Guide to the Coastal Marine Isopods of California

Richard Brusca, Vania R. Coelho, and Stefano Taiti

Table of Contents

Suborder Flabellifera

(Key references: Stimpson, 1857; Richardson, 1899, 1905, 1909; Holmes and Gay, 1909; Hatch, 1947; Menzies, 1951B, 1954A, B, 1957, 1962; Menzies and Barnard, 1959; Schultz, 1969; Brusca, 1978A, 1981, 1983A, 1989; Brusca and Ninos, 1978; Bowman et al., 1981; Bruce et al., 1982; Carlston and Iverson 1981; Delaney, 1982, 1984; Delaney and Brusca, 1985; Bruce, 1986, 1990, 1993; Cookson 1991; Harrison and Ellis, 1991; Brusca and Wilson, 1991; Brusca and France, 1992; Brusca et al., 1995; Wetzer and Brusca, 1997)

Flabellifera comprise a large paraphyletic assemblage of families defined more by the absence of certain features than by any unique attributes. The eyes are usually large and well-developed, but are reduced or absent in cave and deep-sea species. The mouthparts are usually robust, adapted for cutting and grinding, or occasionally for piercing. Both the maxillules and maxillae are biramous. The pereopods are usually subsimilar, but in Serolidae, and some Cirolanidae and Sphaeromatidae, the anterior pairs may be subchelate/prehensile. The pleon comprises 1-5 free segments, plus the pleotelson. The uropods arise laterally, usually forming a distinct tailfan with the pleotelson.

With over 3,000 described species, Flabellifera is the second largest isopod suborder, represented in California by seven families, three of which (Anuropidae, Excorallanidae, Serolidae) have not been reported north of Point Conception. Because of the great diversity of this suborder, it is more convenient to key the families first, and then the species in each family.

Key to California Families of Flabellifera

1. Uropods greatly reduced, with very small, often clawlike exopod; body less than 4 mm long; burrowing in wood or algal holdfasts. Limnoriidae
- Uropods not greatly reduced; body rarely less than 3 mm long; rarely burrowing in wood or algae (a few species of Sphaeromatidae burrow into coastal wood structures, but they are large animals). 2
2. Pleon composed of three or fewer dorsally visible free pleonites, plus the pleotelson. 3
- Pleon composed of four or five dorsally visible free pleonites, plus the pleotelson.
3. Pleon composed of three dorsally visible free (complete) pleonites, plus pleotelson; cephalon fused medially with first pereonite; body strongly depressed and expanded laterally; pereonite 7 tergite incomplete or absent; antennae set very close together; frontal lamina reduced to a small triangular plate visible only by pushing aside antennal bases; pleopods 1-3 small and natatory, basis elongated; pleopods 4-5 large, broadly ovate, suboperculiform. Serolidae
- Pleon composed of one or two dorsally visible free (complete) pleonites plus pleotelson; cephalon not fused with first pereonite (except in Ancinus and Bathycopea); body convex dorsally, not strongly depressed; pereonite 7 tergite complete; antennae not set close together; frontal lamina large and distinct; pleopods subequal, of modest size, basis not elongated; pleopods 4-5 ovate but not operculiform. Sphaeromatidae
4. All pereopods prehensile (dactyli longer than propodi); antennae reduced, without clear distinction between peduncle and flagellum; maxillipedal palp 2-articulate. Cymothoidae
- At least pereopods 4-7 ambulatory (dactyli not longer than propodi); antennae not as above, with clear distinction between peduncle and flagellum; maxillipedal palp of two to five articles. 5
5. Pereopods 1-3 strongly prehensile (dactyli longer than propodi); maxillipeds and maxillules and maxillae with stout, curved, apical setae; lacinia and molar process of mandible reduced or absent; maxilla reduced to a single slender stylet. Aegidae
- Pereopods 1-3 weakly prehensile at best; maxillipeds without stout, curved setae; mandible with or without lacinia and molar process; maxilla not a slender stylet. 6
6. Mandible with distinct lacinia and large bladelike molar process; mandibular incisor generally broad, 3-dentate; maxillule lateral (outer) lobe often with several (ten to fourteen) stout spines, never stylet-like or falcate; maxilla well-developed; pereopods 1-3 not prehensile (dactyli not longer than propodi). Cirolanidae
- Mandible with lacinia and molar process greatly reduced, vestigial, or absent; mandibular incisor narrow; maxillule lateral (outer) lobe simple and falcate; maxilla reduced; pereopods 1-3 weakly prehensile or ambulatory. Corallanidae

Family Aegidae

Aegids are cirolanid-like, with the smooth dorsal surface either vaulted or flattened. The maxillipedal palp is of two, three or five articles, the terminal ones with stout acute setae ("spines"). The mandible is elongate, with a narrow incisor and reduced or vestigial molar process. Coxal plates of pereonites 2-6 are large and distinct. Pereopods 1-3 are prehensile (i.e., the dactyli are as long or longer than the propodi and strongly curved); pereopods 4-7 are ambulatory. The family Aegidae comprises six genera. All are temporary parasites on marine fishes. Adults engorge themselves with food (presumably blood) from their hosts, then dislodge and sit on the bottom to digest their meal. Nine species, in two genera, have been reported from Pacific North America, six of which inhabit California waters. However, only a single species occurs in the intertidal zone, Rocinela signata (figure 36).

Family Cirolanidae

Cirolanids have sleek symmetrical bodies, 2 to 6.5 times longer than wide, with well-developed coxal plates on pereonites 2-7. The mandible has a broad tridentate incisor and a spinose bladelike molar process. The maxillipedal palp typically is 5-articulate and the articles never have hooked or curved setae or spines. All pereopods are ambulatory, although legs 1-3 tend towards a grasping form, with well-developed dactyli. The uropods form a tail fan with the pleotelson.

Cirolanids are all carnivores, either predatory or scavenging. A number of species are known to attack sick or weakened fish, or fish trapped in fishing nets, and some are capable of stripping a fish to the bones in a matter of hours. Stepien and Brusca (1985) review this phenomenon and describe the behavior from Catalina Island. This large family includes fifty-five genera. Eight species (in six genera) are known from California waters, six of which occur intertidally.

(figures 37-38, 39-42)
Key to Intertidal Species of Cirolanidae
1. Antennule peduncle article 1 longer than articles 2 or 3; antennule article 2 arising at right angle to article 1; maxilliped endite barely reaching (or extending barely beyond) first palp article; maxilliped endite without coupling setae; antennae long, extending beyond pereonite 7; lateral margins of pleonite 5 not encompassed by pleonite 4.
(one species in California, E. caudata)
- Antennule peduncle article 2 or 3 longest; antennule article 2 not arising at right angle to article 1; maxilliped endite extending well beyond first palp article, usually to distal margin of second palp article; maxilliped endite with coupling setae; antennae length variable; lateral margins of pleonite 5 variable. 2
2. Antennule peduncle article 2 or 3 longest; clypeus projecting ventrally. 3
- Antennule peduncle article 3 always longer than 1 or 2; clypeus short, broad, flat, and sessile, not projecting ventrally. Cirolana 4
3. Prominent rostral process, apically spatulate, separating antennules. Excirolana 5
- Without prominent rostral process.
(one species, E. arcuata)
4. Uropodal rami without apical notch; rostrum meets but does not overlap frontal lamina; antennule peduncle articles 1 and 2 not fused. Cirolana harfordi
- Both uropodal rami with apical notch; rostrum overlaps frontal lamina; antennule peduncle articles 1 and 2 fused. Cirolana diminuta
5. Pleotelson broadly rounded and crenulate posteriorly; antennule peduncle with articles 2 and 3 subequal in length. Excirolana linguifrons
- Pleotelson obtusely rounded and acuminate posteriorly; antennule peduncle with article 3 longer than article 2. Excirolana chiltoni

Family Corallanidae

Corallanids resemble cirolanids but are even more highly modified as predators. Characteristic features of the family include: very large eyes, absence of an endite on the maxilliped, large falcate apical setae on the lateral lobes of the maxillules (often tended by subapical accessory setae), vestigial uniramous maxillae, and frequently a heavily ornamented dorsum beset with setae, spines, tubercles or carinae (especially in males). There are always five free pleonites. The first three pairs of pereopods are often grasping (dactylus as long or longer than the propodus).

Corallanidae is a small group, with six genera and about seventy species. The family is largely confined to tropical and subtropical shallow-water marine habitats, although some brackish and fresh water species are known. Many species are common on coral reefs (hence the name). Because they are often found attached to large prey, such as fishes, rays, turtles or shrimps, they are sometimes called parasites, but they are actually predators. Two species in the large New World genus Excorallana occur in the California intertidal region. Both can be collected with night lights over rocky bottoms. E. tricornis occidentalis, at least in Costa Rican waters, has nocturnal mass-migrations into the water column, perhaps preying on other microcrustaceans (Guzman et al., 1988). Two species belonging to the closely related family Tridentellidae are easily mistaken for corallanids; Tridentella glutacantha and T. quinicornis both occur in shallow subtidal rocky regions of California's offshore islands.

(figures 43-44)
Key to Intertidal Species of Corallanidae
1. Head of male ornamented with three tubercles; pereonites 4-7 with row of tubercles on posterior margin; pleotelson not densely covered by setae. Excorallana tricornis occidentalis
- Head of male not ornamented with tubercles; pereonites 4-7 without row of tubercles on posterior margin; pleotelson densely covered by setae. Excorallana truncata

Family Cymothoidae

Cymothoids resemble cirolanids and corallanids, but are modified for a parasitic lifestyle — all are fish parasites. The definitive features of the family are that all seven pairs of pereopods are prehensile (with long, strongly recurved dactyli as long or longer than the propodi) and the maxillipedal endite lacks coupling setae. Overall, the mouth appendages are highly modified for the parasitic lifestyle. The maxillipeds are reduced to small palps of two or three articles, the maxillules are modified as slender uniarticulate stylets lying adjacent to one another to facilitate transfer the host's blood to the mouth, and the maxillae are reduced to small bilobed appendages. All of these mouth appendages bear stout, curved, terminal or subterminal spinelike setae that serve to hold the buccal region strongly affixed to the flesh of the host fish. All cymothoid species are probably protandric hermaphrodites, first maturing into males and later transforming into females (unless retained in the male stage by the presence of a female already in place on the host fish).

Cymothoids are parasites on marine or freshwater fishes, and they are commonly found on sport and commercial fishes, such as mullet, jacks, groupers, flounder, perch, anchovies, and many others. Although they are not intertidal species, they are often seen by sport fishers and researchers, hence we provide a key to the California species. Most species attach either epidermally, in the gill chamber, or in the buccal region. However, species in some genera actually burrow beneath the skin where they live in a pocket or capsule formed within the musculature of the host (e.g. Artystone, Riggia, Ichthyoxenus, Ourozeuktes). Aside from some localized damage, in most cases cymothoids do not appear to create a great hardship on their hosts. Host-parasite specificity varies between genera, being high in some (e.g. Cymothoa, Idusa, Mothocya) and low in others (e.g. Anilocra, Nerocila, Livoneca, Elthusa). The only known case of a parasite functionally replacing a host organ occurs in Cymothoa exigua, a species that sucks so much blood from its host fish's tongue that the tongue atrophies and is destroyed, but the isopod remains attached to the remaining tongue stub where the host uses it as a replacement tongue for food manipulation (Brusca and Gilligan, 1983). An extensive radiation of cymothoid genera and species has taken place in the freshwater rivers of the Amazon Basin, and to a lesser extent central Africa and southeast Asia. Forty-three nominate genera and over 400 species of cymothoids exist, but the taxonomy of this family is very poorly understood. Seven species, in five genera, are known from California waters.

(figures 45-48, 49-51)
Key to California Species of Cymothoidae
1. Posterior margin of cephalon trisinuate; pleon not immersed in pereon. 2
- Posterior margin of cephalon not trisinuate; pleon partially immersed in pereon. 3
2. Cephalon not immersed in pereonite 1; uropods generally extend beyond posterior border of pleotelson, and clearly visible in dorsal view.
(one species in California, N. acuminata)
- Cephalon somewhat immersed in pereonite 1; uropods barely or not extending beyond posterior border of pleotelson, and typically held concealed under the pleotelson (not visible in dorsal view).
(one species in California, E. convexa)
3. Basal articles of antennules not expanded and touching. 4
- Basal articles of antennules expanded and touching or nearly touching. Ceratothoa 5
4. Antennule longer than antenna.
(one species in California, M. rosea)
- Antennule shorter than antenna. Elthusa 6
5. Pereopods 4-7 not carinate; posterior margin of pleonite 5 smooth, not trisinuate; labrum with free margin wavy, with wide medial notch. C. gilberti
- Pereopods 4-7 carinate; posterior margin of pleonite 5 trisinuate (except in occasional males); labrum with free margin broadly excavate, without medial notch. C. gaudichaudii
6. Pleotelson in adult female nearly twice as broad as long; eyes medium-size and widely separated; anterior border of head broadly rounded or truncate; antenna of 10-11 articles; juveniles with diffuse dark pigmentation on uropodal exopod and anterolateral areas of pleotelson. E. vulgaris
- Pleotelson in adult female about as broad as long; eyes large, close-set medially; anterior border of head strongly produced, apically blunt; antenna of 8-9 articles; juveniles with pigment granules concentrated in melanophores, lacking distinct color pattern. E. californica

Family Limnoriidae

Limnoriids are a cosmopolitan family of wood and algae-boring isopods (the marine gribbles), distinguished by their minute size (4 mm or less in length), wood/algae boring habits, and several unique anatomical features: the head is set off from the pereon and freely rotates, the mandible incisor process lacks teeth and instead forms a projecting rasp-and-file device used to work wood, the mandibular molar process is absent, the basis of the maxillipeds is elongated and waisted, and the uropods are greatly reduced, with a minute often clawlike exopod.

Over seventy species, in three genera (Limnoria, Lynseia, Paralimnoria) have been described. Four species are known from California waters, one of which is an algal borer (L. algarum) and can be most easily found in the holdfasts of large brown algae such as Macrocystis, Egregia, Laminaria, Postelsia, and Nereocystis. The others infest marine woods, such as pier pilings, docks, boats, driftwood, etc.

(figures 52, 53-55)
Key to Intertidal Species of Limnoriidae
1. Incisor process of mandibles simple, lacking rasp or file; algal holdfast borers. Limnoria algarum
- Incisor of right mandible with filelike ridges, that of left with rasplike sclerotized plates; wood borers. 2
2. Dorsal surface of pleotelson with a median Y-shaped keel at base; lateral and posterior borders of pleotelson smooth. Limnoria lignorum
- Dorsal surface of pleotelson with symmetrically arranged tubercles anteriorly; lateral and posterior borders of pleotelson smooth or tuberculate. 3
3. Four anterior tubercles on pleotelson; posterior and lateral margins of pleotelson not tuberculate. Limnoria quadripunctata
- Three anterior tubercles on pleotelson; posterior and lateral borders of pleotelson tuberculate. Limnoria tripunctata

Family Serolidae

Serolids are quickly recognized by their broadly ovate, very thin, flattened bodies with broadly expanded coxal plates. The head is deeply immersed in the pereon. Some species are quite large (to 80 mm). The mandible lacks a molar process, and the maxilliped lacks coupling setae on the endite. Pereonite 1 is fused dorsally with the cephalon and encompasses it laterally. Pereopod 1 of both sexes, and pereopod 2 of most adult males, are subchelate, with the dactylus folding back upon an inflated propodus.

Serolidae is a cold-water family, primarily Southern Hemisphere in distribution. Deep sea species often have reduced eyes, or are blind. They are carnivores, scavengers or omnivores. Heteroserolis carinata, which ranges form southern California to the Gulf of California, is the only California species (figure 56). It burrows just under the sediment surface, from the low intertidal zone to about 100 m depth.

Family Sphaeromatidae

Sphaeromatid isopods can be recognized by their compact, convex bodies, usually capable of rolling into a ball (conglobation); by their pleon which is consolidated into two or three divisions; and by their lateral uropods in which the endopod is rigidly fused to the basal article and the exopod (if present) is movable. In their ability to conglobate, sphaeromatids resemble certain terrestrial isopods, called pillbugs – a striking example of parallel evolution. Identification of genera and species is often difficult because of marked sexual dimorphism. Hence it is advisable, when making determinations, to have a representative sample including adults of both sexes. Twenty-five species of sphaeromatids, in ten genera, have been described from California waters, twelve of which occur intertidally and are included in the following key. Some workers place Ancinus, Bathycopea and Tecticeps in separate families, while others recognize various subfamilies. However, the relationships of the sphaeromatid genera have yet to be analyzed phylogenetically and such taxonomic opinions are based largely on intuition.

Sphaeromatids are primitive flabelliferans with herbivorous habits. The molar process of the mandible is a broad, ovate grinding structure used to chew algae or other plant material. Smaller species probably feed by scraping diatoms and detritus off sand grains. Paracerceis sculpta, a subtropical species that finds its way north to southern California, is unique in that it is possesses three distinct male morphs (designated alpha, beta and gamma males). Alpha males are large, with a distinct morphology typical of other members of the genus; beta males mimic females; gamma males mimic juveniles. The advantage of the beta and gamma males is thought to be in allowing them to sneak into the harem, protected by a single alpha male, to inseminate females (Shuster, 1992; Shuster and Wade, 1991; Shuster and Sassaman, 1997). In the Sea of Cortez, harems most commonly form in calcareous sponges; the natural history of California populations of P. sculpta has not been studied.

(figures 57, 58-63, 64-67, 68-70)
Key to Intertidal Species of Sphaeromatidae
1. Pereopod 1 prehensile; uropod lacking exopod.
(one species in California, A. granulatus).
- Pereopod 1 ambulatory; uropod with exopod. 2
2. Pleopods 4 and 5 lacking pleats. Gnorimosphaeroma 3
- Pleopods 4 and 5 with pleats on endopods. 4
3. First article of peduncles of right and left antennae touching each other. G. noblei
- First article of peduncles of right and left antennae not touching each other. G. oregonense
4. Pleopod 4 and 5 with branchial pleats on both rami. 5
- Pleopod 4 and 5 with branchial pleats on endopods only. 6
5. Uropods lamellar in females, endopod reduced and exopod elongate-cylindrical in males; ovigerous females with four pairs of oöstegites. Paracerceis 7
- Uropods lamellar in both sexes; ovigerous females lacking oöstegites. 8
6. Uropodal exopod with serrate outer margin. Sphaeroma 12
- Uropodal exopod with smooth or lightly crenulate outer margin. Exosphaeroma 13
7. Male uropods with spines; female pleotelson stout, with four tubercles. P. cordata
- Male uropods without spines; female pleotelson elongate, with three tubercles. P. sculpta
8. Frontal margin of head produced as a quadrangular process; first two articles of antennules dilated. Dynamenella dilatata
- Frontal margin of head not produced; articles of antennules not dilated. 9
9. Uropod rami with crenulate margin (at least in males). Paradella dianae
- Uropod rami without crenulate margin 10
10. Pleotelson with many tubercles. Dynamenella sheareri
- Pleotelson without tubercles. 11
11. Pleotelson with many ridges; uropod rami of similar length. Dynamenella benedicti
- Pleotelson smooth; uropod with exopod (outer ramus) longer than endopod (inner ramus). Dynamenella glabra
12. Pleotelson with many rows of tubercles, posterior extremity without prominent transverse elevation. S. walkeri
- Pleotelson with two rows of tubercules, posterior extremity with prominent transverse elevation. S. quoyanum
13. Pleotelson and uropods relatively small; posterior margin of pleotelson rounded. E. inornata
- Pleotelson and uropods very large; posterior margin of pleotelson acuminate E. amplicauda
About This Page

Richard Brusca
Arizona-Sonora Desert Museum, Tucson, Arizona, USA

Vania R. Coelho
Dominican University of California, San Rafael, California, USA

Stefano Taiti

Correspondence regarding this page should be directed to Richard Brusca at and Vania R. Coelho at

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