Most isopods brood their young in a chamber below the thorax formed from overlapping appendages known as oöstegites.  The eggs are released from paired gonopores into the chamber.  As they emerge from the gonopores, the eggs pass through sperm packets known as a spermatophores, glued to each opening by the male, and are fertilised.  The male deposits the spermatophores during copulation using paired copulatory appendages. Drawing adapted from Brusca et al. 2001 Tree of Life: Guide to the coastal marine isopods of California.

NOTE  lit. “egg cover” G.

The following sections are presented alphabetically by genera.

Cirolana & Excirolana

A study of reproduction of isopods Excirolana chiltoni in sand-beach areas around the Scripps Institution of Oceanography at La Jolla, California reveals an unusual reproductive feature of ovoviviparity.  Thus, in contrast with other isopod species that brood their eggs externally in a pouch formed from overlapping appendages or oostegites in the ventral thorax region, Excirolana deposits its eggs into the paired uteri where they remain through gestation.  As shown in the drawing on the Left, oostigites are present, but they do not function as a brood pouch. Gestation in E. chiltoni requires about 3mo, commencing from mid-February.  Average brood size is 31 and the mancas are comparatively large at birth. Their large size is accommodated in the female by enormous expansion of the ovaries and uteri.  At birthing the juveniles slide out of the expanded genital opening abdomen-first propelled, the author speculates, by hydrostatic forces generated by muscular contraction of the thorax.  At the time of birthing, new eggs are being formed in the overies and the uteri, damaged during birthing, are formed anew.  Species of Excirolana customarily bury themselves in sand near the high tide swashline, and emerge as the tide rises to feed on polychaetes and small crustaceans such as juvenile mole crabs.  The author surmises that oviviviparity may have been selected for as a means of protection of embryonic stages against stresses present in their particular wave-exposed sand-beach habitats.  Klapow 1970 J Zool, Lond 162: 359.

NOTE  the author reports that 6 other world species of Excirolana have similar ovoviviparous-type reproduction to that found in E. chiltoni

On sand beaches near the Scripps Institution of Oceanography at La Jolla, California the isopod Excirolana chiltoni buries itself in the high intertidal area when the tide is out and emerges at high tide to swim and forage.  Where it buries is determined by the level of the preceding high tide.  Thus, the isopods move up and down the vertical extent of the beach on a fornightly schedule.  Moulting is in 2 stages, with the posterior half being cast off before the anterior half.  Amorphous calcium carbonate and calcium-phosphate salts removed from the previous exoskeleton are stored in the integument as localised concretions, termed dermoliths. Moulting is fortnightly and is correlated with the moon cycle.  It occurs during the periods preceding new or full moon when tidal amplitudes are increasing. The author’s results suggest that release of young is on a fortnightly schedule.  During the 2-3mo period when females are gravid, they do not moult.  Unlike in other isopods, the young in Excirolana are borne internally (ovoviviparity). Moulting requires several days to complete, so its synchronicity with rising tides may help ensure that  an individual is not isolated at the top part of the shore without water during the critical period of hardening the exoskeleton.  Klapow 1972 Biol Bull 143: 568.

NOTE  the author’s data show that about 40% of the entire mineral salts in the exoskeleton may be stored temporarily in the dermoliths.  Complete reabsorption of the salts occurs by the midway stage of moulting the anterior part of the exoskeleton

The dermoliths are positioned bilaterally
in thoracic segments 4 & 5

Breeding of the intertidal isopod Cirolana harfordi in Pacific Grove, California occurs throughout the year save for autumn (see graph on Left).  After copulation, eggs are fertilised as they pass from the female gonopores into the brood marsupium.  The eggs remain in the marsupium for 3-4mo.  Females produce 1-2 broods numbering up to about 50 embryos for a large individual during their 2-yr life span (see graph on Right). Cirolana spp. are excellent swimmers and at nighttime can be seen buzzing around looking for food and mates.  Johnson 1976 Mar Biol 36: 343.

NOTE  the marsupium is formed from 5 pairs of overlapping extensions from the thorax known as oostegites. This forms a protective pouch that is outside the body. Note the difference between this situation in Cirolana with that of Exocirolana described in Research Study 1 above

Change in sex during development is relatively common in crustaceans, and most exhibit protandry (lit. “first male” G.),that is, change from male to female.  Protandry is of selective advantage when female fecundity increases with age but male mating success remains independent of size.  Conversely, protogyny (lit. “first female” G.) may evolve when small males are prevented from mating by larger males, making it advantageous to become a male only when a competitive, larger size is reached.  Studies at the Bamfield Marine Sciences Centre, British Columbia reveal that the small intertidal isopod Gnorimosphaeroma oregonense is protogynous.  In field collections, 31% of females have rudimentary penes (see graph upper Right). In laboratory culture a female produces one brood, then passes through a number of moults as an immature male before becoming a sexually mature male.  There is no indication of multiple broods in these laboratory cultures.  Receptive females are each “guarded” in amplexus by a larger male.  While in amplexus the female moults, and copulation is thought to occur immediately following the first (posterior) of the biphasic moults. The male abandons the female following the second half of the biphasic moult, suggesting that copulation has, indeed, occurred.  Embryos appear in the marsupium 1-3d after the anterior moult and develop to hatchlings after 8-10wk at 17^oC.  Large field-collected gravid females have about 40 eggs/embryos (see graph lower Right). After release of the hatchlings or (mancas about 6wk later, the females moult to an immature male, with loss of oostegites and development of penes. In laboratory culture the sex change appears to be facultative because a large proportion of juveniles (>60%) actually develop directly into males.  The authors note that theirs is the first study to document protogynous sex change in G. oregonense.  Brook et al. 1994 Biol Bull 187: 99.

NOTE  the authors list 60 sequentially hermaphroditic species of crustaceans, of which 82% are predominately protandrous. These include barnacles, copepods, isopods, amphipods, and shrimps.  The 18% protogynous species are represented by tanaidaceans and isopods.  Only Order Isopoda has representatives with both reproductive strategies

NOTE  in terrestrial isopods the endopodites of the first 2 pairs of pleopods are modified into rigid, stylet-like sperm-transfer devices.  In Gnorimosphaeroma and perhaps other marine isopod species, the sperm-transfer devices are soft, nipple-like structures bearing a terminal pore through which the sperm is released

Studies on Dillon Beach, California populations of Idotea montereyensis indicate that ovigerous females may be found at any time of year, but with summer peaks and winter lows.  During summer up to 60% of adult females are carrying broods.  There is a small release of juveniles in March from winter brooders, and a much larger release in July from spring brooders.  The author does not indicate the length of incubation time, but it is likely weeks or months, depending upon whether it is a spring or winter brood being carried.  Juveniles spend the summer in protected inshore red-algal beds, while adults spend the winter on lower intertidal surfgrass beds.  Lee 1966 Ecology 47: 930.

Adult Idotea montereyensis clinging to
a blade of surfgrass Phyllospadix sp. 0.7X

All but one species of idoteid isopods are entirely free-living, although many species closely associate with various types of brown algae.  The exception noted is Colidotea rostrata, a southern California/Baja California-inhabiting idoteid that lives symbiotically with 2 sea-urchin species Strongylocentrotus purpuratus and S. franciscanus.  The isopods are about 7mm length, coloured to match the colour of their host, and frequent over half of all urchins at an average year-round density of 6 individuals per host (see graphs on Left). A 2-yr study on reproductive biology and population dynamics of Colidotea at Palos Verdes, California shows that breeding occurs all year long, but with a main spring-summer period (see graph on Right) and an average fecundity of 12 offspring per brood. Juvenile mortality is low and about 80% of individuals survive to reproductive maturity.  Protection conferred by the urchin’s spine canopy against potential fish predators appears to be the main advantage to the isopods in living on the urchins.  Stebbins 1989 Mar Biol 101: 329.

Ligia

In many or all marine and semiterrestrial species, including the semiterrestrial Ligia pallasii, the male carries the female about for several days or weeks prior to copulation, gripping her with a few of his legs.  This is allows the female to moult and become soft for the copulatory act, but under the protection of the male.  Pheromones, not yet identified, signal the female’s premoult copulatory status to the male. In this species, but not in all Ligia species, the male is much larger than the female, perhaps an adaptation enabling the male to protect the female when she is soft or to enable locomotion for the male while keeping the female close during pre-copulation.

NOTE  a detailed account of reproduction in Ligia pallasii can be found at EVOLUTION TO LAND: REPRODUCTIVE MODIFICATIONS

NOTE  known as amplexus lit. “to embrace” L.

A pair of Ligia pallasii in amplexus 2X

Limnoria

Early observations on reproduction in the wood-boring gribble Limnoria lignorum at Friday Harbor Laboratories, Washington suggest that there may be an annual migration or swarming of adults associated with the main breeding season in springtime.  Evidence for swarming comes from regular seasonal infestation of test blocks of wood that are only accessible by swimming individuals originating from infested wood a few meters distant.  Although designated as such by the author, the movement should probably be considered not so much a “migration”, which presumes a direction, but rather just population dispersal to a new habitat.  Most dispersal comes during late winter and spring, when water temperatures range from 7-9^oC (see photographs).   The author notes that gravid females, while present generally throughout the year, are most prevalent in April-May.  Young are released from the female’s brood pouch and immediately bore into the wood substratum.  Johnson 1935 Biol Bull 69 (3): 427.

NOTE  the isopods are about 5mm in length as adults, and a heavily infested piece of wood may contain 25 individuals ^. cm^-3

This sketch is one of "Batham's Cards", a huge collection of biological
records of organisms found during field excursions by Dr. Elizabeth
J. Batham, Director of the Portobello Marine Laboratory in New
Zealand from 1951-74. The species Limnoria lignorum is considered
to be northern-oceans inhabitant, probably explaining the lightly
drawn in species name, awaiting the attentions of a taxonomist

A review of reproduction in Limnoria species includes useful information on several west-coast species, including L. tripunctata, L. quadripunctata, and L. lignorum.  In the female, leaf-like plates or oostegites grow to form a typical marsupium or brood pouch into which the fertilised eggs are deposited.  Although the male’s copulatory appendages are unusually short (for an isopod) the author assumes that sperm are transferred to the openings of the female’s oviducal openings as in other isopod species, with the eggs being fertilised as they transit the oviducts on their way to the brood pouch.  Average brood size in west-coast Limnoria spp.ranges from 7-22, with maximum broods being found in L. lignorum (up to 35),  Young are released as crawl-away juveniles after an undisclosed period of brooding.  Immediately following their release, the female moults and the oostegites are re-formed at their pregravid size.  An interesting comment by the author is that burrows are commonly occupied by a reproductively mature pair for periods of several months (in the laboratory), along with any young produced by the pair.  The behaviour of pairing may relate to the absence of sperm-storage organs.  Menzies 1954 Bull Mus Comp Zool 112 (5): 363.

NOTE  the author notes that a seminal receptacle for temporary storage of sperm is absent in limnorids and indicates (see drawing on Right) that freely swimming sperm are present in the oviducts.  It seems that spermatophores or sperm packets are not employed as in terrestrial species of isopods (oniscids)

In a later investigation the above authors collaborate to investigate reproduction and colonisation of new habitats in gribbles Limnoria tripunctata near the Scripps Institution of Oceanography, La Jolla, California.  The reproductive cycle of this species is similar to that of other limnorids, with females carrying broods (up to 10 individuals in each) in spring and summer, and with young being released after about 30d of incubation (at 20^oC).  In comparison,  females that brood over winter may take 60d (at 15^oC).  Females producing a brood in summer often simultaneously have ripe ovaries and may begin incubating a second brood soon after the first batch of juveniles is released. On release from the adult, which takes place within the burrow, the youngsters immediately begin to construct their own burrows within the walls of the parent burrow.  The authors describe a “migration” or swarming of individuals from old wood habitats in search of new habitats in summer, coincidental with higher water temperatures of 20^oC.  Almost all (97%) of these colonisers are sexually mature, but none of the females is gravid (most have likely just recently produced a brood).  Sexual pairings take place on the new wood block.  A main factor influencing these swarmings appears to be pressure from overcrowding in established populations.  Johnson & Menzies 1956 Biol Bull 110 (1): 54.

NOTE  the authors refer to this as “hatching”, but of course this term should be reserved for the juvenile stage breaking free of the egg capsule in the brood pouch

NOTE  maturity is reached by females at about 0.5mm body width, and by males at about 0.45mm width