Learn About Isopods: Foods, feeding, & nutrition

This sections deals with general nutrition, while growth/moulting is considered in a separate section under the general topic of EVOLUTION TO LAND ISOPOD/MOULTING.

With some exception (see Cirolana below), shore-dwelling isopods are herbivorous, although dead animal matter is often eaten. Juveniles feed on diatoms, filamentous algae, and other microalgae, and adults tend to eat macroalgae. They use their strong mandibles to scrape algae from rocks or take bites from larger food items. The accounts to follow are arranged alphabetically by genera.


Research study 1: Cirolana

photograph of isopod Cirolana harfordi courtesy Gary McDonald, UC Santa Cruz, CaliforniaOn beaches around Pacific Grove, California the colorful intertidal isopod Cirolana harfordi emerges from under rocks at high tide and feeds on small living polychaetes and crustaceans. Included in their diet is a variety of dead animal matter. The small living prey consumed is largely intact in the gut, suggesting that they are consumed with little or no mastication. Simple laboratory tests with diluted juice from macerated fish show a fine sensitivity to food-chemical stimuli. Individuals stimulated from a distance by release of small amounts of fish juice quickly emerge from under-rock hideaways and swim in zig-zag fashion with antennae waving. Females feed little or not at all while brooding their young. Digestion of a full meal is slow, taking up to a month for adults, and the author thinks that a fed individual may sequester itself under rocks for protection during this time. Johnson 1976 Mar Biol 36: 343. Photograph courtesy Gary McDonald, Long Marine Laboratory, UC Santa Cruz, California.

Research study 2: Cirolana

table showing energy budget for a population of Cirolana harfordi in Pacific Grove, CaliforniaIn a companion paper the above author determines an annual energy budget for an arbitrary field population of Cirolana harfordi numbering 1000 individuals at Pacific Grove, California. The population throughout the year averages 607 juveniles, 127 males, and 266 females. Although adults comprise only 40% of the individuals in the population, because of their larger size they account for almost 90% of the standing energy crop. Growth (secondary production) is measured as increase in somatic tissue mass (times calorific value) + egg production (times calorific value) = 106kcal per year (see table on Right). A point of interest from the table is an estimated absorption efficiency of 87-88% for this population, noted by the author to be somewhat on the high side, but not unrealistic for a carnivore eating fish. The budget is presented by the author in balanced form which, based on the known lack of accuracy in extrapolating laboratory data to field populations (especially respiration costs), lack of direct measurements of feeding/defecation rates in field individuals, failure to account for moulting loss, and so on, must be considered as just a rough estimate of what the true budget must be. The budget is balanced by the simple expediency of going backwards from somatic growth combined with reproduction, and adding this total to laboratory-estimated respiration costs to get a value of 262kcal for energy absorbed. From a laboratory-obtained value of 87.3% absorption, total field consumption must therefore be 300kcal (with feces being 38kcal). Although some use might be made of the growth component of the budget, the reader has little confidence in the accuracy of the other entries. Johnson 1976 Mar Biol 36: 351.

NOTE entries required for such a budget are energies of ingestion and egestion (feces) to determine energy absorption (termed assimilation in the paper), respiration to determine metabolic losses, and growth (somatic and reproductive) to determine a secondarily derived estimate of growth efficiency. Food provided in the laboratory is fish, representing a known dietary component of field individuals.Some of these measurements are taken from the companion paper noted in Research Study 1 above (see ISOPODS: REPRODUCTION: CIROLANA & EXCIROLANA). All measurements are done at a temperature of 13°C, reflecting average field temperature in the Pacific Grove area of California

Research study 3: Cirolana

Carnivorous/scavenging dining habits are unusual in isopods, most species of which are herbivores. Food of Cirolana harfordi consists of small crustaceans and worms that are often swallowed whole, but larger animals both live and dead are consumed by biting off pieces of tissue. One might suspect that mouthpart morphologies will have evolved specifically suited to a carnivorous mode of eating, and this is confirmed for Cirolana harfordi in a detailed scanning-electron micrographical study by a researcher at the University of Sydney, Australia. The author’s focus is mainly on setal morphology and, although some adaptations in the 5 pairs of mouthpart appendages are described (see below), with the exception of the mandibles not much is known about how they actually function. The mandibles have overlapping processes that are differently shaped bilaterally so as to form a piercing and slicing device. This differs markedly from the flat, setae-covered surfaces of the molar processes of herbivorous species that function in grinding up plant foods. Thomson 2012 J Microscopy 252 (2): 111.

NOTE the species is found in Australia, Japan, and along the west coast of North America. The Japanese representatives are sufficiently different from the other two to warrant their designation as a subspecies, but Ausralian and American specimens differ only in minor spine counts and are not considered as separate subspecies

The 5 pairs of mouthpart appendages, presented pictorially below in anatomical sequence from outside in towards the mouth, include maxillipeds, maxillae, maxillulae, paragnaths, and mandibles. In each view the appendage preceding the one being considered has been removed in order:

scanning electronmicrograph photos of maxilliped appendage of the isopod Cirolana harfordi

MAXILLIPEDS: these and other mouthparts are adorned with special serrated setae that form comb-like structures used for grooming & possibly filter-feeding

scanning electronmicrograph photos of maxilla appendage of the isopod Cirolana harfordi

MAXILLAE: function to sort, manipulate, and macerate the food, but how the multiple mouthpart appendages actually integrate their function is not known for any "higher" crustacean

scanning electronmicrograph photos of maxillula appendage of the isopod Cirolana harfordi

MAXILLULAE: function in the same way as the maxillae. Note the areas where the outer maxillipeds & maxillae have been removed (arrow)

scanning electronmicrograph photos of paragnath appendage of the isopod Cirolana harfordi

PARAGNATHS: ditto as for maxillulae

scanning electronmicrograph photos of mandible appendage of the isopod Cirolana harfordi

MANDIBLES: bilaterally different incisor processes overlap right & left to form a piercing/shearing device presumably for slicing animal flesh


Research study 1: Idotea

photograph of isopod Idotea montereyensis on a blade of surf grass Phyllospadix scouleriStudies of gut contents of Idotea montereyensis in the Dillon Beach area of California indicate that individuals primarily eat the superficial layers of plant substratum on which they reside, such as surfgrass Phyllospadix scouleri and various red algae. Small plants, including diatoms growing on the surface of these substrate plants, are also consumed and, under conditions of crowding, small crustaceans and even juvenile conspecifics may also be eaten. Lee 1966 Ecology 47: 930.

Research study 2: Idotea

photograph of isopod Idotea resecata courtesy Dave Cowles, Walla Walla University, WashingtonThe isopod Idotea resecata commonly inhabits and feeds on fronds of giant kelps Macrocystis integrifolia. The isopods graze over all parts of the frond surfaces, but a favoured location is the narrow portion close to the pneumatocysts. The author speculates that the higher concentration of nutrients in this narrowing may afford a better meal for the herbivores. However, too high an intensity of grazing at this spot may lead to severance of the frond, and these excised bits are sometimes carried shoreward in the surf, complete with their complement of still feeding isopods. Jones 1971 p. 544 In, The biology of giant kelp beds (Macrocystis) in California (North, ed.) J Cramer Publisher in Lehre. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

Research study 3: Idotea

histogram showing algal preferences of isopods Idotea wosnesenskiiOn beaches around the Shannon Point Marine Center, Washington there are 3 species of green ulvoid seaweeds, Ulva lactuca, U. linza, and Ulvaria obscura. All are eaten by isopods Idotea wosnesenskii, but in preference tests in the laboratory the first 2 are consumed significantly more than Ulvaria (see histogram). On later analysis Ulvaria is found to contain comparatively large amounts (>4% dry mass) of the catecholamine dopamine, a common neurostransmitter/hormonal substance in vertebrates. Other tests using dopamine combined with agar binders in realistic concentrations confirm that it is, indeed, the deterrent substance. Catecholamines are found in higher plants, but their functions are just beginning to be known. In animals they play roles in various physiological and ecological processes, and the authors suggest that the same is likely to be true in plants. The authors note that theirs is the first report of dopamine in an alga, and the first experimental demonstration of a plant two isopods Idotea wosnesenskii with a piece of Ulvacatecholamine functioning as a feeding deterrent. Van Alstyne et al. 2006 Oecologia 148: 304.

NOTE similar results are obtained for other herbivores, such as winkles Littorina sitkana and green urchins Strongylocentrotus droebachiensis, but not included here

NOTE other brown and red algae are included in the tests (see histogram), some with other types of chemical defenses not discussed in the paper

Pair of isopods Idotea wosnesenskii with bits of green alga Ulva sp.1.8X


Research study 1: Ligia

photograph of kelp Nereocystis luetkeana, a favoured food of Ligia isopods

Studies on field diets of sea slaters Ligia pallasii on rocky, wave-swept shores on the outer coast of Vancouver Island, British Columbia indicate a preference for diatoms, filamentous green (Cladophora photograph of a large male isopod Ligia pallasiisp. and Ulva sp.) and red (Bangia sp.) algae, membranous (Porphyra spp.) red algae, and various kelps. In laboratory preference tests, both the green Ulva sp. and brown Nereocystis luetkeana are highly preferred. Both species are accessible in the mid-upper intertidal regions, the former growing there and the latter cast up by waves. Carefoot 1973 Mar Biol 18: 228.

Bull kelps Nereocystis luetkeana are good foods for Ligia pallasii on the west coast of British Columbia

Close view of a large male Ligia pallasii crawling over the edge of a brown kelp Laminaria sp. 2X

Research study 2: Ligia

graph show "apparent" SDA effects of diet in semiterrestrial isopods Ligia pallasiiIn later studies on Ligia pallasii at the University of British Columbia the same author determines the specific dynamic action1 (SDA) effect of 2 seaweed diets. First, individual isopods are placed in Gilson Differential Respirometer flasks and their oxygen uptake recorded over a 1h period to establish a metabolic baseline. Then food, either the brown seaweed Nereocystis luetkeana or the green Ulva lactuca is given and oxygen uptake monitored at intervals over a further 28h (see graph on Left). The results show 2-3-fold increases in oxygen uptake on the diets during the first hour after feeding, with slow diminishment to baseline levels by 6-9h after feeding. This increase is known as apparent SDA, as it includes metabolic costs associated with movement of food through the gut, production of digestive enzymes, behavioral responses to the presence of food and to experimental disturbances lasting beyond the feeding period, in addition to the strictly biochemical processes attributed to SDA (see NOTE1 below). With respect to behavioral responses to food, note in the graph the line labeled “non-eaters”. These animals are ones that, although presented with food, choose not to eat. Yet, oxygen uptake is significantly elevated, most likely general excitement responses to the experimenter opening the respirometry flask and inserting the food2. As part of a series of studies on dietary effects on SDA in Ligia, these non-SDA costs are identified in 4 control treatments as follows (20 animals for each): 1) MECHANICAL COSTS: the diet used here is a mix of indigestible agar and cellulose, with no nutritional benefit, but nonetheless requiring bulk to be moved through the gut tract; 2) CAGED FOOD: algal food is placed in a small cage within the respirometer flask, providing the smell of food, but with access to it denied; 3) EMPTY FOOD CAGES: this controls for the presence of the cages in the previous treatment; and 4) MOCK FEEDING: the respirometer flask is opened, forceps inserted, and then removed without touching the animal. Results show significant elevations in oxygen consumption for several of the control treatments, the integrated3 sum of which represents about 25% of the apparent SDA measured (see graph on Right). Overall, significant SDA effects in Ligia last for no more than 4h and account for about 14% of the food energy ingested during the 1h feeding period or about 20% of the food energy absorbed. The studies are are the first to attempt to quantify all components of apparent SDA graph showing components of apparent SDA in sea slaters Ligia pallasiiin an invertebrate animal. Carefoot 1989 Monitore zool ital (N.S.) Monogr 4: 193; Carefoot 1990 Comp Biochem Physiol 95A: 309; Carefoot 1990 Comp Biochem Physiol 95A: 317; Carefoot 1990 Comp Biochem Physiol 95A: 553.

NOTE1 this is the metabolic effect of diet manifested by an elevated level of oxygen consumption after a meal. The causes of SDA have long been debated but, based on correlations found between dietary level of protein or amino acid and SDA, the costs of amino-acid deamination or costs associated with production of ATP from amino acids are popularly implicated. Also considered are costs of urea and ammonia synthesis, fatty-acid metabolism, and protein anabolism/catabolism. Most scientists now agree, however, that protein synthesis and growth processes, in general, account for most or all SDA effects, a viewpoint not incompatible with the other theories mentioned. SDA can account for between 24-30% of ingested food energy in crustaceans and fishes

NOTE2 the normal instinct of Ligia when disturbed is to run. Within the confines of a respirometry flask, however, this translates into a rigid posture with noticeable rapid trembling, likely the cause of the elevated oxygen consumption

NOTE3 some of the control treatments are over-lapping and thus their effects must be teased apart from one another. For example, MECHANICAL COSTS are statistically significant, but include other possible effects of exposure to food and disturbance caused through food presentation. When these last are taken into account, costs of moving food through the gut are actually negligible

Research study 3: Ligia

Cafeteria-style laboratory studies on feeding preferences of the semiterrestrial Ligia pallasii in Barkley Sound, British Columbia disclose that wrack seaweeds are preferred over fresh seaweeds by adults. The histogram shows data for adult males feeding on discs punched out of either fresh seaweed or wrack with a cork-borer. The bars of the histogram are colour-coded for type of seaweed. Overall, the isopods exhibit a 2-3-fold preference for wrack over fresh seaweed. While wrack is tough in texture, and has low nitrogen content and high mineral content, it does have features that may improve its food value. These include increased presence of bacteria and fungi, lower content of herbivore-deterring chemicals (phenols and terpenes) and, perhaps most importantly, higher organic content per “bite” as compared with fresh seaweeds. Pennings et al. 2000 Can J Zool 78: 1918.

histogram showing amounts of different algal, fresh and wrack (dried), eaten by isopods Ligia pallasii photograph of isopod Ligia pallasii in a food-choice chamber with several algal samples

NOTE these are seaweeds cast up on the shore by waves and often dried to crispy/leathery texture in the sun. More on wrack and on crustacean preference for wrack seaweeds is available in the ODYSSEY: AMPHIPODS: FOOD & FEEDING

NOTE the 2-fold value is for adult females; the 3-fold one for adult males. The females when gravid either don't eat or, if they do, are much more “picky” about their choice of food than males

Research study 4: Ligia

The seaweed foods of marine isopods and other marine herbivores are nutritionally poor. Some species, such as sea urchins and sea hares, partially compensate for this by eating large amounts of several types of algae. In these groups, in abalone, and also in Ligia pallasii, a complement of highly diverse gut microflora (bacteria, fungi, actinomycetes) likely contribute to their hosts’ nutrition. The magnitude of this contribution is mostly unknown. Reasons for our lack of knowledge mostly relate to the difficulty of significantly reducing the numbers of microflora with antibiotics in order to assess their possible role in nutrition. Axenic culture, that is, growing of a species in the absence of any other associated life form, has thus far been restricted to a few insects (e.g., aphids), protists, and rotifers, and has not been done to any great success with any large marine invertebrate.

NOTE for a description of an experiment using external application of antibiotics to reduce gut microflora in green sea urchins Strongylocentrotus droebachiensis see SEA URCHINS: NUTRITIONAL REQUIREMENTS

NOTE lit. “without strangers” G., referring to the presence of only a single life form (no parasites, gut microflora, skin bacteria, or other organisms)

Research study 5: Ligia

graph showing growth of isopod Ligia pallasii on different artificial diets over 50 weeks in laboratory cultureOver 20 different platable types of bacteria have been identified in the gut contents of Ligia pallasii feeding on seaweeds. The species can be maintained through several cycles of reproduction in the laboratory on a diet of brown algae, Nereocystis luetkeana (other kelps including Macrocystis spp. yield similar results). If an artificial diet is formulated from chemicals based on the known nutritional content of kelps and other seaweeds (carbohydrates, amino acids, vitamins, and minerals) and fed in dry form to L. pallasii, it is eaten well. In fact, over 50wk in laboratory culture at 15°C this artificial diet promotes greater growth than a natural seaweed diet (see graph). Note the even better growth on a diet that has twice the content of amino acid. Growth of Ligia on this last diet is 1.75 times better than on the best seaweed, or combination of seaweeds, diet. Best growth of all is attained in a field population at Botanical Beach, British Columbia, which is monitored from springtime, through the summer (rapid growth), over autumn/winter (cessation of growth), and into the springtime. The explanation may be warmer temperatures in the field during the summer and a greater selection of algal foods, especially of diatoms, which are highly nutritious for Ligia species. Carefoot 1984 Symp zool Soc Lond No. 53: 455.

NOTE artificial diets can be compound, that is, made up of a variety of ingredients of unknown or poorly known nutritional content, or they can be holidic, where nutrients are exactly known. An example of the first would be dried and powdered seaweed reconstituted in an agar or alginate gel, which would be expected to be eaten and to give good growth in a seaweed-eating invertebrate. An example of an holidic diet would be a selection of laboratory-shelf chemicals, as described in the above experiment, which would include amino acids, minerals, fatty acids, vitamins, and other nutrients. In the present study, the laboratory-shelf chemicals are formulated in a dry, crumbly texture which, surprisingly, is eaten without any period of adaptation by all post-manca (the first stage after emergence from the brood chamber) life stages of L. pallasii. Such holidic diets have been used to determine the nutritional requirements of rats and mice, and, thus, of humans. The method is tedious, requiring that “knock-out” diets be created in all other respects identical to a control diet, but with a single component missing, and then monitoring physiological performance over long term to determine the nutritional value of the missing component. For macroinvertebrates this has been done for only rotifers and certain insects

NOTE this is a dry chemical diet composed of 59 laboratory-shelf chemicals formulated as closely as possible to resemble the composition of the green alga Ulva (but with other additions, such as cellulose for roughage)

Research study 6: Ligia

histogram showing growth of isopods Ligia pallasii on various artificial dietsAn indirect demonstration of the possible nutritional contribution of gut bacteria in Ligia pallasii is shown in laboratory feeding experiments done at the University of British Columbia. Here, Ligia’s growth in laboratory culture (at 15°C, over 40-50wk) is measured on diets of mixed seaweeds, a chemical diet with all nutrients present (2X1 amino acids), and other chemical diets with various deficiencies. Note in the figure that an artificial diet deficient in all amino acids is lethal, with all test specimens dying2 after 12wk. Where only glycine (non-essential for the rat) is present, growth is quite good. In the absence of glutamic acid (non-essential amino acid for the rat) and histidine (essential for the rat), growth is also good. Growth is also good on diets deficient in all fatty acids and vitamins, but not on one deficient in all minerals, where death comes quickly. If a chemical diet with twice the original content of amino acids is good, then why not increase amino acids to 3X levels? Well, a diet with 3 times the content of amino acids actually promotes less growth than the “complete” diet, likely because some other required component of nutrition must be reduced to make room for the additional amino acids. Unless Ligia has amino-acid requirements completely different from other animals, no growth at all should be possible on the histidine-deficient diet and glycine-only diet, and the animals should have died quickly (as they did on the diet deficient in all amino acids). Death would have been expected on diets deficient in both fatty acids and vitamins, as de novo synthesis of these nutrients would not be expected in an isopod. The data suggest that gut microflora, probably bacteria, are metabolising available amino acids (e.g., glycine) in the diet, and are providing other amino acids and all fatty acids/vitamins essential for growth. Finally, the data show that wiithout minerals3 (especially important is calcium for manufacture of the exoskeleton), survival is not possible. Carefoot 1984 Comp Biochem Physiol 79A: 655; Carefoot 1984 Symp zool Soc Lond No. 53: 455.

NOTE1 the original content of amino acids (9.86% dry mass of diet) is determined from that published in chemical analyses of several seaweeds, but mainly in the green alga Ulva sp. A "2X amino acid" diets has, then, 19.72% content of amino acids, and one with "3X amino acids" has almost 30% content of amino acids. At this level the diet may become too low in other required nutrients to sustain good growth

NOTE2 unless mentioned in the graph, survival on all diets is 100%. Where all animals die within a few weeks, no growth occurs

NOTE3the chemical diet includes 11 different minerals of which, when tested "knock-out style), only calcium, magnesium, and phosphorus are essential for survival and growth. Ligia undoubtedly gets at least some required minerals from seawater in which it routinely bathes its gas-exchanging pleopods. If seawater for such bathing is provided, but the animals still fed a mineral-deficient diet, then survival is increased to 36wk, but still no growth occurs

Research study 7: Ligia

Isopods, including Ligia spp., routinely eat their own feces, a behaviour known as coprophagy. This is thought to be beneficial by providing additional nutrient materials produced in the feces by microfloral metabolism. Coprophagy is also a way of re-establishing the complement of gut microflora should it be lost or diminished. It is well known that the feces of isopods and other soil arthropods actually contain significantly more bacteria and other microorganisms than does the ingested food.

NOTE lit. “dung eat” G. Many animals, including rabbits, amphipods, and other herbivores do this, presumbly for the same nutritional reasons

Research study 8: Ligia

photograph of habitat of Ligia pallasii on Seppings Island, British Columbiagraph showing daily activity pattern of isopods Ligia pallasii in the fieldHow much time does an isopod spend eating? 24-h time and energy budgets calculated for a population of Ligia pallasii living on a vertical rock face on the west coast of British Columbia (see photo on Left) show that a day’s energy expenditure of 14J could be met by consumption of 11mg of scavenged fresh mass of seaweed and diatoms, requiring about 35min of the daily time budget (at 13- 15°C). As can be seen in the figure above Right, the major part of each day is spent resting in protective crevices. For an hour prior to dusk individuals begin to mass at the crevice openings and, at dusk and shortly after, they emerge and make their way down the rock face and into the upper intertidal region to feed. The average daily distance moved in this population is 20m, subdivided into 8m moving to and from the feeding grounds, and 4m while on the feeding grounds. Within 2h of sunrise, the animals begin a slow return to their crevices, but not necessarily to the same ones they emerged from. Nocturnal foraging is common in Ligia spp. and in woodlice, and presumably functions to minimise water loss and to avoid visual predators. The authors suggest that as daytime quiescence is the time during which most digestion occurs, an additional function of nocturnalism may be to maximise the efficiency with which energy and nutrients are derived from nutritionally poor diets, in this case, seaweeds. Carefoot et al. 1998 Israeli J Zool 44: 463.

NOTE joule: a measure of work or energy equivalent to about 0.24 calories. Time budgets are determined by monitoring a population numbering 105-160 individuals over two 24h periods during August. A tally of number of individuals engaged in different activities such as resting, walking, running, copulatory amplexus, and feeding is done every 15min, averaged, and converted to percentages. Energy cost in joules of each activity is determined indirectly from amount of oxygen consumed by animals engaged in the same activity in a laboratory respirometer flask (at 15oC), and applying an oxycalorific coefficient factor to convert from ml oxygen to equivalent joules energy

Research study 9: Ligia

photographs of Ligia occidentalis and L. pallasii courtesy Jackie Soaneshistograms showing feeding preferences of isopods Ligia pallasii and L. occidentalisAlthough distributions of the ligiid species Ligia pallasii and L. occidentalis overlap in a zone from central California to southern British Columbia, their specific habitat preferences are different enough that they rarely come into direct contact. A detailed survey in the Bodega Marine Reserve near the Bodega Bay Marine Laboratory, California show that L. pallasii prefers vertical surfaces with seepage cracks and cooler areas, while L. occidentalis exhibits more general habitat preferences, and seems more tolerant of drier localities. Food algae in both fresh and wrack form available on the beaches fronting the Reserve are similar, and the question arises as to whether feeding preferences of the 2 species are also similar. Indeed, results from cafeteria-style tests in the laboratory reveal no significant differences in food preferences for several common algal species (see histograms). Note that of the macroalgae tested, the kelp Nereocystis luetkeana is most preferred by both species, followed by other species in the same preference order. The brown alga Fucus gairdneri is least preferred by both species. The author confirms in a second food-choice experiment that diatoms are commonly eaten by both species, as well as fish protein in the form of shaved bonito-tuna flakes (both set in agar gel to eliminate textural differences). Eberl 2012 J Nat Hist 46 (29-30): 1779. Photographs courtesy Jackie Sones, Bodega Marine Laboratory, California.

NOTE the seaweeds are offered in naturally dried wrack form, presented as discs cut from the algal fronds with a cork borer