Catholics of the Eastern variety (Melkite, Coptic, Assyrian, or Byzantine Catholics) do not eat fish on Wednesdays and Fridays as fish is considered to be meat since it has verterbrae (a backbone). Seafood such as shrimp, lobsters, crab, and shark are not considered to be fish or meat as they lack a backbone. However, with today's high price of lobster and crab, priests are recommended that parishioners also abstain from these two expensive foods. In addition, it is recommended that one also fast from all forms of meats, gravy, meat broth, dairy, cheese, eggs, and olive oil. During the four great fasts of Great Lent, the Apostles' Fast, the Fast of the Theotokos in honor of her Dormition, and the Christmas (Advent) Fast, we do not partake of meat during those days. However, the calendar does allow for the eating of oil and fish on special feast days.  Of course, your priest will give a dispensation from the fast and abstinence for the sick, pregnant or lactating ladies, or the elderly.

We fast and abstain on Wednesday in reparation for Judas' betrayal of Christ.

We fast and abstain on Friday in reparation for the crucifixion of Christ.

Dominican Tertiaries (and Dominican priests, brothers, sisters, and nuns) also do not eat meat on Wednesdays and Fridays, which was the traditional fast of the Roman Catholic Church before the Vatican I Council. Just before Vatican I, American bishops successfully petitioned Rome to reduce this "black fast" for the citizens of the USA.

We are encouraged to fast as much as we are able, and per the gospel, we fast in silence and in joy, anointing our foreheads with the oil of gladness, so as not appearing to be fasting.

Two things
1- Have never been told shark was ok since it is clearly a fish
2- Have argued with clergy that lobster and crab are contrary to the spirit of the fast to no avail, Byzantines have some different rules, for example the oil thing is observed by Byzantines only. Some also may only abstain from meat during certain fasts and not dairy. In general the fasts and rules are the following

1- no food from midnight to either noon( modern ) or three
2- No meat or dairy (no eggs)
3-Christmas fast,
Nineveh Fast ( Coptic+Syrian)
Great Lent
Apostles Fast (western in origin and not observed as strictly by some)
Dormition Fast
The Wed. and Fri. fast, not observed during Christmas and Easter time. This is observed in the following way, Tuesday evening after vespers @ 6pm begins the fast which ends @6pm Wednesday evening. The same happens for Friday.

Also one is expected to confess during the fast periods and it is not unusual to see no one approach for communion during those times