Exploring evolutionary trends within the Pennellidae (Copepoda: Siphonostomatoida) using molecular data.

Area of Science:

• Marine Biology
• Parasitology
• Molecular Evolution

Background:

• The Pennellidae family includes ecto- and mesoparasitic copepods that infect marine fishes.
• Previous phylogenetic studies based on morphology are limited by the highly modified bodies and reduced appendages of these parasites.
• Objective characterization of morphological traits for phylogenetic analysis is challenging.

Purpose of the Study:

• To analyze the phylogenetic relationships among seven genera and 12 species of pennellids using molecular data.
• To infer evolutionary trends within the Pennellidae family.
• To investigate adaptations related to parasitic lifestyles.

Main Methods:

• Analysis of 18S and 28S ribosomal DNA sequences from seven pennellid genera and 12 species.
• Phylogenetic reconstruction based on molecular data.

Main Results:

• The molecular analysis identified three distinct clades: Clade-I (Peniculus), Clade-II (Haemobaphes, Lernaeocera, Phrixocephalus, Exopenna, Lernaeenicus radiatus), and Clade-III (Pennella, Lernaeenicus spp.).
• Results partially align with previous morphological studies but do not support a sister relationship between Exopenna, Phrixocephalus, and Pennella.
• Clades II and III show reduced body tagmosis and a shift from ectoparasitism to mesoparasitism.
• Specific adaptations like sigmoid bodies and coiled egg strings in Clade-II gill parasites likely evolved for space constraints within host gills.

Conclusions:

• Molecular data provide a robust framework for understanding pennellid phylogeny and evolution.
• The study highlights convergent evolution of adaptations, such as coiled egg strings in eye parasites like Phrixocephalus, potentially from ancestral gill parasites.
• The deep-embedding head structures in Clade-III species represent a significant adaptation for host tissue anchorage.