C H. Loren Fong-2 Abstract Analyses of fecal pellet contents of freshly collected Ligia occidentalis showed the principle foods were a variety of brown, red and green algae wrack Ligia occidentalis prefers to eat the brown algae Macrocystis pyrifora and Gstoseira, when given a choice of several seaweeds. A correlation was established between the fecal pellet contents and algae available in the study sites. H. Loren Fong-3 Introduction The area of transition just above and including the high intertidal zone is characterized by a unique blend of marine and terrestial conditions. It is an area inhabited by few obvious species. One of the dominant forms is the isopod Ligia occidentalis, which inhabits rocky rubble immediately above the highest tide mark (Miller, 1938). Nothing is known of the role Ligia occidentalis plays in the conversion of biomass in this habitat, of its food habits or its food preferences. Studies of Ligia pallasii (Carefoot, 1973) indicate it is an effective consumer of macro algae in the high intertidal and may effect a measurable control on the abundance of certain algae in that zone. This study is a documentation of the dietary habits of Ligia occidentalis both in the field and laboratory. It also reports on preliminary experiments indicating an activity pattern differing from that reported for other Ligia species. Methods Ligia occidentalis was collected along the rocky shore in Pacific Grove, California, at Lovers Point and at Hopkins Marine Station from the large rocky rubble located 1.00-1.25 m above the endocladia zone. This region receives salt-water spray during high tide which collects on pools under this rubble and ensures a supply of water that Ligia may dip their uropods into. In these high tidepools grow various blue-green, and filamentous green algae. The stock supply of isopods was kept in plastic tubs perforated on the bottom with small holes. Each tub was supplied with running sea water at 15 degrees centigrade. Algae were supplied for food. All experiments were carried out at an air temperature of 18-21 degrees centigrade. H. Loren Fong-4 Results Field Studies The following experiment determined what species of algae Ligia consumed in the field. Twenty Ligia were collected from the Hopkins and Lovers Point sites and placed in separate plastic tubs for 24hrs. Fecal pellets from each group were collected, mounted on slides, and compared to reference slides of algae. The presence or absence of algae in each pellet was recorded. The frequency of occurrence of algae which appeared in fecal pellets from both sites was then compared. Examination of fecal pellets collected from the field specimens held for 24hrs, revealed the following (see Fig. 1). Ligia from the Hopkins site consumed significantly more blue-green and green algae than the Ligia from Lovers Point. Macrocystis and Cystoseira were taken more frequently at the Lovers Point site. Porphyra and diatoms were the other food items commonly found in the fecal pellets of Ligia. Granite fragments were present in 55 percent (Hopkins) to 85 percent (Lovers Point) of the fecal pellets, indicating that the Ligia may be actively consuming granite as part of their diet. Red alga were eaten with a greater frequency at Lovers Point, but were still found less than 10 percent of the time. Estimate of the Relative Abundance of Drift Macro Algae Estimates of the relative abundance of drift algae in each study site were made in the following manner. Five 10m transects were placed parallel to the shore contours. Ten .25 x .25m quadrats placed at regular intervals along each transect were then used to sample the area for drift algae. Algae were identified by eye and their presence noted on data sheets. Relative abundance was obtained by dividing the number of times an alga was found in the quadrat by the total number of quadrats taken. H. Loren Fong-5 Examination of Fig. 2 reveals two obvious trends. Lovers Point had a significantly higher amount of the brown algae Macrocystis and Cystoseira, and Hopkins showed the larger abundance of blue-greens and filamentous greens. Phyllospadix and the epiphyte Smithora occurred with similar frequency in both sites. Food Preference Food preference in Ligia was tested in the following manner. Twenty Ligia were starved for 24hrs and placed in a plastic tub containing a chunk of granite, and equal amounts of algae commonly found in the study areas. The animals were fed for 48 hrs and all of the fecal pellets collected. Each pellet was scored using the same method described earlier. Results from examination of the fecal pellets collected in the food preference study are shown in Fig. 3. The frequency of occurence was used as an indicator of "food preference". Significantly more Macrocystis and Cystoseira were consumed in all of the experimental trials. The red alga Porphyra was eaten 25-30 percent of the time. Diatoms and granite fragments were also present. Activity Pattern To monitor activity patterns, 10 Ligia were placed in a plastic tub with various algae for food. A control group of 10 were placed in a separate tub without food. Each received a constant flow of sea-water. The behavior was then monitored in 15 minute intervals over a period of 6 hrs. Those Ligia walking or eating were scored as active. Those Ligia sitting on the rock, sitting on the algae or resting on the bottom of the tub were considered to be inactive. For each 15 minute interval the percent active were calculated, and the results are shown in figure 4. H. Loren Fong-6 The period of greatest activity in this Ligia species occurred during the daylight hours. Most activity occurred from mid-morning to late afternoon. During dusk to dawn the Ligia were inactive, with the majority of individuals clustering around the small chunk of granite in the tub. When no granite was available, the Ligia would group together in the corner of the plastic tubs. This experement was replicated twice, and the results were the same. Although the control animals did not have any food available to them, they still demonstrated the same activity period. Discussion It is apparent from field data and laboratory experiments that Ligia occidentalis feed extensively on the drift algae present in their habitat (Miller, 1938). Although other species of Ligia migrate daily into the intertidal zone during low tide (Carefoot 1973, Hewitt 1907), L. occidentalis does not appear to do so. Instead they remain in the same area and exploit the algae that wash onto the beach. The differences in the algae present in the fecal pellets from the Lovers point site and at Hopkins (See Fig 1) indicate Ligia is an opportunistic forager and will eat the algae which occurs most frequently. Studies by Nicholls in 1931 indicate Ligia have enzymes which act readily on most carbohydrates, proteins, and fats, but do not have the cellulase required to digest cell walls. This would explain why the fecal pellet contents look much like the gut contents. The granite fragments found in the gut may be used to break down the cell walls allowing digestion of the contents of at least some of the cells. My experiments using the brown, red, and green algae, indicate that the Ligia probably utilize each of these algal resources, thereby maximally exploiting available food sources. H. Loren Fong-7 Their activity during the day is a pattern opposite to that reported for Ligia pallasii (Carefoot, 1973) and for other species of Ligia (Saito, 1965). These two Ligia were most active at night. The physiological adaptations of diurnal chromatophore changes (Armitage, 1960) and sensitive visual system (Ruck & Jahn, 1954) give Ligia an adaptive advantage for daylight foraging. Behaviorally, Ligia exhibit a quick startle response, which sends them scurrying for the nearest available cover (Armitage, 1960). Because Ligia can forage underneath the rocky rubble, they may not be vary susceptable to predation by birds. One can speculate that the Ligia may help the dispersal of various types of alga, such as the high intertidal Porphyra. The presence of a healthy baby plant within the fecal pellet (personal observation) and the lack of cellulase in the gut (Nicholls, 1931) may indicate a mechanism whereby Ligia are dispersing rather than limiting the distribution of some high-intertidal algae. Whether or not the granite fragments are utilized to grind and break through the tough cell wall of the algae or are taken accidentally while the Ligia are eating diatoms is a question which remains to be answered. Although the Ligia are eating these various food types, we still do not know what they are utilizing. The gut contents look very much like the fecal pellets and it does not appear that much breakdown of products has occurred. Perhaps the major ecological role of Ligia occidentalis is to reduce large drift down into small bits. There is no doubt that they can process large amounts in very short periods of time (Carefoot, 1973). From the present results it can be speculated that the role of Ligia in the transition zone is geared towards disposal of drift algae and that their impact upon the distribution of intertidal algae may be minimal. H. Loren Fong-8 Acknowledgemer I would like to thank Professor Gilly for professional quidance and uplifting my spirits when the going got tough. Special thanks to Dr. Abbott for the great lessons in Ligia anatomy, and to Dr. Issabella Abbott for the lessons on algae identification and for getting me to say "algae" instead of "algaes." H. Loren Fong-9 Literature Cited Armitage, K.B (1960), Chromatophore Behavior in the isopod Ligia occidentalis Dana 1853, Crustaceana, vol 1:3, 194-207. Carefoot, T.H. (1973) Feeding, Food Preference, and Uptake of Food Energy by the supralittoral isopod Ligia pallasii. Mar Bio 18, 228-236. Miller, M.A. (1938) Comparative Ecological Studies on the Terrestial isopod CRUSTACEA of the San Francisco Bay Region. Univ. Calif. Publs Zool. 43, 113-142. Nicholls, A.G. (1931) Studies on Ligia oceanica. Part II. The process of Feeding, Digestion and absorbtion, with a Description of the Foregut. J. Mar. Biol. Ass. U.K. 17, 675-707. Ricketts, E.F., Calvin, J. (1952) Between Pacific Tides, 3rd Edition. Revised by J.W. Hedgpeth. Stanford Univ Press. 100 90 80 70 + 6( 0 50 o 4 —2 20 100 — 9 O 70 0 40 — 30 20 10 Neidie Abandunce oi brirt Aigae n. Eigia Study Site - — o8 —— — — —.— Hopkins. S — L — — . 6 9 0 — + —1— — Loy — — + —— 4 — — E — Relative Activity: — 0 3 O rcent (%) O O 0 S O 9 8 9 — —— 1O + — 0 —— — — — — S — -1: 8 1 — 3 — — — 2 — — — 1— — — — + 8 S - — — O 00— + — d 3 —— 1 O — —— O O â -— —0— — — O 1 — —— —— 0 8 o —— P —.— -.. —.— - 1OO — — —— 90 a — - 1 N. 8766 t 0 — +—: 40 — 20 — O — — —4 C 0 E — 1 — X — —: — — .. ... — S .. —.. ..... : —+ — .. .-- . — — ——— —— 100 90 80 70 60 9 550 40 30 20 10 0 —100 —90 —80 270 260 250 0 40 30 20 10 Relative Abundance of Drift Algae in Ligia Study Sites 9 Hopkins N-50 + LE 0 9 0 6 Lover Point N=50 — — S Frequency of Occurence of Algae in Fecal Pellets 100 8 90 Hopkins 8 80 N=22 70 8-60 0 0 5 — 40 30 — — 20 10 L E S 8 10 100 8 O 8 90 80 Lovers Point -70 N=3 -60 -50 40 30 +20 10 ( 100 90 880 70 060 50 40 3 20 0 0 Food Preference Study — 1.. —— — 370 — — — + + 8 9 0 0 o O 0 o - — 10 — + — + — + — — —— — — — —0 S â — —— â 0 — — + + ( 0 — — S 4. 00 — +8 8 —— — -â — — 8 P o — 4 a 10 o + — O O (%) uo5le O 10 AAIADVAITE