Why do many cephalopods and nonmarine gastropods develop without a free-swimming larval stage?

Many cephalopods and nonmarine gastropods develop directly without a free-swimming larval stage primarily due to the presence of large amounts of yolk in their eggs, which provides the necessary nutrients for the embryo to develop into a juvenile that resembles the adult.12

In these groups, the evolutionary trend has shifted away from the primitive pattern of releasing gametes into the water to develop into free-swimming, plankton-feeding larvae. Instead, these animals have developed strategies to protect their offspring, such as:13

• Nutrient Provisioning: Eggs are often encased in protective capsules or jelly masses that contain a food supply, allowing the embryo to bypass the need to feed independently in the plankton.13
• Parental Care: Many species exhibit direct care, such as brooding eggs within the mantle cavity, in specialized brood pouches, or attaching them to substrates until they hatch as crawling young.132
• Environmental Adaptation: For nonmarine gastropods, direct development is an adaptation to terrestrial or freshwater environments where a free-swimming larval stage would be hazardous or impractical.13

This direct development allows the young to emerge in a more advanced state, better suited to their specific habitats.12