貝類 (かいるい, kairoi) – shellfish 

貝 (かい, kai) – shell; shellfish

魚介類 (ぎょかいるい, gyokairoi) – marine products; seafood; fish and shellfish

類 (るい, roi) means type, sort, kind… So 貝類 refers to all the various types of shellfish, and 魚介類 refers to all the various types of fish.

鮪 (まぐろ, maguro) – Tuna (Thunnus spp.)

魬 (はまち, hamachi) – young Japanese amberjack; young yellowtail

鯖 (さば, saba) – mackerel (esp. the chub mackerel, Scomber japonicus)

鯛 (たい, tai)
1. sea bream (Sparidae); porgy
2. tai (species of reddish-brown Pacific sea bream, Pagrus major)

烏賊 (いか, ika) – cuttlefish; squid

海藻 (かいそう, kaisou) – seaweed

海鷂魚 (えい, ei) – ray (fish); stingray