# The Histology of Nanomia bijuga (Hydrozoa: Siphonophora)

## TL;DR

This paper describes the cellular anatomy of the siphonophore Nanomia bijuga, revealing unique cell types and structures that enhance understanding of its complex organization.

## Contribution

The study identifies unique cellular populations not connected to a basement membrane, expanding knowledge of epithelial diversity in cnidarians.

## Key findings

- The palpon contains unique absorptive cells indicating specialized intracellular digestion.
- At least two cellular populations lack a basement membrane connection, challenging typical cnidarian epithelial assumptions.
- The anatomical description provides a foundation for future functional and gene expression studies.

## Abstract

The siphonophore Nanomia bijuga is a pelagic hydrozoan (Cnidaria) with complex morphological organization. Each siphonophore is made up of many asexually produced, genetically identical zooids that are functionally specialized and morphologically distinct. These zooids predominantly arise by budding in two growth zones, and are arranged in precise patterns. This study describes the cellular anatomy of several zooid types as well as of the stem and gas-filled float, called the pneumatophore. The distribution of cellular morphologies across zooid types enhances our understanding of zooid function. The unique absorptive cells in the palpon, for example, indicate specialized intracellular digestive processing in this zooid type. Though cnidarians are usually thought of as mono-epithelial, we characterize at least two cellular populations in this species which are not connected to a basement membrane. This work provides a greater understanding of epithelial diversity within the cnidarians, and will be a foundation for future studies on Nanomia bijuga, including functional assays and gene expression analyses.