Ecology of P. stenops NTRODUCTICN Pentidotheastenops (Bencdict 1898) is an isopod (Grustacea) found exclusively in the lower intertidal (+1.0 --2.0 ft. tide level and below), or as more precisely expressed by Doty (1946) between LHLW, the highest level exposed once a day and LLLW, a level rarely exposed. It is reported by Hatch (1947) to be found from Coos Bay, Oregon to Monterey Bay, Calif, Since its color range (Saccardo's olive green to black according to Ridgway's Color Standard) matches that of its plant hosts, Egregia and Laminaria, the animals are not easily seen in the field. Very little is known about this animal beyond the standard anatom- ical description (Menzis, 1950). An investigation of its habitat and its relatidship to the plant hosts together with a study of bchavior atterns of this crustacean was the object of this present study. Ecology of P. stenong MET IELD WORK In order to find the natural habitats and to plot the distri- bution of P. stenops on the Monterey Peninsula, sixteen areas were chosen because of their diversity between the Coast Guard Breakwater. at the western end of Monterey Harbor and Mission Point, at the western end of Carmel Beach which is the midpoint of Carmel Bay, a coastline of 7-8 miles (fig. 1). In this area there are very rough (Pescadero Pt.) to calm (Stillwater Cove) situations, and intermediate locations (table 2). Because of their local diversity, three of these areas (2,6.7) were divided in half giving a total of nineteen stations. Each station was characterized concerning temperature, oxygen, degree of exposure. algal growth and selected physical parameters (table 2). Population densities were scored (table 1) on Egregia and Laminaria which were found to be the favored host plants. Since conditions varied from area to area as to algal growth and physical characteristics these designations turned out to be very flexible and very few arcas actually fit into one category in all respects. Each arca was sampled at least three times and the mean of the values obtained is found in table 2. OXYGEN CONCENTRATION Oxygen concentrations were determined by taking samples in 60nl glass stoppered bottles and performing a standard Winkler oxygen anal- ysis in the lab. Fig. 2 shows the relationship between oxygen concentration and ab- undance in the areas sampled. There is greater than a 99.9% cor- relation between oxygen concentration and number of specimens found Ecology of P. sten (Kendle-Tau Rank-Order correlation test). Ideally, the graph should start in the lower left-hand corner (high temp., low oxygen conc., low abundance) and proceed in a somewhat linear fashion to the upper right- hand corner (low temp., high oxygen conc., high abundance), however fluctuations occur because of the varying conditions of the areas sampled in this study. TEMPERATURE Temperature was recorded on a standard centigrade thermometer. All temperatures were taken at between the -1.0 and -2.0 ft. tidal levels during the early morning hours. A high degree of correlation also exists between temperature and relative abundance (fig. 3). The actual plot, found by plotting all the points separately, is found in the upper right-hand corner while the larger graph relates the mean teperatures over selected ranges to give a better visual representation of the strong correlation which exsists. Temperature seems definitely to be a factor in determining the distribution of the isopod. ap LEGREE OF EXPOSUN Degree of exposure (table 2) was determined by direct observation.. The categories are somewhat arbitrary but they do serve their purpose in further characterizing the areas. Again, a strong correlation exists between degree of exposure and abundance of P. stenops (fig. 4). A rough area seems to be the most preferred. apo pam DUVC PHISICAL CHARACIERISTICS AND ALGAL GRONTA e Ecology of P. stenops The only physical characteristic affecting the distribution of P. stenops is that there must be a rocky substrate availible for the algae to grow on. A greater abundance is found where both Egregia and Laminaria are gia must be present. aminaria may be absent but E present (table 2). I LSCUSSION Fig, 5 sums up the field work by relating all the factors con- sidered on the previous three graphs. The numbers in parenthesis de- note the number of areas with the designated degree of esposure. It should be noted that a population of P. stenops was never found in calm areas where the highest mean temperatures and lowest mean oxygen levels also were recorded. The rough areas, characterized by low temperatures and high oxygen concentrations, seem to be the preferred areas for the existence of larger populations of the isopod. The abundance level rops off as the areas get very rough which must besolclyto the heavy wave action as the oxygen concentrations and temperatures are comparable to those recorded inthe rough areas. Ecology of P. stenops LAB WORK From the field data it was suspected that oxygen, temperature and degree of exposure were possible limiting factors for the distribution of P. stenogs. Lab experiments were undertaken to confirm these sus¬ picions. OXIGEN TULERANCE An oxygen tolerance test was run in which one animal was placed in a 500ml Erlenmeyer flask containing sea water at a recorded temperature and oxygen level. An oxygen probe connected with a rubber plug was used to stopper the flask. The animals were confined in a screen cage and as stirring flea was placed on the bottom of the flask. The temperature was ke puconstant by placing the flask in a water bath. Readings were taken at irregular intervals. The oxygen graph (fig. 6) shows similar curves for all three size sifications of P. stenops. T clas he interesting point is that the animals were still living in oxygen concentrations as low as i-2 pm. During the night in the field it is known that actual oxygen levels approach such low values as the algae do not photosynthsize during the dark hours. It would be of adaptive significance to P. stenops to be able to exist under such conditions. However, it was also noticed that the isopods become weakened when the oxygen concentration level fell to below 3-4 ppm as they gripped their soreen cages loosely and sometimes let go of the wire mesh completely. Atsuch low oxygen levels they open¬ ed their uropods and weakly beat their pleopods, their respiratory mech- anisms. Ecology of P. stenop It was concluded that oxygen concentrations of 3-4 ppm resulted in a weakened state of the isopod, probably reducing its ability to hold on to the algae in the heavy surf zone. TEMPERATURE TOLERANCE A standard LT50 temperature tolerance test was performed to deter- mine sensitivity of P. stenops to temperature variations. Sixteen iso- pods were placed in each of four disposable plastic tubs which were placed in four different temperature environments (fig. 73), A measured amount of E gregia was placed in each tub and the four tubs were oxygenated every four hours. Readings were taken at irregular intervals. The isopods seem to be extremely sensi¬ tive to slight variations in temporature. At 15 C their LT50 was 26 hours, a startling result when one considers that temperatures along the coast exceed this at dif- ferent times of the year. Temperature could definitely be a limiting factor as P. stenops is found to occur predominantly in the rough in- tertidal zones where a constant stirring up of the water insures a rel- atively low temperature,where a calm area remains in a rather stagnant tate and the water is allowed to warm up. EXPOSURE A rough vs smooth water experiment was performed on thirty isopods to determine their preference to water motion and the periodic impact of waves. A wave machine (fig.8) was constructed which created a water dis¬ turbance every 56 seconds. The isopods were evenly distributed according to size and sex along three Egregia rachises, which were immersed in a confined measured area in a large saltwater pool. T The area was divided Ecology of P. stenops equally in half by a chalk line on the bottom of the pool; one side directly oxposed to the periodic wave shock and the other relatively calm. The isopods showed no preference for either the rough or calm areas indicating that perhaps water motion is not a deternining factor but the conditions that result from such wave action are advantageous to the animals' survival. Y" VS CLEAN WATER DRI At the termination of the wave impact experiment the Egregia rachises in the rough area of the tank were noticably cleaner than the rachises in the calm area. It was speculated that perhaps the isopods require a relatively clean environment in which to thrivel A LT50 test to determine survival rates in clean vs "dirty" water (fig. 7) was run. Two disposable plastic tubs were filled with equal amounts of sea water and a measured amount of Egregia was placed in each. A one mil- limeter layer of "dirt", consisting of fecal material of P. stenops and decayed Egregia recovered from the bottom of an aquarium, was placed on the bottom of the Irty tub. Both tubs were placed in a constant temp- erature (12.8•0) water bath. No further water was allowed to enter either tank. Observations were taken at irregular intervals. Oxygen readings were taken at the LT50 time in the dirty tub and at the termin- ation of the experiment in the clean tank. Survival rates under oxygen deprivation conditions were markedly decreased in the dirty water. The final oxygen reading in the dirty water (5.6ppm) was much higher than those concentrations (1-2 ppm) P. stenops Ecology of P. stenops was previously found to survive in. In an even higher concentration the isopods showed signs of weakening. All isopods in the clean tub were still living at the termination of the experiment, 14 hours after the LT50 time of the dirty water. The oxygen level recorded in the water (3.2 ppm) was lower than that in the dirty tub. This implies that there is some other factor involved, other than oxygen deprivation, af- fecting a lower survival rate in the dirty water. SSICATION Another probable factor limiting P. stenor s to the lower inter- tidal zone is perhaps its rate of dessication. A standard LT50 des- sication tolerance test (fig. 7 ) was run on 12 animals. They were placed in a dry glass bowl and kept under constent observation as a short LT50 time was expected. The animals were kept separated, using a metal probe, to reduce clumping, a common method utilized by terrest: rial isopods to reduce water loss. The short LT50 time (1.5 hours) implies that it would not be ad- vantageous to P. stenops to be left outof the water, exposed to the air. for long periods of time, such as during a low tide. This could defin¬ itely be a factor in restricting the animal to the lowest tidal level. ALGAE GROUTH AND FOOD CHOICE Feeding marks were observed on both L inaria and Egregia, in the field and in the lab, and Nereocystis in the lab. Gut analysies were performed on 20 P. stenops collected from various areas. os It was suspected that they may eat di, a surfgrass com¬ mon to low intortdal levels, since one small piece was found in one of Ecology of P. stenops the guts analyzed from field collected isopods. Eight stenops were placed in an aquarium containing only scraped (to remove possible epiphytes) llospadix. An analysis of these gut contents after a week of exposure showed Phyllospadix exclusively. Egregia, the various epiphytes that grow upon it and Laminaria seem to be the most common algae used for food by P. stenops. They ap¬ parently will eat x, Nereoc hyllosre tis and assorted Red algae (table 3) but prefer not to when the more preferred food is availible. DED PERFORMANCE A performance test relative to feeding and temperature was run to assegs the limiting factors determining the distributions of P. stenops. It had been noticed in lab kept animals that as conditions deteriorated dueto oxygen depletion, a rise in temperature and a change han ono of food, animals lost their ability totheir host plants. It was reasoned that a test of their strength to hang on would be indicative of their physical condition. A seawater fire hydrant was utilized in order to simulate a strong current in the following way: A rachis of Egregia was placed in an aluminum gutter (fig.12), one end of which received an onrush of seawater from the spout of the hydrant. Twelve animals were grown on each of three different substrates, one at a different temperature (fig.8) for 48 - 72 hours before testing. It was felt that food and temperature variances were most convenient to test. It would have been proper to test also oxygen levels but this presented too many complications, All twelve anpimals grown under cach condition were placed on the Ecology of P. stenops 10 rachis at one time. An initial problem arose during the first runs in that it became clear that the isopods showed erratic responses to their new conditions and some repeatedly fell off. A proceedure was developed whereby after the twolvo were placed on the rachis, the water was turned on so that only a slight current was created. Those that did not hang on were removed. If such animals numbered not more than half (6) then the run was continued and the water was turned on full. The flow was kept up until half the isopods were swept away. The time was recorded from the point of the first rush of water when turned on full to the time when the last isopod was swept off. The numbers in the columns of fig. 8 refer to the numberof animals left after the initial wash, The results seem to indicate that indeed food and temperature do affect the isopod's physical well-being as sharp drops occur in the time able to hang on as tho diet and temperature were varied from the normal. This is more evidence that both food (algae) and temperature are limiting factors in the distribution of P. stenops. DISCUSSION The results of the lab work correlate quite well with observations and recordings made in the field. It is obvious that not one single factor is resposible for P. stenops occuring in abundance where they do but rather it is a combination of most of the factors considered that determines their presence in a given area. It has been shown that it is not the actual movement of water in a rough area that is beneficial for the success of a population of P. stonops, but the oxygen levels, temp- erature and cleanliness of water resulting from the effects of the heavy. Ecology of P. stenops 11 periodic water motion. The dgree to which P P. stenops can survive in a heavy surf zone can be noted in that its abundance diminishes as the area becomes very rough. That stenons occurs only at the lowest intertidal levels explains why it is susceptible to dessication. Even though Egregia, its major host and food source, occurs up to the ti.0 ft. tidal level, P. stenops can be observed only from approximately the-O.5 ft. level and on down. From the data collected an "ideal" area for the maintenance of a sizable population of I stenons can be described. It is the following: A generally rough area with a temperature range between 10' and 15°G. he oxygen concentration of the water must be above 8.00 ppm during the day and the water should be relatively clean (note: area 6b, sewer out- fall). Both Egregia and I inaria should be plentiful. Ecology of D. stenops 12 NATURAL HISTORY NOTE Some interesting behavior patterns in the field were noted dur- ing sampling excursions and they will be considered here. ORIENTATION The first concerns the particular orientation of P, stenops on Egregia stipes. No matter which way the current is flowing in the ocean there is always a flow of water outward from the holdfast to the end of the rachis on Egregia. It was observed that most of the stenops oriented themselves so that they were facing in the direction of. i.e.. antennae towards,the apex-ofthe stipe. A current tube (fig. 10) was constructed to test the significance of theis observation and the re¬ sults (fig. 7 ) show that a significant number of isopods prefer this orientation. Animals were placed both facing towards and away from the current. four at a time, on a rachis of Egregia and exposed for two hours. From the data (fig. 7 ) it is seen that P, stenops do prefer to face away from the current a significant amount of the time and this result agrees with the field observations. It was also noted that when facing towards the apex of the rachis the isopod assumes a more hydrodynamically favorable position as com- pared with facing the current head on. The telson is pressed down upon the surface of the rachis in such a way as to allow the water to pass over and around the body with very little lifting and pulling of the animal. This would appear to be extremely advantagcous to P. stenops for its survival in the wave swept areas in which it is most commomly 0 Ecology of P. stenops 13 found. PHCTOTROPY P. stenops showed obvious photonegativism in the field as they were predominantly foundon the underside of Egregia rachises and when held exposed to the sun would scurry back around to the shady side. A light vs dark experiment was devised (fig.11) to test the significance of this behavior. A disposable plastic tub was covered with black paper and holes were punched at either end. A rachis of) a, lipped of its blades in the middle three-quarters of length, was placed longi- tudinally in the box and fixed in position by running the two ends through the two holes. The box was clamped, submerged, in an aquarium. open to the window of the aquarium. Two one-hundred watt bulbs were placed facing the open side and an isopod was placed either facing the open lighted side or on the dark side, always facing up. The lights vere left on for two hours per trial. The results of this experiment (fig. 7 ) show conclusively that sienops is photonegative which explains the field finding of the predominance of isopods on the underside of ia rachises. P stenops found on Laminaria will often times have a paraeopod hooked on another leaf and have pulled it over itself to cover its body in a kind of sandwich of Laminaria blades. This curious be- havior can also be explained by their photonegativism, ADAPTION TO HOST A direct correlation between size of isopod and size of rachis or blade of algae was found through observations made in the field. It Ecology of P. stenops 14 seems that P. stenops can not puncture the algae from the top with their hook-like, semi-chelate legs but rather must hook them around the opposite side of the rachis or blade where it can and does punc¬ ture the alga. This phenomenoowas demonstrated by placing an isopod on a wide blade of-Laminaria that it could not reach around. It was very easy to push off, not being attached to the alga by any means. In the field one often finds an isopod on a blade too wide for it to stretch both legs around. It will, in such instances, be to one side so that it can at least loop the paraeopods of one side of its body around to secure itself. This behavior is further evidenced by baby Stenops which are found exclusively on the basal portion of the lateral blades of Egregia (Robert G. Sellers - unpublished work), These tiny blades are perfect for the small stenops to hang onto SWIMIING P. Stenops is a rather poor swimmer for an animal which spends most of its life underwater. The only function that swimming seems to serve is that of finding something to grab onto. aStenops were never seen swimming in the field. Even when observed underwater during high tides the animals were always seen clutching to either Egregia or Laminaria. This observation was further evidenced in the lab. When P. Stenops were placed in an empty dish thoy would engage in short swimming bursts in all directions, presumably searching for something to grab on to. Any material that was introduced int the dish, includ- ing: metal forceps, other stenops, fingers, glass slides and wire mesh 0 Ecology of P. stenops 15 became a temporary home for the isopods as they would immediately grasp onto such materials after having run into it during the course of one of their short swimming bursts. I am under the impression that stenops utilizes its limited ability to swim only under extreme circumstances such as when it becomes dislodged from its algal host, COLOR VARLATION Color variation in P. Stenops is another highly adaptive character- istic it possesses. The current two species of Egragia found in the gata and E. menzisii, range in color from a light area studied, E. laevis olive green to almost a black, including all variations of green and brown. As was mentioned in the introduction, the stenops observed in the study area were found to possess color variations in the exact same range. This enables them to utilize camouflage as a defense mechanism against predators (and collectors I may note). GENERAL The general activity of P. stenops in the field consists of feeding (algae browsing) and copulation. As in other Idotheas the male grasps onto the back of a non-brooding female and waits until she molts, which time copulation occurs. The male and female then separate, and, to what extent they could be observed, they do not encounter each other again until the female delivers her young and becomes, once again, non- brooding. Males and females in the field were found to occur in equal numbers. Although it appears that many parameters of the lifestyle of the marine isopod P. stenops have been investigated here, a great deal more remains to be investigated into this crustacean. Ecology of P. stenon 16 SORMARY Populations of P. stenops were observed and sampled along selected treas on the Central California coast. Ecological factors affecting their population distribution as well as characteristic behavior pat- ns were noted. Lab experiments were performed to test the signi- ficance of such observations on the general well-being of the isopod. Ecology of P. stenop OTTPROTA ACKNOWLELGEMENTS I would first like to thank EMBO Sellers, without whose help on those cold, foggy, dark and sometimes wet mornings the data for this project would never have been completed. I would next like to thank Chris Harrold, without whose brilliant ideas some of the more original apparatus would never have gotten off the ground. I would like to ac- knowledge Mr. John Kono for his helpful assistance irseeing me through some of my more adventrous projectsandSteve Pratt, Bob Barmeyer and Porter Storey for their watchful eyes when I just had to get some sleep deserve recognition. I would especially like to thank Dr. I. A. Abbott for her as- sistance throughout the quarter, her fantastic and timely epicurean preparations and most of all her patience and understanding. I offer my final thanks to the 24 hour bakery on Lighthouse ave. Ecology of P. stenops 18 FIGURS LE 1) A map noting the specific aicas along the Monterey Peninsula that were sampled in this study (5cm = imi.). 2) A combined line and bar graph showing the strong correlation between oxygen concentration and abundance of P. stenors in the areas sampled, Temporature and oxygen are also related and fluctuations in an idcally llnear plot occurred because of thevariable conditions in the arcas. 3) A bar graph relating selected mean temperatures observed to abun- E stenops. Included is a line graph relating the actual dance pr1. mperature values obtained to abundance. 4) A line graph relating the mean degree of exposure of all the areas sampled to the abundance of animals in the field. 3) A bar graph summarizing all the information gathered in the field (Temp., oxygen levels, degree of exposure, abundance) raph showing the respiration rates of three sizes of stenops, 6) A line 7) Data tables summerizing the lab experiments. 8) A bar graph comparing the performances of stenops exposed to the en- vironments indicated. 9) Pictures of the gut contents of random sampled stenops. 10) Current tube used to simulate currents of water which occur along Egregia rachises. 11) Black-box setup used for light/dark experiment. 12) Trough and Egregia orientation for performance test. 13) Wave machine used for causing periodic impacts of water in the rough vs calm, water experiment. *All drawings are schematic. Ecology of P. stenops TABLE 71 Standards: - Er plants checked 10 per area per sample -Average size of plant 20 stipes - Laminarian plants checked 10 per area per sample - size of P. stenop O. LOmm Abundance P. stenops per Presene of stenops per minaria plant Egregia plant feeding marks on Laminaria Abundant 90-100 Many 71, 22 1, 42 50-757 Some 0.5. 2 1 90.5. 81 25-50% Few 20. &a 0.5 0, 2 0.5 70, £ 25% None 0% Those areas that could not be assigned one of these specific catagories for various reasons (e.g. different algal growth) were listed: many-abundant, some-many, etc. 19 Ecology of P. stenops TABLE 42 Area Degeee of Exposure oC Temp. 10.) pum semi-protected 10.50 9.10 semi-protected 11.00 8.80 rough 10.45 9.50 semi-rough 10.90 8.40 rough 10.60 8.37 calm 11.30 7.30 62 rough 10.70 8.82 6b semi-protected 11.00 6.17 rough 10.40 9.15 70 calm 11.30 7.75 rough 10.60 9.17 rough 10.80 8.90 10 semi-rough 10.80 8.86 11 very rough 10.60 9.40 12 calm 12.00 7.05 rough 10.80 8.90 14 semi-rough 11.00 8.50 15 semi-rough 11.00 8.70 very rough 8.75 11.00 Abundance Some-many few-some many few-some some none many none abundant none abundant many-abun, many many none many some some some Eg./Lam. Sg./Lam. Eg./Lam. 2./Lam. Eg./Lam. gregia Eg./Lam. ./Lam. 2g./Lan. Macrocystis g./Lam. Eg./Lam./Nereocy Phyll./Eg. Eg. Ham. regia Eg./Lam. Egregia Egregia Eg./Lam. 0 Egregia iphytes Leathesia Acrochaetium Ectocarpus Diatoms Laminaria Phyllospadix Red Algae Nereocystis Ecology of P. stenop TABLE 43 CIOAI AL BUNVRY fig. 1 - *.--...... - -.--.. stendp Ecology of.P. NONTEREY . /6 8 4 7 ) N.- P.- . 1 — a. kata- —4 4 — i Va . 2)— CAA . — 4. ARNEL. 11 +7 5 1 . . . . grs POTNEKO 1 51710 KX + O . ). 17/155 7 7 . " . —. — . 7.... . .. r ABUNE MANY SOME FEW L.91se K00021 — Ecology of P. stenops fig. 2 — 23 fig. 3 uppm 11 restt.n fig. 4 5. Orzereen Dogree Exposure" 1 Temp in?c fig. 5 10 De11ae E1posura ean Q-tems. S-0 EAbansaaca -Eetadla 0 fig. 6 123 Ecology of P. stenops T12.2% Time in haurs ALIVE o Small 3.0en O Medium 3 OemsLs 40em * Larzo 4.01 A 24 ELL Ecology of P. stenops fig. 7 LICHT/BAI END POSITION STARTING TOVA TRIAI POSITION AWAY TOWARDS AWAY 15 CURREN UDE — TOWARDS AWAY 20 OLERANGE MP LT-SCehrs 500 -20— LEOSICATION 30 1000 IT.50-hrs 15% 12 1.5 200 8.5 25 SIGNIFICANCE 05»P9.01 P.01 05) P9.01 P01 DIRTTOLEA TYPE 1750.hrs 02 5.6 12 28 3.2 fig. 8 Ecology of P. stenons Enrn aiiati nve. 12.800 EGREGIA 26 649 Ecolgy of Psteneps fa 4 Ecolezy ot Pstenops 28 fig. Ecology of P. stenon: fig. 11 fig. 13 1 — — — — k - Ecology of P. stenops LITERATURE ( TED IN THIS STUDY Benedict, J. E.. 1898. Two new isopods of the genus Idotea from the coast of California. Proceedings of the Biological Society of Washington, 12:53-55. Doty, M. S.. 1946. Critical Tide Factors That Are Correlated With the Vertical Distribution of Marine Algae Along The Pacific Coast, Ecology, 27(4):315-328 . Hatch, M. H.. 1947. The Chelifera and Isopoda of Washington and adjacent regions. University of Washington Publications in Biology, 10:155-274. Menzies, R. J.. 1950. The Taxonomy, Ecology, and Distribution of Northern California Isopods of the Genus Idothea with the De- scription of a New Species. The Wasmann Journal of Biology. 8(2):171-173. Sellers, R. A.. 1973. Unpublished work. 30