A Nicothoid Copepod Parasite of Cancrid Rock Crabs

Overview

In an invertebrate biology teaching laboratory in October 2021, a demonstration of crab egg predators turned up something that was not supposed to be there. Brooding Pacific rock crabs from the Gaviota Coast carried a nicothoid copepod egg predator (Nicothoidae, cf. Choniosphaera sp.), sitting among the host’s eggs at nearly the same size and color as the eggs themselves.

Follow-up collections found it on three commercially important cancrid crabs, at intensities high enough to turn an entire brood maroon. This work is the first record of a nicothoid egg predator on cancrid crabs anywhere in the Eastern Pacific. It describes the parasite’s life cycle across two microhabitats, the egg mass and the gills, and asks what a near-universal egg predator means for the reproductive output of a fishery nobody is closely watching.

Published in Ecology

Orli, J. E., S. M. Lecuona, G. O. Plewe, C. N. Gadler, A. M. Kuris, D. Tang and Z. L. Zilz. 2025. Discovery of an unidentified species of nicothoid copepod infesting cancrid crabs in Santa Barbara, California. Ecology 106(12): e70263.

Read the paper Data and code on Dryad Videos on Zenodo

Quick Facts

Role: Lead author, Kuris Lab, University of California, Santa Barbara
When: 2021 to 2025
Hosts: Metacarcinus anthonyi, Romaleon antennarium, Cancer productus
Where: baited traps at 66 to 100 meters along the Gaviota Coast, plus additional Southern California sites
Project type: Scientific Naturalist: discovery, natural history and fishery context
Skills: field collections, crab identification, dissection, brood stage scoring, parasite life history description, figure and media design, open data curation

Background and Questions

Externally brooding crustaceans carry a long list of egg predators, from nemerteans to copepods, and those predators can take large fractions of a brood. Some have been implicated in fishery collapses. Nicothoid copepods are specialized crustacean associates that commonly mimic their host’s eggs and feed on yolk through the egg shell, which is exactly what makes them easy to miss: at a glance, an infested brood looks like a brood.

How common is this nicothoid on California rock crabs, and which host stages carry it?
How does its life cycle use the egg mass and the gills as separate microhabitats?
What does near-universal infestation mean for the fecundity and management of a data-poor fishery?

Methods

Discovery and collections. The first specimens came out of a teaching demonstration rather than a survey. Targeted collections followed: ovigerous cancrids taken by local fishers in baited pots at 66 to 100 meters along the Gaviota Coast, then held in flow-through seawater so that broods could be inspected repeatedly rather than once.

Host and parasite observations. Each crab was identified to species, with sex, size and reproductive status recorded. Egg masses were scored by egg developmental stage and examined for adult female nicothoids, their egg sacs and embryos, nauplii and copepodids, and for co-occurring egg predators such as Carcinonemertes species. Gill lamellae were inspected in both sexes, which is where the second half of the life cycle turned out to be.

Life cycle synthesis. Adult females were tracked through their color and size change, from yellow and orange when small to maroon when large and heavily gravid, and their egg sacs were characterized from structure through development: one to four embryos per sac, embryo to nauplius to copepodid. Timing of copepodid peaks in the egg mass, and their presence in gills before and after hatching, is what makes it possible to infer how the parasite crosses from one brood to the next.

Key Results

A novel nicothoid on Eastern Pacific cancrids

The parasite matches Choniosphaera on the traits that matter in an adult female: globular body, ventral mouthparts, leg and antennule morphology. It differs enough to remain unidentified at species level.

It is the first nicothoid egg predator reported on cancrid crabs in the Eastern Pacific, despite decades of egg masses being opened in the search for nemertean predators.

Nearly every breeding host carried it

Across all three host species, almost every adult examined had at least one nicothoid life stage, on the egg mass, in the gills, or both.

Ovigerous females routinely carried reproductive adult females on their broods. Only a handful of juveniles and long-held males were completely clear.

The eggs are being eaten

Adults sit among the host’s eggs, pierce the shell and consume the yolk, leaving empty or degraded capsules behind and opening the way for microbial invasion.

A heavily infested brood reads as maroon, the color of dense pink nicothoid eggs laid over the crab’s own. Copepodid densities peak late in host egg development, which is what cumulative exposure across a roughly forty five day brooding period looks like.

Gills bridge one clutch to the next

Copepodids occur on the gill lamellae of both sexes, including on females that went on to extrude a new clutch.

In some of those females, copepodids appeared in the new egg mass within a day and adults within nine, which points to transfer from gill to egg at oviposition, and to the gills as the reservoir in between.

A data-poor fishery, and an open question

A conspicuous parasite appearing suddenly on a well-studied crab assemblage has two plausible explanations: a native species that becomes abundant in decadal pulses, as reported for Choniosphaera in the Atlantic, or a recent host shift onto cancrids.

Either way, California rock crabs are under-managed and data-limited, and an egg predator this common could erode reproductive output and year-class strength without anyone measuring it.

Figures

Both figures are from the published paper. Click any panel to open it full size; hover over a sequence to pause it.

Figure 1. Adults on the egg mass

Small yellow adult nicothoid on crab eggs — 1a. Adult females from *Metacarcinus anthonyi*, in sequence: growth and color change from yellow and orange to maroon as they enlarge and become heavily gravid.

Slightly larger orange adult nicothoid — 1a. Adult females from *Metacarcinus anthonyi*, in sequence: growth and color change from yellow and orange to maroon as they enlarge and become heavily gravid.

Adult nicothoids and their eggs among crab eggs — 1b. Adults with their pink egg sacs nestled among the host’s eggs. Empty capsules mark where embryos have been consumed.

Gravid adult nicothoid with multiple egg sacs — 1c. A gravid female carrying multiple egg sacs, tangled among the host’s egg fascicles.

Heavily infested egg mass with dense nicothoid coverage — 1e and 1f. Two broods at a similar developmental stage: one deep maroon under dense nicothoid eggs, the other carrying few parasites.

Lightly infested egg mass at similar developmental stage — 1e and 1f. Two broods at a similar developmental stage: one deep maroon under dense nicothoid eggs, the other carrying few parasites.

Figure 2. Larvae, and the gills

Nicothoid embryos in egg sacs attached to adult female — 2a to 2d. The larval series in order: embryos in egg sacs, nauplii, copepodids on the egg mass, and copepodids attached to excised gills of *Metacarcinus anthonyi*. The last panel is the one that makes the life cycle close.

Nicothoid nauplii stages among crab eggs — 2a to 2d. The larval series in order: embryos in egg sacs, nauplii, copepodids on the egg mass, and copepodids attached to excised gills of *Metacarcinus anthonyi*. The last panel is the one that makes the life cycle close.

Videos

Adults on the brood. An adult moving among host eggs and feeding, showing egg mimicry and attachment.

Copepodids on gills. Larval stages on the gill lamellae, the microhabitat that carries the parasite between broods.

A heavy infestation. Dense clusters of adults and egg sacs through a single brood.

Higher resolution versions and the raw files are archived in Zenodo, linked at the top of the page.

Acknowledgments

This project builds on work with the Kuris Lab at the University of California, Santa Barbara, collaborators who helped collect crabs and identify copepods, and the Coastal Fund at the University of California, Santa Barbara (CF-202204-02367), which supported field and laboratory work.