You're right, the first form is not correct. However the second form also has a problem, because of the pluralisation of "eggs" vs "that of". To make the second one correct it would need to be something like: "Unlike the eggs of other fish, those of anchovies are not spherical but are about twice as long as they are wide." You might go one step further: "Unlike the eggs of other fish, those of anchovies are not spherical but are *instead* about twice as long as they are wide." You could correct the first one correct with: "Unlike the eggs of other fish, anchovy eggs are not spherical but are about twice as long as they are wide." [Here anchovy refers to the animal as a species, so is singular] or: "Unlike the eggs of other fish, anchovies' eggs are not spherical but are about twice as long as they are wide." [Here 'anchovies' refers to anchovies as animals in the world rather than as an abstract species, and the eggs belong to them (plural) - note the possesive apostrophe on the plural form of anchovy] Hope that helps :)

It should be "those of anchovies." We say, "those eggs" not "that eggs." "Or you could say,"Unlike the eggs of other fish, the eggs of anchovies are not spherical but are about twice as long as they are wide."

Unlike the eggs of other FISHES, those laid by anchovies are not spherical but are about twice as long as they are wide. Fishes >different species of fish

Right idea, but it needs to be plural to match "eggs": "...those of anchovies..."

It should be "those of" instead of "that of", but otherwise your understanding is correct.