AN ECOLOGICAL STUDY of PHYLLAPLYSIA Taylorl DALL, 1900 GASTROPODA : OPISTHOBRANCHIA with an EMPHASIS on ITS REPRODUCTION Robert Beeman
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AN ECOLOGICAL STUDY OF PHYLLAPLYSIA TAYLORl DALL, 1900 GASTROPODA : OPISTHOBRANCHIA WITH AN EMPHASIS ON ITS REPRODUCTION Robert Beeman To cite this version: Robert Beeman. AN ECOLOGICAL STUDY OF PHYLLAPLYSIA TAYLORl DALL, 1900 GAS- TROPODA : OPISTHOBRANCHIA WITH AN EMPHASIS ON ITS REPRODUCTION. Vie et Mi- lieu , Observatoire Océanologique - Laboratoire Arago, 1970. hal-02959363 HAL Id: hal-02959363 https://hal.sorbonne-universite.fr/hal-02959363 Submitted on 6 Oct 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. AN ECOLOGICAL STUDY OF PHYLLAPLYSIA TAYLORl DALL, 1900 GASTROPODA : OPISTHOBRANCHIA WITH AN EMPHASIS ON ITS REPRODUCTION by Robert D. BEEMAN Marine Biology Department, San Francisco State Collège, San Francisco, California INTRODUCTION Phyllaplysia taylori DALL, 1900, a small, bright-green, striped aplysid (Order Anaspidea, Family Aplysiidae — the " sea-hares ") (figs. 1-6), is often very numerous on Zostera marina LINNAEUS, 1758, the broad- blade eelgrass which is common in several embayments of the north- eastern Pacifie. As with many of our local marine invertebrates, relatively little is known of the natural history of this mollusk. In conjunction with, and as part of, a five year study (BEEMAN, 1966) of the reproductive biology of P. taylori in Elkhorn Slough, California (36° 48' N, 121° 47'15" W) I was able to add to our ecological knowledge of this animal. My emphasis was, of course, on the reproductive aspects of its ecology. METHODS AND MATERIALS I. — COLLECTION AREA. Elkhorn Slough is a shallow, muddy embayment of the east side of Monterey Bay with its mouth at the head of the Monterey Submarine Canyon. The Slough once served as the outlet of the Salinas River, but — 190 — PIQ. l. Phyllaplysia taylori, dorsal aspect of a 4.5 cm individual, from life (BEEMAN, 1968a). — 191 — it is now without a continuous source of fresh water. Fresh water does enter the Slough, but only from intermittent streams and ground drainage. ADDICOTT (1952) recorded a salinity range from 32.97 °/<» to 8.70 °/oo over a tidal cycle from higher high water to lower low wàter in the main channel. The nature of the Slough and its ecological aspects were considered in some détail by MAC GINITIE (1927, 1935). Subséquent industrial development of Elkhorn Slough has been accompanied by the décline of its Zostera beds, as a resuit of dredging, and probably pollution, to a level far below that of the lush growth reported by MAC GINITIE. Ail of the animais used in this study, unless otherwise noted, were collected in the Zostera marina beds of the northern arm of Elkhorn Slough during the years 1961-1966. This location probably corresponds to station three of MAC GIXITIE'S 1935 report. II. — COLLECTION AND PRÉPARATION OF SPÉCIMENS. Collections were made at roughly monthly intervais, depending on the tides. During each collection two buckets of Zostera plants were picked at random throughout the beds by breaking off the plants near the surface of the mud. The plants were taken to the laboratory and carefully examined under bright light. An attempt was made to search the entire surface of each leaf examined, under constant conditions, so the population could be sampled without a bias toward largei animais. In obtaining monthly population samples, every Phyllaplysia located was removed with a spatula, relaxed by immersion in isotonic magnésium chloride solution or injection of succinylcholine chloride (BEEMAN, 1968b) and then flxed in sea-water BOUIN'S fluid. Weight of fixed spécimens was found to be the most reliable size indicator. Each spécimen was allowed to stabilize its weight by remaining at least one month in 70 per cent ethyl alcohol. It was then blotted dry and promptly weighed on a Mettler analytical balance. The 2351 animais in the popula- tion samples ranged in weight from less than 0.001 g to 1.505 g Sériai sections, 7 \i thick and stained with hematoxylin and eosin, were made of représentative animais from each of the weight classes indicated in figure 12 that were présent in each monthly sample. In addition to the population sample preserved each month, a number of living animais were removed from Zostera and placed in running-seawater tanks for further observation and expérimentation. RESULTS AND DISCUSSION I. — HABITAT. Almost ail of the Phyllaplysia taken during minus tides were found tightly fltted in the spaces between the Zostera leaf bases. 15 FIG. 2. — Contrast between a typical Phyllaplysia taylori taken from Elkhorn Slough (left) and a Phyllaplysia taylori reared in outdoor seawater tanks at Hopkins Marin:; Station. Scale in millimeters. FK;. 3. — Phyllaplysia taylori and a nidosome on a blade of Zostera marina. Internai organs are visible through the dorsal body wall. Scale in millimeters (Modified from BEEMAN, 1968 a). FIG. 4.— Dorsal view of a Phyllaplysia taylori post-settlement larva covered bv a shell 2 mm long; the cephalic tentacles project anteriorlv (Modifled from BEEMAN, 1968a). FIG. 5. — Ventral view of above Phyllaplysia taylori larva. FIG. 6. — Phyllaplysia taylori, shell-less juvénile, and nidosome. Scale marks are millimeters. — 194 — This habit of concealment, plus the manner in which the animais mimic the surface of Zostera (Fig. 3), hides the species so well that even persons collecting in the Zostera may be unaware of the tremendous abundance of this opisthobranch. The 18 April 1963 measurement of 100 Phyllaplysia per 850 g of wet drained Zostera from Elkhorn Slough is typical. Phyllaplysia taylori probably receives its greatest ecological stress at the time of extremely low tides when many Zostera plants may be fully exposed to air, sunlight, and/or rain. Only a detailed study of the micro-environments of individual animais at such times would give température, salinity, oxygen, and other measure- ments of any real ecological significance. Such an approach was not germane to the présent study, therefore few data of this sort were gathered. Seawater samples from the Zostera bed on the overcast morning of 21 June 1962 gave the following results : surface température 15.0 °C; dissolved oxygen (WINKLER method) 2.22 ml 0,/l at the surface and 2.04 ml/1 at the bottom; pH 7.7 at both surface and bottom. This may be compared with a sample taken from the nearby océan surf the same morning : 14.0 °C; 4.55 ml 02/l; and pH 8.0. Another sample from knee-deep water in the same Zostera bed on the overcast day of 1 July 1965, when the surface water température was 17.0 °C, showed higher oxygen ratings : 4.19 ml 02/l at the surface and 3.70 ml/1 at the bottom. The abundant mixing of slough water with the nearby océan water tends to maintain fully marine conditions within the Zostera beds. At the présent time, rains seem to cause the most significant salinity changes. On 7 February 1963, after a week-long storm, accompanied by heavy rains, I recorded a salinity of 23.95 °/00 in the above-mentioned Zostera bed. Surface température readings in the Zostera bed from July 1962 to August 1965 ranged from 13.5 °C to 21.0 °C. Readings were typically in the 15 - 17 "C interval. ADDICOTT (1952) and MAC GINITIE (1935) listed surface water température ranges of 11.05- 20.50 °C and 11-25 °C for their respective studies in the Slough. Phyllaplysia taylori must be a rather eurythermic organism as no mortality has been observed to coincide with thèse extrêmes. It is generally agreed that the normal habitat of Phyllaplysia taylori is on Zostera marina in bays and estuaries (BEEMAN, 1963). Its vertical range coincides with that of the Zostera, extending from the low intertidal zone to subtidal depths of a few meters. Interes- ting additions to the above have been made by MAC FARLAND (1966) and by NETTIE MAC GINITIE (personal communication, 1966). MAC FARLAND reports Phyllaplysia taylori " dredged from the Monterey Wharf in 1894 ". This is the only known record of the animal in the open bay itself; the dredging probably touched a former stand — 195 — of Zostera not more than 7 m deep. MAC GINITIE reports taking several spécimens of Chrysaora gilberti Kishinouye, a large scypho- zoan, winch were floating in Newport Bay, California on 25 July 1953. Upon excising and opening some " tumor-like " growths in the oral tentacles of thèse jellyfish she found several small spéci- mens of what she considered to be Phyllaplysia. I have examined two of thèse preserved spécimens which differed little from each other. One of this pair is a 12 mm, 0.03 g spécimen without color or external markings. Its gênerai structure, radula formula (36 X 32-1-32), and shell structure (0.69 mm X 2.07 mm) do not give any reason to believe that thèse animais are anything but strangely located Phyllaplysia taylori. They may have transferred from Zostera to Chrysaora while thèse jellyfish were drifting in Zostera beds. Their rôle from that point is enigmatic. The normal habitats of other species of Phyllaplysia and Petalifera are very poorly known. The gênera must be considered together due to their taxonomic confusion (BEEMAN, 1968 a). Almost ail of the literature on thèse animais has been systematic. Many références give no ecological information at ail; some make vague comments : THIELE (1931) refers to Phyllaplysia " im Mittelmer ", RANG (1828) says of Petalifera " Elle vient des mers de Nice.