Most crustaceans found in the sand have special pouches in which they can brood young. Interstitial cladocerans ("water fleas") typically reproduce by parthenogenesis (like gastrotrichs) and brooding. The young are then released just before the mother molts. Also like the gastrotrichs, cladocerans can combine asexual amplification with sexual reproduction.

Copepods, another type of crustacean that is abundant in the sandy beach, are strictly sexual reproducers. They also keep their embryos in specialized brood sacs.

One can find other brooding crustaceans, such as mysids ("opossum" shrimps) and cumaceans, at the sandy beach by sieving the uppermost layer of sand with a fine net.  Mysid shrimps and cumaceans, as well as closely related isopods and amphipods, have a brood chamber on their thorax (called a marsupium) where their eggs develop.   After a few days or even up to months of development, the young hatch out of as fully formed larvae.

Barnacles

Conspicuous members of the rocky promontory, especially at higher tidal heights, are the sexually-reproducing barnacles. Although barnacles remain fixed in place throughout their lives, they manage to fertilize internally by copulation with close neighbors. Individuals (all hermaphrodites) extend a relatively long penis and deposit spermatophores into the mantle cavities of close neighbors, where fertilization takes place.  After fertilization, eggs are brooded and later released as nauplii larvae that can spend weeks or months in the plankton. The nauplii larvae metamorphose into cyprid larvae, which search for a good settlement site. Upon settlement, the cyprid will attach itself permanently to the rocky substrate and spend its life kicking its legs into the water to feed .

Some gastropods, including snails in the genus Nucella, lay benthic egg capsules. These snails' larvae develop completely inside the egg capsules and then hatch out as fully-formed but tiny Nucella juveniles.

Other organisms found in this habitat have not one, but two modes of reproduction.  The sea anemone Anthopleura elegantissima can reproduce repeatedly by asexual fissioning, where an individual will dive itself in two. The same anemone can also reproduce sexually by free-spawning, which results in pelagic, feeding planula larvae that disperse much farther than a fissioned clone. Sea anemones like A. elegantissima often form large mats of individuals, called clones, in mid-intertidal pools and crevices that stay wet at low tide.  

Anthopluera elegantissima clone in a crevice

Polychaetes

Benthic polychaete worms of the family Nereidae undergo a life-history transformation of the adult body into a stage called an epitoke.  Epitokes are sexually mature worms that become highly specialized for swimming and releasing gametes into the water column.  Spawning times are usually highly synchronized within species to improve chances for successful fertilization.

Worms of the family Lumbrinaridae are also abundant in the eelgrass. Unlike the nereids, these worms do not form a specialized stage and instead deposit benthic eggs, which are brooded and may produce crawl-away juveniles that resemble adult worms.

Gastropods
Two very similar grazing snails, Lacuna vincta and Lacuna variegata, are very conspicuous and abundant on Phyllospadix .  One way to tell the two species apart is to look at the egg masses that they deposit.  L. vincta's egg masses are donut shaped (with a hole in the middle), while L. variegata's, although circular in shape, have a less distinctive hole.

Isopods and amphipods are both abundant under the loose cobble, and each have marsupium for brooding young.  In the photo to the left, the young isopods being brooded, and perhaps released prematurely upon handling, are visible near the mother.

Shore crabs (Hemigrapsus sp.), porcelain crabs (Petrolisthes sp.), and hermit crabs (Pagarus sp.) also brood their eggs, but they release free-swimming larval stages called zoea. In the plankton, the zoea increase their size by feeding and molting.  As they become ready to settle, they molt into a post-larval stage called a megalopa.  As you can see from the picture, megalopa look similar to a little crab with a prominent abdomen, which is reduced and tucked under the cephalothorax in adults.