , 1877), and larvae of E. boltbnisi America. Emerita analoga (Stimpson) has been Sankolli have been studied in the laboratory recorded from Vancouver Island, British Colum­ by Sankolli (1965) . bia (Butler, 1959) to Magdalena Bay, Baja Californ ia, and from Salavery, Peru to Eden METHODS Harbor, Territory of Aysen, Chile (Haig, 1955) . Dr. Ian Efford (personal communica­ An ovigerous female of Emerita ratbbunae tion of unpublished observations) has found was collected from a sandy beach near La Playa, that the northern limit of the species may be Mazatlan, Mexico, on 20 September 1963 dur­ Kodiak Island, Alaska, and that the southern ing a cruise in the Gulf of California aboard limit of its range in South America is the Strait R/V "Alexander Agassiz" of the Scripps In­ of Magellan. E. ratbbunae Schmitt has been stitution of Oceanography. The female was held found from La Paz, Baja California to Capon, aboard ship in a 3-gallon aquarium . Hatching Peru (Schmitt, 1935) . Specimens of E. ratb­ of the eggs began 12 hours after capture. The bunae have also been collected at San Francis­ larvae were maintained in groups of 10- 25 in quito Bay, on the east coast of Baja California 4-inch glass finger bowls of 1200 cc capacity above La Paz (Steinbeck and Rickets, 1941). or in plastic containers of 400 cc capacity. They The larvae of Em erita analoga were described were transferred daily to fresh sea water and by Johnson and Lewis ( 1942) . Th e first zoea fed newly hatched nauplii of A rtemia salina was obtained from eggs hatched in the labora­ (1.) . All larvae molted once during the six tory and one individual molted to the second days' culture period aboard ship. In addition, stage.Later stages were described from pre­ one second zoea of the species was sorted from served plankton samples. Larvae of the species a plankton tow taken earlier near shore below have subsequently been cultured from egg to Cape Corrientes, Mexico (19° 22'N , 105° 03' megalopa by Dr. Ian Efford (pe rsonal com­ W ) , and was maintained in isolated culture munication) . During the present study, larvae aboard ship for 11 days. A surface temperature of the tropical species Em erita ratbbun ae were of 29.6° C and salinity of 33.9%0 were cultured in the laboratory and compared with recorded for the water from which the larva specimens from the plankton to provide a was taken. The salinity of water used for cul­ detailed description of the sequence of larval tures was 33. 8- 33.9%0' As the ship returned development and a means of differentiating the north from the collecting area, the temperature larvae from those of the amphi-tropical species of the water held in the cultures dropped from E. mzaloga. 29° to 23° C, subjecting the larvae to consider­ The larvae of three other species of Emerita able cooling during the early zoeal stages. have been investigated. Menon (1933) ob­ At the conclusion of the cruise, the approxi­ tained five zoeal stages of E. em erita (1.) from mately 100 larvae hatched aboard ship (then in the plankton, the larvae of E. talpoida (Say) stages II and III) , and the single larva taken have been described both from laboratory cuI- from the plankton were transferred to the lab­ oratory and isolated either in compartmented plastic trays holding 50 cc per compartment, or 1 Contribut ion from Scripps Institution of Ocean­ ography, Uni versity of California , San D iego. Manu­ in plastic boxes of 400 cc capacity. They were script received January 18, 1966. transferred daily to sea water filtered through 58 Larval Development of Emerita rathbtmae-KNIGHT 59 glass wool, and fed newly hatched Artemia majority passing through 8 zoeal stages. The nauplii. The laboratory cultures were main­ stages became progressively longer and the dura­ tained at room temperature which, because of tion of the last stage was twice that of the pre­ seasonal cooling, decreased gradually from 22° ceding one (Table 1). Of the larvae cultured to 19° C, with daily fluctuations of 1° C or less. 42% completed zoeal development and molted The salinity range during the culture period was successfully to megalopa. The highest mortality 33.5-33.8%0' The larvae were kept under (20%) occurred in the terminal stage. natural illumination but away from direct sun­ Within stages I, II, and III morphological light. development was similar for all individuals. In Both aboard ship and in the laboratory, all subsequent stages, slight variation was found exuviae and some specimens of each develop­ among individuals that completed development mental stage were removed and preserved in in seven zoeal molts, but larvae passing 5% formaldehyde buffered with hexamethylene through eight or nine stages showed consider­ tetramine. The casts were transferred to glyc­ able individual variation in relative growth and erine for study. The cultures, the preserved setation of appendages, so that the number of specimens, and the exuviae were maintained in stages through which an individual had pro­ such a way that the individual history of each gressed could be positively ascertained, beyond larva could be followed. In the course of the stage IV, only by a study of its molting history. study, 328 exuviae and 50 specimens of reared Despite this variability, the order of develop­ larvae were examined and dissected. mental events was similar for all larvae. The In order to compare zoeal stages occurring degree of growth attained with each molt be­ naturally in the plankton with those obtained yond stage III decreased with an increase in the in the laboratory, 143 zooplankton samples number of zoeal stages in the larval period, (taken in August-September and November­ but the terminal zoeas were alike in develop­ December in the area between Cape San Lucas ment of appendages (addition of pleopods, and Cape Corrientes, north into the Gulf of growth of thoracic appendages and flagellum California to Tiburon Island, and along the of second antenna, etc.) and differed mainly in west coast of Baja California north to Magda­ details of setation and size. No larvae inter­ lena Bay) were examined. The majority of mediate between the terminal zoea and mega­ samples were taken with a I-meter net towed lopa were observed. obliquely from 140 m to the surface, filtering The variation among the cultured larvae in approximately 500 m", during cruises 5612 number of zoeal stages, and in morphology of (SIO Ref. 61-22, 1961), 6108 (SIO Ref. 62­ individuals with similar molting histories, 16, 1962), 6208-9, Azul II and El Golfo prompted detailed examination of specimens (Snyder and Fleminger, 1965). A total of 150 from preserved plankton samples. The ratios of specimens were obtained for comparison with carapace length to rostrum length (Table 2), cultured larvae. In addition, 70 larvae of together with study of appendages, were used Emerita analoga were sorted from zooplankton for identification of planktonic specimens. The samples taken off Point Conception, on cruises stage of development was determined by exami­ 32 (SIO Ref. 52-1, 1952a) and 33 (SIO Ref. nation of the natatory setation of the first and 52-7, 1952b) and off the Coronado Islands, second maxillipeds and of relative change in for detailed comparison of the two species. other appendages. The cultured larvae, hatched Larvae were dissected in glycerine. Drawings with four natatory setae on the exopodites of of whole specimens and appendages were pre­ the first and second maxillipeds, added two pared with the aid of a camera lucida. Young setae with each molt through stage III. Either stages were stained with lignin pink to facilitate one or two setae were added with each succes­ dissection and study. sive molt. Rees (1959:368) and Sankolli (1965:39) found similar patterns of progres­ RESULTS sive increase in setation in cultured larvae of The cultured larvae molted 7, 8, or 9 times E. talpoida and E. boltbuisi. Planktonic larvae before metamorphosis to megalopa, with the of E. ratbbunae, with rare exceptions, had only 0\o TABLE 1 D URATION ANDN UMBER OF ZOEAL STAGES FOR LARVAE COMPLETING THE MOLT TO MEGALOPA (Means given for isolated cultures only) DAYS IN STAGE NO. OF ZOE AL NO. OF STAGES LARVAE I II III IV V VI VII VIII IX Cultured lar vae 7 4 mean - - - 5.3 9.3 11.5 31.8 range 5 5-6 4-6 5-6 8-10 10-13 22-43 'i:i 8 22 mean - - 5.3 6.5 9 .0 8 .9 12.4 30.7 > range 5 5-6 4- 6 5-8 7- 11 7- 12 11-16 22-43 () 9 3 mean - - - 7 8 9 .7 9.7 13.3 29.3 -'T.I ran ge 5 5-6 4-6 7 7- 9 8-12 9-10 11-16 27-31 ()- C/l Plankto nic larva () 8 8 14 ?30 tTl 3 3 7 Z- D uration megalopa stage for 6 individu als: mean 12.5 days () range 10- 13 days Fl <: ~ X X .r- '-'I>' ;:) I: I>'.... '< ...... \0 0\ -...J Larval Development of Em erita rathbtmae- KNIGHT 61 an even number of setae on the exopodites of the maxillipeds. This setation, therefore, ap­ X T peared to be a reliable indication of the number XT OT of molts through which an individual had ron ( Xl progressed. ( 0) In samples examined, 67% of the planktonic 6 larvae of E.