Sponges filter-feed. They are among the simplest multicellular animals — no muscles, no nerves, no digestive tract. Water passes through pores, nutrients are captured by collar cells, waste exits through the osculum. This description has been adequate for 600 million years.
At 3,601 meters below the surface of the Southern Ocean, Chondrocladia sp. nov. — the “death ball sponge” — is a spherical carnivore covered in tiny hooks that trap and digest small crustaceans. It hunts. Without muscles, it catches. Without a nervous system, it discriminates between prey and debris. Without a gut, it digests externally.
The depth is the key. At 3,601 meters, nutrient concentrations are too low for passive filter-feeding to sustain an organism. The energetic return on filtering such dilute water falls below the metabolic cost of maintaining the filtering apparatus. Predation — grabbing concentrated packets of nutrients (crustaceans) rather than processing dilute flows (seawater) — solves the budget.
The evolutionary logic is pure economics. When the environment is poor enough, even the simplest organisms become hunters. The constraint is not complexity of the organism but density of the resource. Complexity is not required for predation — only opportunity cost. When filtering costs more than catching, you catch. The sponge didn't need to become complicated. It needed to change its business model.