Some time prior to a mature female’s moult to form the brood chamber a male is attracted to her through perception of sex pheromones.  The male uses his large gnathopods (backward-facing claws on the 2nd pair of walking legs) to hold the female in copulatory amplexus until she moults, and then applies sperm packets or spermatophores to the oviducal openings. At the time time of egg laying, which occurs shortly after copulation, the eggs pass through the spermatophores and are fertilised. Females carrying eggs or young in their brood chambers can be found throughout spring and summer, depending upon geographical region.  After several weeks of development in the brood chamber, the youngsters crawl out of, or perhaps in some way are released from, the brood chamber and take up the adult way of life.

NOTE  lit. “embrace” L., where the male holds the female beneath his body until she moults, then he applies the spermatophores (= copulation)

NOTE  in semiterrestrial isopods a female arches its back upwards, which springs open the brood chamber at the back, allowing the youngsters to crawl out.  Whether something similar happens in amphipods is not known

Studies on skeleton shrimps Caprella laeviuscula and Dentella californica at Friday Harbor Laboratories, Washington show that a male holding a female with its 5th pair of walking legs will fight off other males using its large 2nd gnathopods as clubs.  Mean brood sizes in the 2 species from females in lab culture are 31 for C. laeviuscula and 16 for D. californica.  The species have life spans of about 8mo.  Caine 1979 J Exp Mar Biol Ecol 40: 103. Photograph courtesy Pearson College, Vancouver Island, British Columbia Pearson College.

NOTE the taxonomy of this species is uncertain

Skeleton shrimps Caprella laeviuscula on
a piece of submerged equipment 1.5X

Amphipods Megalorchestia californiana dig burrows within the sand within which they hold females, either singly or sometimes in a harem.  The males are larger than females, and use their antennae and gnathopods to guard the females prior to mating. Invading males, or males in desirable burrows, may be grabbed by another male in its tentacles and/or clubbed with its gnathopods.  Prior to the moult the males use their antennae and gnathopods to hold a female in copulatory position, or amplexus.  Copulation occurs usually within an hour after the female moults while her exoskeleton remains flexible.  Females survive for about a year and only mate once.  In comparison, males are able to mate many times.  In experiments done at Friday Harbor Laboratories, Washington, researchers find that while females show only random preference for a particular burrow on successive days, males display significantly greater burrow fidelity and may return several times to the same one.  When groups of 3 individuals are placed within arenas, whether or not they share burrows depends upon sex ratio and size of males.  For example, 2 females with a single male will usually end up in the same burrow, while 2 males with a single female usually end up with the males in separate burrows and the female in the burrow of the largest male.  In these and other small-group trials, male competition seems more important than female choice in explaining mate selection.  However, in larger mixed-sex groups, choice by females of larger males becomes more important. Note in the histogram that males selected by females are 17% larger than males not selected. The authors note that male size correlates positively with length and intensity of redness of antennae, so their quality as mates may be communicated visually.  This is the first study of its kind on sexual selection in M. californiana. Iyengar & Starks 2008 Behav Ecol 19: 642. Photograph courtesy Dave Cowles, Walla Walla University, Washington wallawalla.edu.

NOTE  other secondary sexual characteristics in the males are the enlarged red-coloured antennae and large gnathopods, the latter used to club away competitors and to hold the female during amplexus

Male Megalorchestia californiana in single-sex groups tend to space themselves out.  High-quality males
(large body size, bright red antennae) compete for burrows in the best locations and females are attracted
to them.  Smaller males settle nearby
to maximise their chance of “sneaking”
in to grab a female. Here, a male
is about to enter a burrow 4.5X

A female M. californiana 9X