What might lead to the development of this shared body shape among the four bacterial species?

Marine debris mostly originates from oceangoing ships is a false statement.  

Explanation:

The main cause for marine debris is intentional dumping and littering in the coastal waters. All the waste dumped into the internal water bodies like streams, rivers, canals are carried away by the water and reach the ocean.  All these debris are carried by the ocean currents to the deep waters and till the ocean surface bed.

About 60% to 80% of the debris is constituted by plastic, paper, metal, rubber, wood, glass waste materials, organic wastes, cigarette butts, and many other types of wastes left behind the coastline.  

The oceangoing ships do contribute to marine debris by dumping its waste material but the majority of the debris is caused due to external intentional dumping .

Marine debris not only is a great potential danger to the entire marine ecosystem but also to the mankind.